3.4.2 Causes and laboratory investigation of hypothyroidism

Hypothyroidism is the clinical state that develops as a result of the lack of action of thyroid hormones on target tissues (1). Hypothyroidism is usually due to impaired hormone secretion by the thyroid, resulting in reduced concentrations of serum thyroxine (T₄) and triiodothyronine (T₃). The term primary hypothyroidism is applied to define the thyroid failure deriving from inherited or acquired causes that act directly on the thyroid gland by reducing the amount of functioning thyroid tissue or by inhibiting thyroid hormone production. The term central hypothyroidism is used when pituitary or hypothalamic abnormalities result in an insufficient stimulation of an otherwise normal thyroid gland. Both primary and central hypothyroidism may be transient, depending on the nature and the extent of the causal agent. Hypothyroidism following a minor loss of thyroid tissue can be recovered by compensatory hyperplasia of the residual gland. Similarly, hypothyroidism subsides when an exogenous inhibitor of thyroid function is removed.

Peripheral hypothyroidism may also arise as a consequence of tissue resistance to thyroid hormones due to a mutation in the thyroid hormone receptor. Resistance to thyroid hormones is a heterogeneous clinical entity with most patients appearing to be clinically euthyroid while some of them have symptoms of thyrotoxicosis and others display selected signs of hypothyroidism. The common feature is represented by pituitary resistance to thyroid hormones, leading to increased secretion of thyrotropin that in turn stimulates thyroid growth and function. The variability in clinical manifestations depends on the severity of the hormonal resistance, the relative degree of tissue hyposensitivity, and the coexistence of associated genetic defects (see Chapter 3.4.8).

A list of the causes of primary hypothyroidism is given in Box 3.4.2.1. Autoimmune thyroiditis is the most common cause of spontaneous hypothyroidism in areas with adequate iodine intake. Iatrogenic hypothyroidism is responsible for many hypothyroid patients in these regions and inborn errors of thyroid hormone synthesis, goitrogens, and other destructive processes of the thyroid gland account for a few cases. Iodine deficiency is crucial in the pathogenesis of endemic cretinism and of adult hypothyroidism in areas in which an efficient iodine prophylaxis has not been undertaken.

Autoimmune thyroiditis includes a spectrum of diseases that are distinguished for their clinical course, the degree of thyroid dysfunction, and the changes of thyroid size. All these variants recognize an immune-mediated pathogenesis and usually present with high titres of circulating antithyroid antibodies. As in most organ-specific autoimmune reactions, the aetiology of autoimmune thyroiditis is still unknown but is somehow linked to genetic and environmental factors, and is influenced by the gender and the age (see Chapter 3.2.6).

Chronic thyroiditis is the most common among the autoimmune thyroidites. Historically, two clinical variants of the disease are described. A goitrous variant (Hashimoto’s thyroiditis), characterized by heavy lymphocytic infiltration and thyroid enlargement, and an atrophic variant (primary myxoedema) with progressive fibrosis and reduction of thyroid size. In clinical practice, a clear distinction between the two forms is not always possible. In the initial stage of the disease the two variants commonly do not present distinctive features. Moreover, atrophy may be a destructive end result of goitrous thyroiditis with the thyroid gland showing near complete replacement with fibrosis. When overt hypothyroidism has occurred, thyroid volume shows a unimodal distribution, with thyroid atrophy and goiter being extremes within the distribution (2). Overall, these observations suggests that the two variants do not represent separate disorders.

Thyroid failure usually develops very slowly and, as thyroid function fades, the resulting increase in serum thyroid-stimulating hormone (TSH) limits the decline in thyroid secretion. Thus overt hypothyroidism is commonly preceded by a variable period of time in which elevated TSH is the only hormonal abnormality (subclinical hypothyroidism) (3, 4). The transition from euthyroidism to hypothyroidism may pass unrecognized and initial symptoms may be attributed to ageing, menopause, or other chronic concomitant diseases (5). Thus, it is not uncommon that chronic thyroiditis is diagnosed when clinical manifestations of thyroid failure become severe or complications of hypothyroidism have occurred. The circumstances leading to early diagnosis of the disease include family history for autoimmune thyroid diseases, appearance of goiter, blood testing for screening of autoimmune diseases in patients with polyglandular autoimmunity, routine diagnostic protocols for patients with menstrual dysfunction, or hyperlipidaemia. Occasionally hypothyroidism may be due to TSH-receptor blocking antibodies preventing thyroid cell stimulation by TSH. TSH-receptor blocking antibodies are more frequent in atrophic thyroiditis than in goitrous thyroiditis (6). Hypothyroidism may be reversible if the TSH-receptor blocking antibody titre declines and enough thyroid tissue remains for thyroid hormone synthesis. Graves’ hyperthyroidism may develop in hypothyroid patients with chronic thyroiditis because of a change in the nature of TSH-receptor antibodies from blocking to stimulating (7).

In some instances the disease may be preceded by a transient phase of thyrotoxicosis (hashitoxicosis) due to the discharge of preformed thyroid hormones, as a result of an unusually intense inflammatory process. The gland is tender and sometimes painful, resembling subacute thyroiditis. Hypothyroidism usually develops in a short time and may be permanent, especially in patients with elevated thyroid peroxidase antibody.

Features of thyroid-associated ophthalmopathy may occur in patients with chronic thyroiditis and hypothyroidism. This condition is termed ‘hypothyroid Graves’ disease’ and may represent a distinct entity with pathogenetic mechanisms common to Graves’ disease, or may be the endstage of Graves’ disease after spontaneous remission of hyperthyroidism.

Focal thyroiditis is characterized by spotty collections of mononuclear cells within thyroid tissue, and minimal changes in follicular epithelium or stromal fibrosis. Most patients with focal thyroiditis are euthyroid and only 10–20% have subclinical hypothyroidism (8). The disease may be suspected at ultrasound examination in patients with circulating thyroid autoantibodies, or may be a histological occurrence in surgical or autopsy specimens. In the presence of circulating thyroid autoantibodies, focal thyroiditis may represent the earliest stage of chronic autoimmune thyroiditis, whereas the clinical significance of nonspecific isolated lymphocytic infiltration in patients without circulating autoantibodies has still to be clarified.

Juvenile thyroiditis (autoimmune thyroiditis in childhood and adolescence) is described as a separate entity because follicular oxyphilia is usually mild or absent, goiter is soft, and thyroid antibody titres are not as high as in adults. Fine-needle aspiration biopsy is sometimes required to establish the diagnosis. Spontaneous resolution is relatively common but hypothyroidism may develop during the course of the disease (9).

Pregnancy is known to influence the clinical course of various autoimmune disorders, including autoimmune thyroid disease. Typically, amelioration during pregnancy is followed by aggravation after delivery. This phenomenon is thought to depend on the physiological need of inhibiting maternal immune reactions that might cause rejection of the fetus. Thus, thyroid peroxidase antibodies, thyroglobulin antibodies, and TSH-receptor antibody titres decrease or may even disappear during pregnancy. Following delivery, a rebound of autoimmune processes occurs and may result in destructive thyroiditis with release of preformed thyroid hormones and transient thyrotoxicosis. This clinical entity is named postpartum thyroiditis and occurs in 5–9% of unselected postpartum women (10). (see Chapter 3.4.6).

Spontaneous hypothyroidism may develop during the course of Graves’ disease whenever destructive processes of thyroiditis predominate over thyroid-stimulating events (burnt out Graves’ disease). This may occur after long-term remission of hyperthyroidism associated with disappearance of TSH-receptor stimulating antibodies, or following prolonged therapy with antithyroid drugs (11). TSH-receptor blocking antibodies may also appear and neutralize TSH-receptor stimulating antibodies, leading to hypothyroidism.

Subacute thyroiditis is an inflammatory disease of viral origin (12). Although the disease is relatively uncommon it must be suspected any time a patient presents with anterior neck pain. Recovery is complete in most patients but in rare cases (1–5%) epithelial loss is severe, resulting in permanent hypothyroidism (see Chapter 3.2.7).

Riedel’s thyroiditis is an extremely rare chronic disease of unknown aetiology, characterized by progressive fibrosis of the thyroid and surrounding tissues (13). Hypothyroidism develops when fibrosclerosis has involved most of the gland (see Chapter 3.2.7).

Thyroid ablation for therapeutic purposes is a common cause of primary hypothyroidism in the adult. Thyroid failure is an obvious consequence of total or subtotal thyroidectomy for thyroid cancer, goiter, or Graves’ disease, but clinical hypothyroidism does not develop as long as substitutive therapy is started shortly after surgery. Similarly, ¹³¹I therapy for Graves’ disease is directed to destroy thyroid tissue. However, the success rate of radio-iodine therapy and the time of onset of hypothyroidism are not fully predictable; they depend on several factors including the dose of radiation delivered, the size of the goiter, and the underlying autoimmune phenomena (14, 15). Drug-induced hypothyroidism is also common. Excessive inhibition of thyroid hormone synthesis commonly occurs during therapy for hyperthyroidism with antithyroid agents. Furthermore, primary hypothyroidism may develop as a side effect of several drugs administered for different purposes.

Total thyroidectomy is performed for thyroid cancer, Graves’ disease, and large diffuse or multinodular goiters, occasionally also harbouring Hashimoto’s thyroiditis. However, hypothyroidism does not develop as long as l-thyroxine replacement therapy is started soon after thyroidectomy. In patients with thyroid cancer, thyroid hormone therapy must be discontinued at an appropriate time before ¹³¹I scanning and therapy. Thus, patients develop transient, and usually not severe, clinical hypothyroidism. The availability of recombinant human TSH for clinical use will avoid hypothyroidism due to thyroid hormone withdrawal in thyroid cancer patients (see Chapter 3.5.6).

The frequency of hypothyroidism after subtotal thyroidectomy varies depending on the mass of remaining tissue and the degree of its autonomous function. A small thyroid residue may be sufficient for maintenance of the euthyroid state in Graves’ disease. On the other hand, a large residue of a multinodular or Hashimoto’s goiter may not be enough for adequate thyroid hormone secretion. Partial thyroidectomy or lobectomy for multinodular goiters or solitary nodules are usually not associated with permanent hypothyroidism.

Among different radioactive isotopes of iodine, ¹³¹I is the agent of choice in the treatment of thyroid hyperfunction. After oral administration, radio-iodine is completely absorbed, rapidly concentrated, oxidized, and organified by thyroid follicular cells. The biological effects of radio-iodine include necrosis of follicular cells, shorter survival and impaired replication of undestroyed cells, and vascular occlusion, leading to atrophy and fibrosis of thyroidal tissue.

The goal of radio-iodine therapy for hyperthyroidism is to destroy sufficient thyroid tissue to cure the hyperthyroidism with one dose of ¹³¹I. This dose is calculated on the basis of thyroid size and uptake of ¹³¹I. Because of radiation safety restrictions, in some centres small repeated doses of radio-iodine are administered. In other centres standard fixed doses are given. Small glands are destroyed more readily by radio-iodine than larger ones, and toxic adenoma or toxic multinodular goiter are usually more radioresistant than Graves’ glands. Radio-iodine has a delayed effect and several months may be required for the complete control of hyperthyroidism.

In the case of Graves’ disease, the goal of radio-iodine should be to destroy as much thyroid tissue as possible (15). This strategy has been adopted because residual tissue, necessary to ensure euthyroidism, is responsible for the relapse of hyperthyroidism in a large proportion of patients. A strict control of thyroid function is required during the first 6–12 months following ¹³¹I therapy for Graves’ disease to avoid the appearance of symptoms of hypothyroidism, which may be rapidly progressive and severe. Early postradio-iodine hypothyroidism may be transient, and hyperthyroidism may relapse during l-thyroxine replacement therapy.

Radio-iodine-induced hypothyroidism is less frequent after treatment for toxic adenoma or multinodular goiter because nonfunctioning thyroid tissue should not receive the radioisotope. Yet, hypothyroidism may develop whenever TSH is not completely suppressed at the time ¹³¹I is administered. Furthermore, a small degree of iodine uptake is maintained in normal thyroid cells even in the absence of TSH stimulation, and this may be the cause of hypothyroidism many years after radio-iodine administration.

External irradiation to the neck for nonthyroidal neoplasias (lymphomas, tumours of the head and neck, spinal tumours, or metastases) may produce hypothyroidism in up to 50% of patients (16). Thyroid failure may develop after a variable interval, depending on the dose of radiation that has been administered. Hypothyroidism after total body irradiation for acute leukaemia or aplastic anaemia has also been reported (17). An increased risk of hypothyroidism has been found in older breast cancer patients treated with radiation, since a portion of the thyroid gland may be included in the treatment fields (18).

Transient hypothyroidism is common in the course of medical treatment for hyperthyroidism with thionamides, and quickly subsides with adjustment of the dose. Excess iodide, such as in disinfectants, radiographic contrast agents, and seaweed-containing preparations, may precipitate hypothyroidism in autoimmune chronic thyroiditis, due to failure of the thyroid to escape from the Wolff–Chaikoff effect. Animal studies suggest that excessive iodide increases the incidence of thyroid autoimmunity but evidence in humans is controversial. Amiodarone is an antiarrhythmic agent containing about 37 mg iodine per 100 mg drug. Amiodarone may produce hypothyroidism by the excess iodine released with metabolism of the drug. As in other cases of excess iodine administration, an underlying autoimmune thyroid disease is a prerequisite (19). Amiodarone may also induce destructive thyroiditis in an otherwise normal thyroid gland. The pathogenetic mechanisms of this phenomenon are not clear, and hypothyroidism follows a transient thyrotoxic phase. Distinction of the two forms of amiodarone-induced hypothyroidism is important for choosing the right treatment measures and method of follow-up.

Lithium inhibits thyroid hormone synthesis and secretion, and long-term lithium therapy for psychiatric disorders may result in subclinical (up to 23%) or overt (up to 19%) hypothyroidism (20). The risk of development of hypothyroidism is increased in patients with positive antithyroid antibodies or with minor thyroid abnormalities, which reduce the ability of the thyroid gland to override the inhibitory effects of lithium. Goiter is also common in lithium-treated patients, even when serum thyroid hormones and TSH are within normal limits.

Tyrosine kinase inhibitors are newly developed drugs approved for the treatment of several tumours. The first observation of hypothyroidism after sunitinib treatment has been reported in 2006 (21). Since then several studies have been published and have confirmed that various tyrosine kinase inhibitors can affect thyroid function tests through different physiopathological mechanisms impairing thyroid function or thyroid hormone metabolism (22).

Several other drugs have been reported to be capable of inducing primary hypothyroidism (23). Treatment with interferon-α or interleukin-2 may produce hypothyroidism, thyrotoxicosis, or the biphasic pattern of silent thyroiditis. Pre-existent thyroid autoimmunity increases the risk of thyroid dysfunction during treatment with these agents. Other medications occasionally reported to induce hypothyroidism include sulfonamides, sulfonylureas, ethionamide, p-aminosalicylic acid, phenylbutazone, and nicardipine, but the antithyroid potential of these drugs is weak and an underlying thyroid abnormality or concurrent iodine deficiency are usually associated.

Environmental iodine deficiency is common in many areas throughout the world, particularly in inland mountainous areas. Goiter is the most common disorder due to iodine deficiency and its prevalence is inversely related to the median iodine intake of the population. Endemic goiter is usually not associated with hypothyroidism. However, the pattern of circulating thyroid hormones in the population from areas of severe iodine deficiency differs from that found in iodine-sufficient areas (24). The mean serum T₄ is reduced while serum T₃ is unchanged or increased and an inverse correlation between serum TSH and T₄ is found. The low iodine content within the thyroid gland and the increased TSH stimulation lead to preferential secretion of T₃, which is far more potent than T₄ in terms of metabolic responses. Thus, the relative increase in T₃ secretion enables a patient to maintain the euthyroid status in spite of reduced availability of iodide.

Cretinism is the result of an insufficient supply of thyroid hormones to fetal tissues and is due to severe iodine deficiency in both the mother and the fetus during early stages of gestation (25). Fetal hypothyroidism is not compensated by transplacental passage of maternal T₄ and is responsible for severe physical and neurological damage.

Adult hypothyroidism may occur in rural populations living in areas of severe iodine deficiency where isolation prevents access to iodine-rich foodstuff. In this case, hypothyroidism is rapidly reversed by iodine supplementation. Consumption of food containing antithyroid agents, such as thiocyanate in cassava meal and flavonoids in a variety of plants, may aggravate the effects of dietary iodine deficiency and add to the development of goiter and hypothyroidism. Phloroglucinol, a potent antithyroid compound contained in some species of seaweeds, may play an additional role to that of iodine excess in the development of iodine-induced hypothyroidism. More recently, attention has been focused on environmental endocrine disrupters (pesticides and industrial pollutants) as a possible cause of thyroid imbalance, but their effects on human thyroid function have not been fully elucidated (see also Chapter 3.2.2) (26).

A high prevalence of primary hypothyroidism has been described in patients with thalassaemia major. The incidence and severity of thyroidal dysfunction appears related to the degree of iron overload. Hypothyroidism may contribute to deterioration of heart function, and regular iron chelation therapy should be advised for these patients (27, 28).

Congenital hypothyroidism occurs in 1/3000–4000 neonates worldwide, and may be classified as permanent or transient. Primary congenital hypothyroidism accounts for most affected children, whereas central congenital hypothyroidism is rare (29) (see Chapter 3.4.7).

Both the fetus and the neonate are particularly sensitive to the block of thyroid function induced by excess iodide since the immature gland is not able to escape from the Wolff–Chaikoff effect (30). Iodide-induced transient hypothyroidism is most common in premature infants and in low-birthweight babies, and has occurred more in relatively iodine-deficient areas of Europe (31), than in iodine-sufficient North America.

Transient fetal–neonatal hypothyroidism and goiter may develop in babies born to hyperthyroid mothers with Graves’ disease treated with excessive doses of propylthiouracil or methimazole. Both hypothyroidism and goiter resolve spontaneously with the clearance of the drug from the circulation of the neonate. TSH-receptor blocking antibodies may be present in patients with autoimmune hypothyroidism; the antibodies compete with TSH and inhibit the biological effects of TSH on thyroid cell function and growth (6). These antibodies have been found mainly in patients with autoimmune atrophic thyroiditis, and contribute to the development of thyroid failure and atrophy. The maternal TSH-receptor antibody responsible for thyroid failure in the neonate inhibits TSH binding to its receptor and therefore blocks the effect of TSH on adenylate cyclase stimulation, iodine uptake, and thyroid cell growth (32). TSH-receptor blocking antibodies may also occur in women with Graves’ disease and be transmitted to the fetus. Although thyroid-stimulating antibodies usually predominate in these patients, transient hypothyroidism is possible in the offspring of women with Graves’ disease due to very high TSH-blocking antibody titres and relatively low concentrations of thyroid-stimulating antibody. Because TSH-induced growth is blocked, these infants do not have a goiter.

Central hypothyroidism is the consequence of anatomical or functional disorders of the pituitary or the hypothalamus. Several of the causes reported in Box 3.4.2.2 may affect both the pituitary and the hypothalamus, and in many instances the main anatomical site of the dysfunction cannot be identified. Thus, the former terms of secondary hypothyroidism (of pituitary origin) and tertiary hypothyroidism (of hypothalamic origin) are no longer recommended (33). Central hypothyroidism is rarely isolated, being part of a generalized disorder involving the secretion of other pituitary hormones. Permanent central hypothyroidism is rare, its prevalence being about 0.005% of the general population. However, transient functional abnormalities of TSH secretion are relatively common, and often pass unrecognized due to rapid recovery of the normal thyroid hormone balance.

Pituitary adenomas represent the most common cause of central hypothyroidism. Reduced secretion of TSH is usually a consequence of mechanical compression of nontumorous cells and of adenohypophyseal blood vessels by the adenoma (33). The pituitary stalk and the hypothalamus may also be involved by suprasellar extension of the tumour. The tumour may be nonfunctioning or secrete other hormones. Thus, the resulting syndrome will depend on the extent of hypopituitarism and on the particular hormone secreted by the adenoma. A sudden enlargement of pituitary adenomas may occur as a result of haemorrhage within the tumour, leading to pituitary apoplexy. Several other causes may produce central hypothyroidism, by acting at the hypothalamic or pituitary level. Primary extrasellar brain tumours or metastatic tumours originating from other sites may produce a variable degree of hypopituitarism, depending on the location and the extension of their mass. Among brain tumours, craniopharyngiomas should be suspected when central hypothyroidism is diagnosed in young people. Craniopharyngiomas are usually extrasellar but they may extend inferiorly causing destruction of the bony margins of the sella. Pituitary infarction may develop postpartum following excessive blood loss during delivery (Sheehan’s syndrome), or in patients with severe shock or during systemic anticoagulation therapy. Various degrees of pituitary insufficiency may be observed in these cases. Traumatic head injuries can lead to central hypothyroidism because of hypothalamic or pituitary infarction or haemorrhage. Iatrogenic causes of central hypothyroidism include external radiation and surgery for pituitary or brain tumours. The empty sella syndrome is caused by a defect of the sellar diaphragm leading to cisternal herniation within the pituitary fossa and flattening of the pituitary. Hypopituitarism develops slowly along with expansion of the cisternal herniation caused by transmission of cerebrospinal fluid pressure.

Hypothalamic or pituitary lesions may derive from any of the infectious or granulomatous diseases listed in Box 3.4.2.2. Chronic lymphocytic hypophysitis may be responsible for pituitary insufficiency, and has been described in association with autoimmune thyroiditis or adrenalitis. Recent studies have demonstrated a high prevalence of hypothyroidism following traumatic brain injury or subarachnoid haemorrhage, although TSH deficiency is less common than growth hormone, luteinizing hormone/follicle-stimulating hormone, and adrenocorticotropic hormone deficiencies (34). Bexarotene, a selective ligand for the retinoid X receptor which has been approved for the treatment of cutaneous T-cell lymphoma, may cause central hypothyroidism, with marked reductions in serum TSH and thyroid hormone levels in a significant proportion of treated patients (35). Pituitary aplasia or hypoplasia is a rare congenital defect, usually associated with other severe malformations. In most instances these patients die shortly after birth. Genetic abnormalities in TSH synthesis may cause central hypothyroidism characterized by inherited isolated TSH deficiency. Mutations in the TSH β-subunit gene or in a pituitary-specific transcription factor (Pit1/GHF-1) have been described in a few families (36, 37). Inactivating mutations in the thyrotropin-releasing hormone (TRH) receptor gene have also been reported (38, 39). In some patients no demonstrable pathology can be found to explain TSH deficiency, and the term idiopathic central hypothyroidism is therefore applied. Impairment of TRH secretion, TSH synthesis, or TSH release have been hypothesized in the pathogenesis of this disorder.

Transient impairment of TSH secretion is commonly observed and may depend on a variety of causes (see Box 3.4.2.2). The recognition of these conditions is essential to avoid unnecessary and expensive diagnostic procedures. In most instances replacement therapy is not necessary or is contraindicated.

The diagnosis of hypothyroidism and of its cause requires the evaluation of several clinical, laboratory, and instrumental parameters to manage the patient properly (Table 3.4.2.1).

A small decrease in thyroid secretion may produce only minor changes in serum concentrations of thyroid hormones that remain within the normal range. The most sensitive index of a reduction in serum thyroid hormone concentration is serum TSH because of a decrease in feedback inhibition of pituitary TSH secretion. Thus, elevated serum TSH is the earliest laboratory abnormality in patients with primary hypothyroidism. The combination of normal thyroid hormones and elevated TSH is defined as subclinical hypothyroidism (3). With the progression of thyroid dysfunction, serum levels of T₄ fall below the normal limit while serum T₃ may still be normal. This is because high TSH levels induce preferential secretion of T₃ by residual thyroid tissue.

The lack of TSH response to reduced thyroid hormone levels complicates the diagnosis of central hypothyroidism, and the finding of low serum T₄ is a prerequisite for the diagnosis of this condition. Usually in central hypothyroidism, basal serum TSH concentrations are inappropriately low with respect to reduced serum thyroid hormones. Yet, in some instances serum TSH may be slightly elevated due to secretion of immunoreactive but biologically inactive TSH (40).

Assays for measurement of total thyroid hormones in serum are gradually being replaced by methods that determine the free (unbound) fraction of T₄ and T₃ (41). Although measurement of free T₄ and free T₃ concentrations is more cumbersome as compared to that for total T₄ and T₃, free T₄ and free T₃ determinations are preferred because free thyroid hormones are those capable of entering the cell and therefore represent the biologically active hormone. Indeed, the concentrations of total thyroid hormones may be elevated or reduced in spite of normal free fractions, due to changes in the concentrations of serum transport protein (see Chapter 3.1.2) (42).

Measurement of the serum TSH response to TRH (200–500 μg intravenously) may be useful in selected patients with a borderline to low value of T₄ and borderline to high or borderline to low values of basal TSH, to identify subclinical primary or central hypothyroidism, respectively. An exaggerated response will be observed in primary hypothyroidism whereas in central hypothyroidism the serum response of TSH may be reduced or abnormally prolonged. The TRH test may be useful also to measure the increase in serum T₃ levels following the rise in serum TSH. In people with normal thyroid function, serum T₃ increases 30–100% above the baseline value 120–180 min after the injection of 200 μg TRH. In central hypothyroidism, the T₃ response may be impaired or absent in spite of a normal peak of TSH, indicating secretion of biologically inactive TSH (43). Evaluation of the nocturnal surge of TSH in samples taken every 30 min from 11.00 p.m. to 2.00 a.m. may be useful to confirm the diagnosis of central hypothyroidism. At variance with people with normal thyroid function, the TSH surge is blunted or absent in central hypothyroid patients (44).

A transient phase of central hypothyroidism may occur in patients with nonthyroidal illness, particularly hospitalized patients with medical or psychiatric illnesses. In these cases repeated hormonal measurements are useful since values usually become normal as patients recover from that illness.

Antithyroglobulin and antithyroperoxidase antibodies are sensitive markers of thyroid autoimmunity. Thus, if present, they may contribute to the diagnosis of autoimmune thyroiditis, represent a prognostic index for the development of postpartum thyroiditis, and help to predict the outcome of iodine- or drug-induced hypothyroidism. Antithyroglobulin antibodies are found in up to 70% of patients and antithyroperoxidase antibodies in 80–95% of patients with chronic autoimmune thyroiditis. Low titres can be found in 20–35% of patients with other nonautoimmune thyroid diseases and sometimes also in people with normal thyroid function (45). l-thyroxine therapy has been shown to reduce serum levels of antithyroglobulin antibodies and antithyroperoxidase antibodies in patients with autoimmune chronic thyroiditis. TSH-receptor antibodies can either have stimulating activity (thyroid-stimulating antibody), as in Graves’ disease, or block the receptor (TSH-receptor blocking antibody) preventing TSH stimulation of the follicular cell. TSH-receptor blocking antibodies are highly specific for autoimmune thyroiditis. They are found in up to 30% of patients with chronic autoimmune thyroiditis and can produce or add to the development of hypothyroidism by blocking the thyroid response to TSH (6). Hypothyroidism produced by TSH-receptor blocking antibodies can spontaneously remit following disappearance of antibody from serum. Assays for TSH-receptor antibodies measure the ability of a patient’s IgG to inhibit the binding of ¹²⁵I-TSH to its receptor in thyroid membrane preparations. Radioreceptor assays are now easy to perform, inexpensive, and provide reliable results but do not distinguish thyroid-stimulating antibodies from TSH-receptor blocking antibodies. For this purpose methods that assess the capacity of IgG to stimulate or to prevent TSH-induced cAMP production in thyroid preparations are necessary.

Endogenous antibodies against thyroid hormones may develop in patients with autoimmune thyroiditis (46). These antibodies usually have no clinical relevance, but may interfere on assays for serum total and free T₄ and T₃, producing artefactual results depending on the technique used to measure the hormones. The presence of T₄ or T₃ antibodies should always be suspected in autoimmune patients with unexpected results of thyroid hormone assays. These antibodies can be detected easily by immunoprecipitation of radiolabelled T₄ or T₃ with the patient’s serum.

Thyroglobulin is present at low concentrations in serum of people with normal thyroid function, and is elevated in all states associated with enlargement, hyperfunction, or injury of the thyroid gland (47). Measurement of serum thyroglobulin has no meaning for the diagnosis or the management of hypothyroidism, but may be useful to estimate the amount of residual thyroid tissue after surgery or other thyroid destructive events. Furthermore, detectable serum thyroglobulin in congenital hypothyroidism excludes thyroid agenesis. Antithyroglobulin antibodies in serum interfere with measurement of thyroglobulin and therefore this test should not be performed in such patients.

Measurement of urinary iodide provides information about the daily iodide intake in epidemiological studies (48). The demonstration of elevated concentrations of urinary iodide in a hypothyroid patient may be useful if exposure to excessive iodide is suspected.

Thyroid ultrasonography may be helpful in determining the cause of hypothyroidism by providing important information on location, size, structure, and vascularity of the gland. In autoimmune thyroiditis a gross inhomogeneity and low echogenicity characterize the echo pattern of the gland. Areas of apparently normal tissue of variable size may be observed, whereas true nodules reflect a different aetiology and should raise the possibility of coexisting nodular goiter, adenomas, or malignancies. A diffuse low thyroid echogenicity is indicative of diffuse autoimmune involvement of the gland and is associated with or may predict the development of hypothyroidism (49). Studies using colour flow Doppler show a variable degree of vascularity in goitrous autoimmune thyroiditis, whereas vascularity is decreased in the atrophic variant of the disease. In subacute thyroiditis the gland is usually enlarged and presents large hypoechoic areas with poorly defined boundaries, mainly within the painful lobe. A large diffuse or multinodular goiter can be documented by ultrasonography in hypothyroidism with inherited defects in thyroid hormone biosynthesis. No evidence of thyroidal tissue in its appropriate location and the demonstration of an ectopic gland are helpful in the diagnosis of congenital hypothyroidism due to thyroid dysgenesis.

Thyroid scintiscan may be helpful in the evaluation of hypothyroid patients to indicate the location of functioning thyroid tissue and to provide an estimation of overall thyroid size, although in this regard better evidence is usually obtained by thyroid ultrasonography. Occasionally scintiscan may reveal ectopic thyroid tissue not discernible by other means (e.g. lingual thyroid). Thyroid scintiscan can also be used to reveal substernal thyroid tissue when hypothyroidism is associated with a large goiter.

Radio-iodine uptake is expressed as the percentage of radioactivity that is trapped by the thyroid at a given time after administration of a tracer quantity of inorganic radio-iodine. Early radio-iodine uptake measurements (3–6 h) provide information on the rates of transport and organification of iodide within the gland, whereas 24- and 48-h radio-iodine uptake measurement reflects the rate of release of radio-iodine from thyroidal tissue. It is also a way of estimating the extrathyroidal pool of iodide, being low to absent after intake of excess iodide but increased in iodine deficiency. An exception is represented by amiodarone-induced hypothyroidism in which radio-iodine uptake is preserved despite iodine excess (50). Radio-iodine uptake is increased if hypothyroidism is caused by defective synthesis of thyroid hormones since TSH stimulates all steps in hormone synthesis capable of response. In chronic autoimmune thyroiditis values of the radio-iodine uptake depend on the amount of residual functioning thyroid tissue and the serum concentration of TSH. Radio-iodine uptake may be normal or even increased during the initial phase of chronic thyroiditis, whereas it tends to decrease as the disease progresses. Very low values of the radio-iodine uptake are characteristic of the early phase of destructive thyroiditis (e.g. subacute thyroiditis) which is usually associated with thyrotoxicosis caused by follicular disruption. In these cases, return of radio-iodine uptake to within the normal range may be helpful to indicate recovery of thyroid function. Radio-iodine uptake measurement, which is obviously reduced in postablative hypothyroidism, may be used occasionally to estimate the amount of residual thyroid tissue after thyroidectomy or radioactive treatment.