15 The thyroid

Thyroid problems are frequent topics in both undergraduate and postgraduate exams. It is therefore well worth investing some time in understanding the thyroid.

The thyroid begins its development at the foramen caecum at the base of the tongue. The foramen caecum lies at the junction of the anterior two-thirds and the posterior third of the tongue in the midline ( see Fig. 15.1).

The thyroid descends through the tissues of the neck and comes to rest overlying the trachea. This descent leaves a tract behind it—this can be the source of pathology in later life (e.g. thyroglossal cysts— see ‘Congenital neck remnants’, p. 266).

The thyroid gland is surrounded by pretracheal fascia and is bound tightly to the trachea and to the larynx. This means the gland moves upwards during swallowing. The recurrent laryngeal nerves (branches of the vagus) lie very close to the posterior aspect of the thyroid lobes. These nerves have ascended from the mediastinum in the tracheo-oesophageal grooves and they are at risk in thyroid operations. They may become involved in thyroid malignancy—in cases of malignancy a patient will most often present with a weak and breathy hoarse voice.

The thyroid gland has a very rich blood supply—trauma or surgery to the gland can lead to impressive haemorrhage into the neck.

The parathyroid glands—important in calcium metabolism—lie embedded on the posterior aspect of the thyroid lobes.

Goitre simply means an enlargement of the thyroid gland. It is not in itself a diagnosis. Both physiological and pathological conditions may cause a goitre.

This is a diffuse enlargement of the thyroid and may result from iodine deficiency. Diffuse enlargement of the gland also occurs in Graves’ disease.

This benign goitre is the commonest thyroid problem. It is caused by episodic periods of thyroid hypofunction and subsequent thyroid-stimulating hormone hypersecretion which leads to hyperplasia of the gland. This is followed by involution of the gland. Prolonged periods of hyperplasia and involution are thought to be responsible for the nodular enlargement of the gland found in a multinodular goitre.

A finding of a single nodular enlargement of the thyroid raises the question of malignancy. This should be managed as described ( see Fig. 15.2).

A partial thyroidectomy may be necessary but only in a patient with one or all of the following signs:

This is an autoimmune condition where antibodies are produced that mimic the effect of thyroid-stimulating hormone (TSH). A hyperthyroid state develops and there is often a smooth goitre. The patient’s eye signs may be most impressive (the actor Marti Feldman had this condition). ( See ‘Thyroid investigations’, p. 280) for eye signs in Graves’ disease.

Hormonal manipulation with carbimazole. Surgery to correct the proptosis may be achieved via a transnasal orbital decompression. Here, the medial wall of the bony orbit is removed to allow the orbital contents to herniate into the nasal cavity.

This is an autoimmune condition where there is often hyperthyroidism and where many patients develop a goitre. Thyroxine replacements may be necessary. Patients with this condition have an increased risk of developing a thyroid lymphoma.

Thyroid tumours may arise from either the follicular cells or the supporting cells found in the normal gland. They are quite common and each of these tumours has its own particular characteristics ( see Fig. 15.3). Papillary and follicular adenocarcinomas are frequently referred to as ‘differentiated thyroid tumours’.

These usually affect adults aged 40–50 years. There are usually multiple tumours within the gland. 60% of affected patients have involved neck nodes.

If the disease is limited to the gland, 90% of patients will survive 10 years or more. If the disease has spread to involve the neck nodes, 60% of patients will survive 10 years or more.

Involves a near-total thyroidectomy, plus a neck dissection where there are involved nodes. Postoperative radio-iodine may be given to ablate any viable thyroid tissue or tumour left behind after the surgery. After surgery, patients will need lifelong thyroid replacement at TSH suppressing doses.

It usually affects adults aged 50–60 years. There is a well-defined capsule enclosing the tumour and it spreads via the bloodstream. Up to 30% of patients will have distant metastases at presentation, and hence the prognosis is less good than in papillary adenocarcinoma.

As above for papillary adenocarcinoma.

This condition occurs in adults over 70 years of age, and is more common in women. It involves rapid enlargement of the thyroid gland and pain. The patient will have airway, voice, or swallowing problems due to direct involvement of the trachea, larynx, or oesophagus.

The prognosis is very poor: 92% of patients with this condition will die within 1 year, even with treatment.

This arises from the parafollicular C cells (or calcitonin-secreting cells). The patient’s serum calcitonin level is raised while their serum calcium level remains normal. Neck metastases are present in up to 30% of patients.

Involves a near-total thyroidectomy and radiotherapy.

These can be functioning or non-functioning:

They produce thyroxine and will take up iodine and technetium. They appear bright or ‘hot’ on isotope scanning. Symptoms of thyrotoxicosis may develop. They are rarely malignant.

Treatment is usually medical via thyroid suppressing drugs, but may be treated surgically via excision. Radiotherapy and ablation may be required.

These adenomas do not take up iodine. They appear ‘cold’ on isotope scanning. 10–20% will be malignant. Treatment will be via a surgical excision.

Before performing any special investigations look for signs of abnormal thyroid function. Classic signs are given Table 15.1.

Graves’ disease gives rise to particular ‘eye signs’. These include:

T4 (thyroxine) and T3 (tri-iodo-thyronine) are both bound to plasma proteins in the blood, but a proportion of both remains unbound and these are physiologically active. Bear this in mind when interpreting these results in conditions where the free-to-bound ratio may be disturbed, e.g. nephrotic syndrome or pregnancy.

Thyroid-stimulating hormone (TSH) controls the production of thyroid hormones via a negative feedback mechanism. TSH is usually raised in hypothyroidism and reduced in hyperthyroidism.

This is the carrier protein for T4. Its levels can be measured directly in the blood. It is most frequently used as a tumour marker for the differentiated thyroid carcinomas.

This is produced by the medullary C cells of the thyroid. Levels are raised in medullary thyroid carcinomas.

This is a tumour marker of medullary carcinoma of the thyroid.

Specific thyroid autoantibodies can be identified in Graves’ disease and Hashimoto’s thyroiditis.

Radiolabelled [¹²³I]iodine or [⁹⁹Tc^m]technetium is given to the patient orally. Then radiology is used to assess its subsequent uptake into metabolically active thyroid tissue. A thyroid nodule may take up the marker—it will appear bright or ‘hot’, or it will fail to accumulate the marker—and it will appear ‘cold’.

This is an excellent investigation to demonstrate the thyroid. It will readily distinguish solid and cystic masses inside the thyroid. Often, a USS will show that what appears clinically as a single nodule is in fact part of a multinodular goitre.

These scans may be helpful in determining the extent of a retrosternal swelling. They may confirm airway distortion or compression from a large goitre or they may demonstrate nodal metastases.

This test will differentiate solid from cystic masses and may diagnose malignancy. A residual mass noted after cyst aspiration should be tested again by FNA to exclude malignancy. It is difficult to distinguish between follicular adenoma and follicular carcinoma. This difference relies on demonstrating capsular invasion which is impossible on cytological features alone. Formal histology is usually required to confirm this diagnosis.

This is best shown diagrammatically, see the flow chart in Fig. 15.2.

Patients experiencing hypothyroid states and after thyroidectomy may need to take thyroxine for life. Doses of levothyroxine sufficient to suppress the TSH production are given in well-differentiated thyroid cancers to reduce tumour growth since these tumours are also TSH-driven.

May be given in hyperthyroidism since these inhibit the formation of T3 and T4.

Most well-differentiated thyroid tumours will trap iodine. This ability can be put to therapeutic effect by administering radioactive iodine. The patient is first rendered hypothyroid by thyroidectomy. The tumour cells then become hungry for iodine and as such will avidly take up the radioactive iodine to their own cytotoxic demise! Radio-iodine therapy can also be used to control a persistent hyperthyroid state.

Thyroid surgery is generally safe and well tolerated by patients.

This involves the removal of one thyroid lobe. It is indicated in benign thyroid conditions and as an excisional biopsy procedure where malignancy is suspected but not confirmed.

This is indicated in thyroid malignancy. Because it increases the risks to the recurrent laryngeal nerves and to the parathyroid glands, some surgeons will perform a near-total thyroidectomy, leaving a small amount of thyroid tissue behind in the area of the recurrent laryngeal nerve.

Some of the most common and important risks of thyroid surgery are given below.

This is due to recurrent laryngeal nerve damage. Patients will present with a weak and breathy voice. All patients should undergo a vocal cord check preoperatively to document cord mobility before the procedure.

This will lead to medialization of the vocal cords resulting in life-threatening airway obstruction. Facilities for re-intubation and tracheostomy must be readily available.

Haematoma after thyroid surgery is another potentially serious complication. This is because the vascular nature of the thyroid can lead to a rapid accumulation of blood in the neck, resulting in compression of the airway. For this reason, all thyroidectomy patients should have stitch/clip removers located at the bedside. If a patient’s neck begins to swell rapidly after thyroid surgery, the wound should be re-opened (on the ward if necessary), the clot evacuated, and the airway restored. Once the airway has been secured, the bleeding point can be found and controlled.

This should be anticipated whenever a total thyroidectomy has been performed. Daily calcium levels should be checked and the patient should be observed for the signs of hypocalcaemia such as:

As soon as hypocalcaemia is suspected the patient should be given IV calcium gluconate and started on oral replacement therapy.