The relationship between serum TSH and total free T4 concentrations is shown in normal subjects (N) and in various typical abnormalities of thyroid function: primary hypothyroidism (A); central or pituitary-dependent hypothyroidism (B); thyrotoxicosis due to autonomy or abnormal thyroid stimulation (C); and TSH-dependent thyrotoxicosis or generalized thyroid hormone resistance (D). Note that linear free T4 responses correspond to logarithmic TSH changes. Areas A and C represent primary thyroid abnormalities, while results that fall in areas B and D suggest a primary pituitary abnormality. Abnormal findings that fall in the intermediate areas suggest non-steady state sampling conditions due to the large difference in half-lives of TSH and T4, an assay artefact, an altered T4–TSH relationship, or the presence of another agonist, e.g. T3.
