In primary thyroid failure, the low circulating thyroid hormone levels stimulate the pituitary to increase TSH output (Fig. 15.30). The combination of low T4 and a high basal TSH is therefore almost diagnostic of primary hypothyroidism. A normal basal TSH excludes primary hypothyroidism if the pituitary is intact. The TSH response to TRH is typically exaggerated, but the rise in TSH is proportional to the basal level, and the TRH test is seldom necessary. Serum T3 levels may be normal, even though T4 levels are low and TSH is raised. The preservation of normal T3 levels may be interpreted as a compensatory mechanism of thyroid hormone production.
