Thyroid Physiology in Health and Disease

Abstract

The pituitary thyrotrophin reserve in 55 patients was tested with thyrotrophin-releasing hormone, and only 28% with deficient responses were hypothyroid; in three patients with hypothalamic disorders the response was delayed but otherwise normal. Three pituitary hormones, thyrotrophin, luteinizing hormone, and follicle-stimulating hormone, contain the same alpha polypeptide chain but different beta chains that confer specificity. Although the serum triiodothyronine (T₃) concentration is 1/75th that of thyroxine (T₄), its greater volume of distribution (threefold) and disappearance rate (sixfold) results in a T₃ turnover one third to one fourth that of T₄. Approximately 20% to 33% of T₃ comes from the thyroid gland, the remainder from T₄. But T₄, a prohormone for T₃, also contributes hormonal effects. The features of Graves' disease suggest it is an autoimmune disorder, but long-acting thyroid stimulator does not correlate with level or control of thyroid function. The serum thyroglobulin level is higher in women (6.0 ng/ml) than men (3.4 ng/ml); the levels are stable, higher in the newborn than in their mothers; and they rise after thyroid stimulation and fall after suppression. Thyroglobulin levels are elevated during active subacute thyroiditis and are persistently high in hyperthyroidism.