Central hypothyroidism and hyperthyroidism

Thyroid function is maintained by tonic secretion of TSH by the pituitary. TSH secretion, in turn, is dependent on hypothalamic TRH production. Therefore, diseases of the hypothalamus and pituitary are frequently associated with TSH deficiency, producing central hypothyroidism. All patients with hypothalamic or pituitary disease should have thyroid function tests including a serum TSH by radioimmunoassay (RIA). In central hypothyroidism the TSH RIA is inappropriately low in relationship to the degree of hypothyroxinemia but is not always undetectable. In fact, because of the production of biologically inactive TSH, the TSH RIA may be in the high range of normal. Therapy of central hypothyroidism includes the management of associated pituitary hormone deficiencies, particularly secondary adrenal failure, and neurologic defects. A rare cause of hyperthyroidism is excessive TSH secretion. This may be due to a TSH-secreting pituitary tumor or to a functional disturbance in TSH secretion. TSH-secreting pituitary tumors are often large and locally invasive. Selective pituitary resistance to thyroid hormone is the most common cause of functional TSH-induced hyperthyroidism. It is important to rule out generalized thyroid hormone resistance before use of antithyroid drugs or thyroid surgery in patients suspected of this disorder. This is because antithyroid treatment is contraindicated in generalized thyroid hormone resistance.