Disorders of the hypothalamic-pituitary-thyroid axis

This chapter reviews disorders that are not primarily related to thyroid gland pathology. Thyroid hormone synthesis and secretion is stimulated by the action of thyrotropin (TSH), which is secreted into the general circulation from the anterior pituitary thyrotrope cells. Thyrotropin (TSH) is a member of the glycoprotein hormone family, which includes luteinizing hormone (LH), follicle-stimulating hormone (FSH) and human chorionic gonadotropin (hCG). Hyperthyroxinemia in the setting of a nonsuppressed TSH level is secondary to two different clinical disorders that include resistance to thyroid hormone (RTH) and TSH-secreting pituitary adenoma (TSHoma). The cause of RTH is a germline defect, and as such there is no cure for this condition. As many patients are clinically euthyroid, those compensated patients without symptoms do not require any treatment. TSH-secreting pituitary adenoma is a rare tumor of the anterior pituitary. Nearly three-quarters of all TSHomas secrete only TSH. Patients with hypopituitarism have decreased quality of life, despite replacement hormonal therapy.