In primary hyperthyroidism, the thyroid produces large amounts of T3 and T4, which, through negative feedback inhibition, suppress TSH secretion from the anterior pituitary. Hypothyroidism, on the other hand, happens due to inadequate thyroid hormone production. Hyperthyroidism occurs when there is excessive thyroid hormone synthesis or release. A thyroid problem can exist in the form of hyperthyroidism or hypothyroidism. In contrast, secondary thyroid disease refers to central problems arising from the anterior pituitary that indirectly affects thyroid function. Primary thyroid disease refers to problems arising from the thyroid gland itself. In this review, we discuss the physiology, biochemistry, and clinical relevance of TSH.

T4 and T3 can then exert negative feedback on the anterior pituitary with high levels of T3/T4 decreasing TSH secretion and low levels of T3/T4 increasing TSH release. When released into the circulation, it forms T3 through the process of de-iodination. Tetraiodothyronine, also known as thyroxine or T4, constitutes more than 80% of the secreted hormone.

Though it represents only 20% of the released hormone, the majority of T3 comes from the peripheral conversion of T4 to T3. Triiodothyronine, or T3, is the active form of thyroid hormone. TSH, in turn, stimulates thyroid follicular cells to release thyroid hormones in the form of T3 or T4. Specifically, neurons in the hypothalamus release TRH, or thyroid-releasing hormone, which stimulates thyrotrophs of the anterior pituitary to secrete TSH. The hypothalamic-pituitary axis regulates TSH release. It also exerts growth effects on thyroid follicular cells leading to enlargement of the thyroid. It is the primary stimulus for thyroid hormone production by the thyroid gland. Thyroid-stimulating hormone, also known as TSH, is a glycoprotein hormone produced by the anterior pituitary.

Tumor that secretes adrenaline