THE pituitary glycoprotein hormone TSH is synthesized and secreted by the thyrotropes in the anterior pituitary. TSH acts on the thyroid gland to increase the production of the thyroid hormones T₃ and T_4, which then act on organs throughout the body to modulate growth and various metabolic processes. High levels of the thyroid hormones in turn feed back at the level of the pituitary to suppress synthesis and secretion of TSH, and the thyroid hormone concentration is the most important physiological regulator of TSH levels (1–5). Additional regulation can occur via hypothalamic hormones such as TRH and dopamine, which stimulate and suppress TSH levels, respectively (1, 2) (Fig. 1). Thyroid and hypothalamic hormones interact to modulate both circulating levels of TSH and the transcription of the genes encoding the hormone.

TSH is one of three pituitary glycoprotein hormones that consists of two dissimilar noncovalent subunits, α and β. The α-subunit is common to all three, including TSH and the two gonadotropin hormones LH and FSH, while the β-subunits are unique for each hormone and confer biological specificity (3). The subunits are synthesized as separate peptides from distinct messenger RNAs (mRNAs) (6–9). The α-subunits have molecular weights of approximately 20,000–22,000 and contain two relinked carbohydrate groups. The TSH subunits molecular weights are approximately 18,000 and contain one Nlinked complex carbohydrate (1).