GLOSSARY

Thyroid-stimulating hormone (TSH, thyrotropin) is a glycoprotein from thyrotroph cells of the anterior pituitary; by stimulating TSH receptors on thyroid follicular cells, it regulates iodine uptake, the synthesis and secretion of T4 and T3, and thyroid growth.

Structure

An alpha subunit (shared with LH, FSH, and hCG) and a specific beta subunit.

Effects on the Thyroid

• Increases iodine uptake (via the NIS transporter).
• Increases synthesis of thyroglobulin and thyroid peroxidase (TPO).
• Releases T4 and T3 from thyroglobulin.
• Thyroid hypertrophy and hyperplasia (goitre with chronic stimulation).

Regulation

• Stimulation: TRH.
• Inhibition: T3 and T4 (very sensitive negative feedback; a two-fold change in FT4 → about a 100-fold change in TSH), somatostatin, dopamine, and cortisol.

Diagnostic Importance