A concise review of thyroid hormone synthesis, regulation, and key clinical correlations relevant to practice and academic study.
The thyroid gland produces thyroxine (T4) and triiodothyronine (T3), hormones essential for basal metabolic rate, thermogenesis, and neurodevelopment. Iodide is transported into follicular cells via the sodium–iodide symporter, oxidized by thyroid peroxidase (TPO), and incorporated into thyroglobulin to form mono- and diiodotyrosine residues. Coupling reactions yield T4 and T3, which are stored in the colloid and released after thyroglobulin endocytosis and proteolysis. Thyroid-stimulating hormone (TSH) from the anterior pituitary is the principal trophic regulator, itself controlled by hypothalamic thyrotropin-releasing hormone and negative feedback from circulating free T4 and free T3.
Most circulating hormone is protein-bound; free hormone drives biologic activity and feedback. Peripheral conversion of T4 to T3 by deiodinases explains why T4 functions largely as a prohormone. In illness, conversion patterns may shift (e.g., reduced T3), complicating interpretation of thyroid tests. Clinically, understanding the axis helps interpret discordant results such as elevated TSH with normal free T4 (often subclinical hypothyroidism) or suppressed TSH with normal free T4/T3 (possible subclinical hyperthyroidism or non-thyroidal factors). A structured approach—confirming assay validity, reviewing medications (biotin, amiodarone), and correlating with symptoms—improves diagnostic accuracy and prevents over-treatment.
Most circulating hormone is protein-bound; free hormone drives biologic activity and feedback. Peripheral conversion of T4 to T3 by deiodinases explains why T4 functions largely as a prohormone. In illness, conversion patterns may shift (e.g., reduced T3), complicating interpretation of thyroid tests. Clinically, understanding the axis helps interpret discordant results such as elevated TSH with normal free T4 (often subclinical hypothyroidism) or suppressed TSH with normal free T4/T3 (possible subclinical hyperthyroidism or non-thyroidal factors). A structured approach—confirming assay validity, reviewing medications (biotin, amiodarone), and correlating with symptoms—improves diagnostic accuracy and prevents over-treatment.
Published February 17, 2026