Thyroid hormone synthesis, regulation and action
How the thyroid traps iodide and makes T4 and T3, how the HPT axis and deiodinases set hormone levels, and how these steps explain thyroid function tests, iodine and amiodarone effects, drug actions and sick euthyroid patterns.
Tier 1Medical sciences concept
Written with AI assistance and fact-checked against the sources below. For exam study, not clinical decisions. How notes are made
At a glance
- The thyroid makes mostly T4 (about 100 µg/day) and a little T3. About 80% of circulating T3, the active hormone, comes from peripheral deiodination of T4.
- Synthesis depends on iodide trapping by the sodium-iodide symporter (NIS) and on thyroid peroxidase (TPO), which organifies and couples iodine on thyroglobulin.
- TSH drives every step from iodide uptake to secretion. T3 feeds back on the pituitary and hypothalamus, and TSH responds log-linearly to free T4, so it is the most sensitive test of a stable axis.
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Sources
- Williams Textbook of Endocrinology, 15th editionElsevier, 2024
- Guyton and Hall Textbook of Medical Physiology, 14th editionElsevier, 2021
- 2016 American Thyroid Association guidelines for diagnosis and management of hyperthyroidism and other causes of thyrotoxicosisAmerican Thyroid Association, 2016
- Guidelines for the treatment of hypothyroidism: prepared by the American Thyroid Association task force on thyroid hormone replacementAmerican Thyroid Association, 2014
- Endocrinology (Therapeutic Guidelines)AU/NZTherapeutic Guidelines Limited, 2026
- Maia AL, Kim BW, Huang SA et al. Type 2 iodothyronine deiodinase is the major source of plasma T3 in euthyroid humans. J Clin Invest 2005;115:2524-33Journal of Clinical Investigation, 2005
- Last updated
- 14 November 2026
What changed
- 14 November 2026 Personal emails, About story and Australian-style copy