Thyroxine (T4) and triiodothyronine (T3)
End hormone or mediator · T4 prohormone D2 makes T3 Nuclear receptor
T4 is the prohormone, with a 7-day half-life and over 99.9% protein binding; T3 is the active ligand at the nuclear receptor. About 80% of circulating T3 is generated outside the thyroid by 5'-deiodination.
Traced from the start
- Thyrotrophin-releasing hormone (paraventricular nucleus)
- Thyrotroph — thyroid-stimulating hormone
- Thyroid follicular cell
- Thyroxine (T4) and triiodothyronine (T3)
Detail
- Deiodinases
- D1 in liver and kidney supplies plasma T3; D2 in pituitary, brain and brown fat makes T3 locally, so the thyrotroph effectively senses T4; D3 inactivates T4 to reverse T3
- Binding
- Thyroxine-binding globulin carries most of it, so oestrogen raises total T4 while free T4 stays normal
- Effects
- Raises basal metabolic rate and thermogenesis, up-regulates beta-adrenergic receptors, and is essential for fetal and infant brain development and for linear growth
- Conversion blockers
- Propranolol, propylthiouracil, glucocorticoids, amiodarone and severe illness all reduce T4 to T3 conversion
When it goes wrong
Severe non-thyroidal illness (sick euthyroid syndrome)
Low T3 with normal or low TSH and later a low T4; thyroid function should not be tested in acute illness and this pattern should not be treated
Untreated congenital hypothyroidism
Irreversible intellectual disability and growth failure — the justification for newborn screening and thyroxine within the first two weeks
Practise this structure
4 questions in the bank tagged Thyroid.
A 42-year-old man was treated with radioactive iodine for Graves disease three days ago. He now reports dry mouth and painful swelling in front of both ears, wo…
Clinical rotations · common
A 34-year-old woman taking carbimazole for Graves disease presents with fever, malaise and two painful oral ulcers. She has a temperature of 38.7°C and mild cer…
Clinical rotations · common
A 35-year-old woman has palpitations and a 9 kg weight loss over 3 months. Her free thyroxine is 48 pmol/L (reference 10–22 pmol/L), TSH is less than 0.01 mIU/L…
Mechanisms of disease · common
A 38-year-old woman presents with two weeks of pain in the front of her neck radiating to her jaw, together with palpitations, sweating and 2 kg of weight loss.…
Clinical rotations · common