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
5 questions in the bank tagged Thyroid.
A 38-year-old woman presents with three weeks of aching pain in the front of her neck that worsens when she turns her head or swallows and radiates up to both e…
Clinical rotations · middle twist
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