Iodine-containing hormones that control the body's metabolic rate.T₄ major form in blood (longer half-life), conversion to T₃ (more potent)

Source: Follicles of thyroid. 5'-deiodinase converts T₄ (the major thyroid product) to T₃ in peripheral tissue. T₃ binds nuclear receptor with greater affinity than T₄. Peripheral conversion is inhibited by glucocorticoids, β-blockers and propylthiouracil (PTU). Thyroid peroxidase is responsible for oxidation, organification of iodide and coupling of monoiodotyrosine (MIT) and diiodotyrosine (DIT).

Propylthiouracil inhibits both thyroid peroxidase and 5'-deiodinase. Methamizole inhibits thyroid peroxidase only.

Function: - Bone growth (synergism with GH)- Brain maturation- β-adrenergic effects. ↑ β₁ receptors in heart → ↑ CO, HR, SV, contractility- Basal metabolic rate via ↑ Na⁺/K⁺-ATPase activity → ↑ O₂ consumption, RR, body temperature- Blood sugar (↑ glycogenolysis, gluconeogenesis)- Break down lipids (↑ lipolysis)- ↑ cholesterol clearance

Thyroxine-binding-globulin (TBG) binds most T₃/T₄ in blood. Bound T₃/T₄ = inactive. ↑ TBG in pregnancy or OCP use (estrogen → ↑ TBG) → ↑ total T₃/T₄↓ TBG in hepatic failure, steroids, nephrotic syndrome