Thyroid Hormones (T3 and T4)
The thyroid gland produces two primary hormones:
- T4 (thyroxine) — the storage/transport form; produced directly by the thyroid and requires conversion to T3 to exert most of its biological effects.
- T3 (triiodothyronine) — the biologically active form; more potent than T4; generated primarily by peripheral conversion of T4 via selenoenzyme deiodinases (requiring selenium).
Both hormones require iodine for their synthesis (T4 contains 4 iodine atoms; T3 contains 3).
Regulation
- The hypothalamus releases TRH (thyrotropin-releasing hormone).
- TRH stimulates the pituitary to release TSH (thyroid-stimulating hormone).
- TSH acts on the thyroid to produce T4 and T3.
- T4 and T3 provide negative feedback to suppress TRH and TSH.
In graves-disease, TSH-receptor antibodies bypass this regulation, driving uncontrolled hormone production.
Clinical Significance
- Excess production → hyperthyroidism / thyrotoxicosis
- Deficient production → hypothyroidism (treated with levothyroxine, a synthetic T4)
Measurement
TSH, free T4 (FT4), and free T3 (FT3) are the standard blood tests. Note: high-dose biotin supplements can give false readings on these tests.