What is the correct relationship between T3 and T4?

Correct answer: C. T3 is the more biologically active hormone

• A. Both act on the same nuclear thyroid hormone receptors; they differ in affinity, not in receptor identity.
• B. T4 is more ABUNDANT and has a much longer half-life (about 7 days versus 1 day), but it is considerably LESS potent at the receptor.
• C. CORRECT. T3 (triiodothyronine) is roughly three to four times more potent than T4 and binds the nuclear thyroid hormone receptor with far greater affinity. However, the thyroid secretes mostly T4 — about 80–90% of output — which functions as a long-acting PROHORMONE. The majority of circulating T3 is generated in PERIPHERAL tissues (liver, kidney, muscle) by 5'-deiodinase removing an iodine atom from T4.
• D. They differ substantially in both potency and half-life, so they are not equivalent.

Note: This is exactly why LEVOTHYROXINE (synthetic T4) is the standard replacement therapy: its long half-life gives stable levels on once-daily dosing, and the body performs its own conversion to T3 according to tissue demand. It also explains PTU's advantage in thyroid storm — it blocks that peripheral conversion step.