Which test is used for monitoring thyroid hormone replacement therapy?

• A. TSH
• B. FT3
• C. FT4
• D. rT3

Answer: (A)

The main idea is that TSH best reflects the body's overall thyroid status after starting or adjusting hormone replacement. In primary hypothyroidism, the pituitary senses circulating thyroid hormones and adjusts TSH accordingly. When the replacement dose is right, TSH sits in the normal range. If the dose is too low, TSH rises as the pituitary signals for more hormone; if the dose is too high, TSH falls due to suppressed pituitary output. This makes TSH the most sensitive and practical marker for fine‑tuning therapy.

Free T4 and Free T3 measure circulating hormone levels, which is useful information, but they don’t mirror the feedback balance as reliably as TSH does. Small changes in dose may shift TSH before FT4 or FT3 move significantly, so TSH is preferred for monitoring. Free T4 can help in special situations or to corroborate a decision, but it’s not the primary monitoring tool. Free T3 varies more with peripheral conversion and isn’t routinely used to guide dosing. Reverse T3 is not used for monitoring replacement because it doesn’t consistently reflect thyroid hormone status or treatment needs.