A 42-year-old woman has fatigue, weight gain, dry skin and constipation. TSH is 52 mIU/L and free T4 is undetectable. Anti-TPO antibodies are 1200 IU/mL. She is started on levothyroxine. After 6 weeks, the MOST appropriate monitoring parameter to guide dose titration is:
- A Free T3 level
- B TSH level ✓
- C Free T4 level
- D Anti-TPO antibody titre
Correct answer: B. TSH level
Explanation
TSH is the most sensitive parameter to guide levothyroxine therapy in primary hypothyroidism because the hypothalamo-pituitary axis integrates small changes in circulating thyroid hormone levels; the target is a TSH in the lower half of the normal reference range. Free T4 levels may normalize before TSH, and free T3 is not routinely monitored. Anti-TPO titres reflect autoimmune activity and do not guide dose adjustment.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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