A 45-year-old woman with primary hypothyroidism is started on levothyroxine replacement. The dose is titrated primarily based on monitoring of:
- A Serum TSH ✓
- B Serum free T4
- C Serum total T3
- D Serum thyroglobulin
Correct answer: A. Serum TSH
Explanation
In primary hypothyroidism, TSH is the most sensitive marker for monitoring levothyroxine replacement because the hypothalamic-pituitary axis responds to small changes in thyroid hormone levels. The therapeutic goal is to normalize TSH. Free T4 is used in central hypothyroidism where TSH is unreliable. Total T3 and thyroglobulin are not used for dose titration.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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