In the evaluation of subclinical hyperthyroidism, which TSH level defines Grade 2 (overt) suppression and carries the highest risk of atrial fibrillation and fracture?

• A TSH 0.1–0.4 mIU/L
• B TSH < 0.1 mIU/L
• C TSH < 0.01 mIU/L ✓
• D TSH 0.4–0.5 mIU/L

Explanation

Subclinical hyperthyroidism is classified by TSH level: Grade 1 is TSH 0.1–0.39 mIU/L (low-normal suppression) and Grade 2 is TSH < 0.1 mIU/L (profound suppression). Grade 2 (TSH < 0.1 mIU/L) carries significantly greater risk of atrial fibrillation (3-fold), osteoporosis/fracture, and cardiovascular mortality compared with Grade 1. Treatment thresholds are lower for Grade 2 subclinical hyperthyroidism and treatment is recommended in patients over 65 or those with cardiac risk factors regardless of symptoms.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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