The most sensitive test for detecting primary hypothyroidism in the preoperative evaluation is which of the following?

The most sensitive test for detecting primary hypothyroidism in the preoperative evaluation is which of the following?

• A. Thyroid-stimulating hormone (TSH) level
• B. Total plasma T3 level
• C. Total plasma T4 level
• D. Resin T3 uptake

Answer: (A)

When diagnosing primary hypothyroidism, the body’s most sensitive signal is the pituitary’s TSH level. If the thyroid isn’t producing enough hormone, circulating levels drop, and the pituitary who senses this through feedback increases TSH to stimulate the thyroid. This rise in TSH often occurs before measurable drops in T4 or T3, so TSH can be abnormal even when free T4 or total T4/T3 are still normal. That makes TSH the best screening test in the preoperative setting, as it detects both overt and subclinical hypothyroidism. Total T3 and T4 levels can be misleading because they depend on binding proteins and may remain normal in early disease; resin T3 uptake is an older, indirect measure and not as reliable for assessing thyroid function.

When diagnosing primary hypothyroidism, the body’s most sensitive signal is the pituitary’s TSH level. If the thyroid isn’t producing enough hormone, circulating levels drop, and the pituitary who senses this through feedback increases TSH to stimulate the thyroid. This rise in TSH often occurs before measurable drops in T4 or T3, so TSH can be abnormal even when free T4 or total T4/T3 are still normal. That makes TSH the best screening test in the preoperative setting, as it detects both overt and subclinical hypothyroidism.

Total T3 and T4 levels can be misleading because they depend on binding proteins and may remain normal in early disease; resin T3 uptake is an older, indirect measure and not as reliable for assessing thyroid function.