The recommended initial thyroid function test for screening in asymptomatic individuals is:

• A. Free thyroxine (free T4)
• B. Thyroid-stimulating hormone (TSH)
• C. Total thyroxine (T4)
• D. Triiodothyronine (T3)

Answer: (B)

TSH is the best initial screen for thyroid function in people without symptoms because the pituitary-thyroid feedback system makes it the most sensitive indicator of changes in thyroid hormone levels. Small decreases in circulating thyroid hormones cause a noticeable rise in TSH, and small increases cause a drop, often before free T4 or T3 change enough to be detected. This sensitivity makes TSH a reliable, cost-effective first test to screen for both under- and overactivity of the thyroid.

If TSH is abnormal, checking Free T4 helps determine the cause: a high TSH with a low Free T4 points to primary hypothyroidism, whereas a low or suppressed TSH with a normal or high Free T4 suggests hyperthyroidism. Other options like free T4, total T4, or T3 are less suitable for screening because they can be normal in early disease, are influenced by binding proteins, or don’t reflect pituitary response as directly as TSH.