What is the first and most important screening test for evaluating thyroid function?

• A. TSH
• B. T4
• C. T3
• D. TRAb

Answer: (A)

TSH testing is the primary screening tool for thyroid function because it is the most sensitive indicator of the body’s thyroid status through the pituitary’s negative feedback response to circulating thyroid hormones. When thyroid hormone levels drop, TSH rises to stimulate the thyroid; when they rise, TSH falls. This means TSH often becomes abnormal before changes in T4 or T3 are detectable, making it the best first test to screen for both hypo- and hyperthyroidism. If TSH is abnormal, measuring free T4 helps distinguish whether the issue is hypothyroidism or hyperthyroidism and to assess severity. In contrast, measuring T4 or T3 alone can miss early or subclinical cases, and TRAb is a specific test for Graves’ disease, not a general screening test.