What is the primary screening test used to evaluate thyroid function?

What is the primary screening test used to evaluate thyroid function?

• A. T3
• B. T4
• C. TSH
• D. Calcitonin

Answer: (C)

Measuring TSH is the primary screening test for thyroid function. The pituitary gland releases TSH to stimulate the thyroid, and the amount of TSH in the blood shifts in the opposite direction of thyroid hormone levels due to negative feedback. Because of this tight feedback, TSH is extremely sensitive to even small changes in thyroid hormones, making it the most reliable early indicator of thyroid dysfunction. If TSH is elevated, it usually points to hypothyroidism and is typically followed by checking free T4 to gauge severity; if TSH is low, it suggests hyperthyroidism and is followed by free T4 (and sometimes free T3) testing. The other options aren’t ideal for initial screening: T3 and T4 measure hormones themselves but can be normal in early disease or influenced by binding proteins, so they’re less sensitive as screening tests; calcitonin is a marker used mainly for evaluating certain thyroid cancers, not general thyroid function.

Measuring TSH is the primary screening test for thyroid function. The pituitary gland releases TSH to stimulate the thyroid, and the amount of TSH in the blood shifts in the opposite direction of thyroid hormone levels due to negative feedback. Because of this tight feedback, TSH is extremely sensitive to even small changes in thyroid hormones, making it the most reliable early indicator of thyroid dysfunction. If TSH is elevated, it usually points to hypothyroidism and is typically followed by checking free T4 to gauge severity; if TSH is low, it suggests hyperthyroidism and is followed by free T4 (and sometimes free T3) testing. The other options aren’t ideal for initial screening: T3 and T4 measure hormones themselves but can be normal in early disease or influenced by binding proteins, so they’re less sensitive as screening tests; calcitonin is a marker used mainly for evaluating certain thyroid cancers, not general thyroid function.