The primary serum test to screen for thyroid disease is:

The primary serum test to screen for thyroid disease is:

• A. Free T4
• B. TSH
• C. Total T4
• D. Autoimmune antibodies to thyroid tissue

Answer: (B)

The most informative initial test for thyroid function is TSH because the pituitary responds to circulating thyroid hormones and TSH is extremely sensitive to even small changes in those levels. In primary thyroid disorders, TSH typically becomes abnormal before free T4 or free T3, making it the best screening marker. A high TSH suggests hypothyroidism and leads you to check free T4 to gauge severity, while a low TSH suggests hyperthyroidism and prompts checking free T4 (and sometimes free T3) to assess the degree of excess. The other tests have limitations for screening. Total T4 can be influenced by binding proteins and may not reflect active hormone levels reliably. Free T4 is useful for confirming and classifying thyroid status but can be normal in early or subclinical disease, so it’s not the best standalone screen. Autoimmune antibodies indicate autoimmune thyroiditis but do not measure current hormone status and are not used as a general screen for thyroid function.

The most informative initial test for thyroid function is TSH because the pituitary responds to circulating thyroid hormones and TSH is extremely sensitive to even small changes in those levels. In primary thyroid disorders, TSH typically becomes abnormal before free T4 or free T3, making it the best screening marker. A high TSH suggests hypothyroidism and leads you to check free T4 to gauge severity, while a low TSH suggests hyperthyroidism and prompts checking free T4 (and sometimes free T3) to assess the degree of excess.

The other tests have limitations for screening. Total T4 can be influenced by binding proteins and may not reflect active hormone levels reliably. Free T4 is useful for confirming and classifying thyroid status but can be normal in early or subclinical disease, so it’s not the best standalone screen. Autoimmune antibodies indicate autoimmune thyroiditis but do not measure current hormone status and are not used as a general screen for thyroid function.