In primary hypothyroidism, which pattern of TSH and free T4 is typically observed?

• A. Low TSH and low free T4
• B. Elevated TSH with low free T4
• C. Elevated TSH with elevated free T4
• D. Low TSH with elevated free T4

Answer: (B)

The pattern reflects failure at the thyroid gland with intact pituitary signaling. In primary hypothyroidism, the thyroid cannot produce enough thyroid hormone. The pituitary senses this low level of free T4 and responds by increasing TSH to stimulate the thyroid. Because the thyroid cannot respond adequately, free T4 stays low despite the high TSH. So the typical lab picture is elevated TSH with a low free T4.

This contrasts with patterns you’d see in other situations: if the thyroid were making too much hormone, both TSH would be suppressed and free T4 would be high; if the problem were in the pituitary (central hypothyroidism), TSH would be low or inappropriately normal with a low free T4. But in primary thyroid failure, the high TSH plus low free T4 is the classic combination.