In primary hypothyroidism, which lab pattern is most typical?

In primary hypothyroidism, which lab pattern is most typical?

• A. Low TSH with low T4
• B. High TSH with low T4
• C. High TSH with high T4
• D. Low TSH with high T4

Answer: (B)

The key idea is how the hypothalamic–pituitary–thyroid axis uses negative feedback to keep thyroid hormone levels in check. In primary hypothyroidism the thyroid gland itself cannot produce enough T4. Because T4 is low, the pituitary responds by increasing TSH in an attempt to stimulate the thyroid. However, since the problem is in the thyroid, it can’t produce enough hormone despite the high TSH, so T4 remains low. This combination—elevated TSH with low T4—is the classic lab pattern for primary hypothyroidism. If the issue were in the pituitary, TSH would be low or inappropriately normal with a low T4, and if the thyroid were overactive, TSH would be suppressed with a high T4. (There can be early, subclinical cases with high TSH but normal T4.)

The key idea is how the hypothalamic–pituitary–thyroid axis uses negative feedback to keep thyroid hormone levels in check. In primary hypothyroidism the thyroid gland itself cannot produce enough T4. Because T4 is low, the pituitary responds by increasing TSH in an attempt to stimulate the thyroid. However, since the problem is in the thyroid, it can’t produce enough hormone despite the high TSH, so T4 remains low. This combination—elevated TSH with low T4—is the classic lab pattern for primary hypothyroidism. If the issue were in the pituitary, TSH would be low or inappropriately normal with a low T4, and if the thyroid were overactive, TSH would be suppressed with a high T4. (There can be early, subclinical cases with high TSH but normal T4.)