What distinguishes secondary hypothyroidism from primary on lab testing?

• A. Low or normal TSH with low free T4.
• B. Elevated TSH with high free T4.
• C. Low TSH with high free T4.
• D. High TSH with high free T4.

Answer: (C)

The main idea is how the pituitary–thyroid feedback loop responds when thyroid hormone is low. In primary hypothyroidism the thyroid gland itself can’t produce enough T4, so low T4 removes the negative feedback and the pituitary pumps out more TSH. In secondary hypothyroidism the pituitary (or hypothalamus) isn’t signaling properly, so even though T4 is low, the TSH response is blunted or inappropriately normal.

Because of that, the lab pattern that distinguishes secondary from primary is low free T4 with a low or inappropriately normal TSH. You wouldn’t expect a high TSH in secondary hypothyroidism. This contrasts with primary hypothyroidism, where TSH is typically elevated despite the low free T4.

So the key takeaway is: when free T4 is low and TSH is not elevated (low or inappropriately normal), that points toward secondary hypothyroidism rather than primary.