What is the classic laboratory pattern for primary hypothyroidism?

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

Answer: (A)

The key idea is the thyroid–pituitary feedback loop. When the thyroid gland fails to produce enough hormone (free T4) in primary hypothyroidism, the pituitary senses the low T4 and responds by increasing TSH to try to stimulate the thyroid. That leads to a high TSH level alongside a low free T4 level—the classic laboratory pattern for primary hypothyroidism. If TSH were low or normal with high free T4, that would point away from primary hypothyroidism (toward hyperthyroidism or another cause), and a normal TSH with normal T4 would indicate euthyroidism. Elevated TSH with high free T4 would suggest a different, non-classic scenario (such as a TSH-secreting pituitary process) and is not the typical pattern for primary hypothyroidism.