How do you differentiate primary from secondary hypothyroidism using labs?

• A. Secondary shows low or normal TSH with low free T4; Primary shows high TSH with low free T4
• B. Primary shows high TSH with low free T4; Secondary shows low or normal TSH with low free T4
• C. Primary shows low TSH with high free T4; Secondary shows high TSH with high free T4
• D. Secondary shows high TSH with high free T4; Primary shows normal TSH with normal free T4

Answer: (B)

Differentiating primary from secondary hypothyroidism hinges on the pattern of TSH and free T4. When the thyroid gland itself is failing (primary hypothyroidism), the pituitary ramps up TSH in an attempt to stimulate the thyroid, so TSH is high and free T4 is low. If the problem lies in the pituitary or hypothalamus (secondary hypothyroidism), TSH production is insufficient or inappropriately normal, leading to a low or normal TSH with a low free T4 because the thyroid isn’t receiving adequate TSH stimulation.

So the best lab pattern to tell them apart is: high TSH with low free T4 for primary hypothyroidism; low or normal TSH with low free T4 for secondary hypothyroidism. (Note: a high TSH with normal free T4 suggests subclinical primary hypothyroidism, which is a different scenario.)