Clinical inquiries: which lab tests are best when you suspect hypothyroidism?

Thyroid-stimulating hormone (TSH) level is the preferred test for initial evaluation of suspected primary hypothyroidism (strength of recommendation [SOR]: expert opinion). If TSH is abnormal, a free thyroxine (T4) level will further narrow the diagnosis. Obtain a triiodothyronine (T3) level if TSH is undetectable and free T4 is normal.