Thyroid Function Tests: TSH, Free T4, Free T3—and When You Actually Need Each One
Thyroid Function Tests: TSH, Free T4, Free T3—and When You Actually Need Each One TSH is almost always enough. Free T3 is almost never needed. And sick euthyroid syndrome fools everyone. This post complements the thyroid antibody workup from earlier in the series. That post covered which antibody to order when you suspect autoimmune thyroid disease. This one covers the function tests—TSH, free T4, and free T3—and the interpretation patterns that trip up clinicians daily. The Hierarchy: TSH First, Always TSH is the single best screening test for thyroid dysfunction in outpatient primary care. The pituitary amplifies small changes in thyroid hormone levels, making TSH far more sensitive than direct hormone measurement. A normal TSH essentially rules out primary thyroid disease in an outpatient setting. TSH Free T4 Free T3 Interpretation ↑ High ↓ Low — Overt hypothyroidism. Treat with levothyroxine. ↑ High Normal — Subclinical hypothyroidism. Repeat in 6–12 w...