Free T4 (fT4)

Free T4 reflects the thyroid gland's production capacity and serves as the primary screening test for thyroid dysfunction. It provides early detection of both hypothyroidism (low fT4) and hyperthyroidism (high fT4) before symptoms become obvious. Because it's unaffected by protein-binding changes, free T4 gives a true picture of thyroid hormone availability, helping distinguish between thyroid disease and conditions that merely affect hormone transport.

Most adults with normal TSH don't need routine free T4 testing. When monitoring thyroid medication, free T4 is reassessed alongside TSH at 6 to 8 weeks after any dose change.

At baseline / for screening: Once every 12 months from age 30. More frequently if you have a family history of thyroid disease, are pregnant or planning pregnancy, have an autoimmune condition, or notice symptoms suggesting thyroid dysfunction (energy, weight, mood, temperature regulation).

When monitoring an intervention or change: Retest 6 to 8 weeks after starting or adjusting thyroid medication (levothyroxine, liothyronine, antithyroid drugs), since that's how long TSH takes to reach a new steady state. Acute illness and severe caloric restriction can shift free T3 downward independent of thyroid function, so wait for those to resolve before reading the trend.

Note: Biotin supplements above 5 mg per day interfere with the assay, so pause biotin for 48 to 72 hours before testing. Severe illness lowers free T4 transiently (non-thyroidal illness); retest 4 to 6 weeks after recovery before reading as a stable thyroid value.