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.

Routine / 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 free T4 together with TSH 6 to 8 weeks after starting levothyroxine or changing the dose. Have your blood drawn before you take that day's tablet, since free T4 rises for a few hours after each dose. If you are being treated for an overactive thyroid, free T4 is checked every 4 to 6 weeks at first, because TSH can stay low for months.

Note: If you take biotin (vitamin B7), for example in hair, skin and nail supplements, stop taking it at least 2 days before your test, as it can interfere with the free T4 measurement. A standard multivitamin is fine. If your doctor has prescribed high-dose biotin, don't stop without speaking to them first. Severe illness lowers free T4 transiently (non-thyroidal illness); retest 4 to 6 weeks after recovery before reading as a stable thyroid value.