Free T3 (fT3)

Free T3 directly controls metabolic rate, body temperature, heart rate, and energy production in every cell. Low levels can cause fatigue, weight gain, cold intolerance, depression, and sluggish metabolism, while high levels may lead to anxiety, weight loss, heat intolerance, and rapid heartbeat. Unlike total T3, free T3 isn't affected by changes in binding proteins from pregnancy, medications, or illness, making it a more accurate measure of thyroid function at the tissue level

Most adults with normal TSH don't need routine free T3 testing. When monitoring thyroid replacement therapy, free T3 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: Free T3 falls during acute illness and severe caloric restriction (non-thyroidal illness syndrome); this is not true hypothyroidism. Wait 4 to 6 weeks after illness recovery or refeeding before reading free T3 as a stable thyroid marker.