Free Triiodothyronine

Free T3 (free triiodothyronine) is the small, unbound fraction of T3 circulating in your blood — the biologically active thyroid hormone that actually enters cells and drives metabolism. Most T3 in the body is bound to carrier proteins and inactive; the free fraction is what your tissues can use. It's a more direct read of active thyroid status than T4 or TSH alone.

A typical reference range is roughly 2.3–4.2 pg/mL for adults, though the exact numbers vary by lab and assay. Many thyroid-focused clinicians consider the upper half of the range optimal for people with lingering hypothyroid symptoms. Always interpret free T3 alongside TSH and free T4 rather than in isolation.

Low free T3 can reflect hypothyroidism or impaired conversion of T4 to T3, but it's also commonly driven by things unrelated to the thyroid itself — a state called non-thyroidal illness (or "low T3 syndrome") seen in acute illness, and by chronic calorie restriction or aggressive dieting, which down-regulate T3 to conserve energy. Because of this, a single low free T3 during a diet or illness isn't automatically a thyroid problem. Pair it with TSH, free T4, and symptoms, and discuss the pattern with your physician.

Elevated free T3 usually points to hyperthyroidism (an overactive thyroid, as in Graves' disease or a toxic nodule) or to taking T3-containing thyroid medication. Symptoms can include a racing heart, weight loss, tremor, heat intolerance, and anxiety. High free T3 with a suppressed TSH is the classic hyperthyroid picture and warrants medical evaluation.

TSH is a sensitive screen but can miss cases where the pituitary looks satisfied yet active thyroid hormone at the tissue level is low. Some men convert T4 to T3 poorly and feel hypothyroid — fatigue, low libido, brain fog — despite a normal TSH. Checking free T3 (and free T4) rounds out the picture; those symptoms overlap heavily with low testosterone, so it's worth ruling out thyroid before blaming your TRT protocol.