Tri-Iodothyronine Free (FT3)

If FT3 (free T3) is high, it indicates excess thyroid hormone activity causing hyperthyroidism. Symptoms include weight loss, heat intolerance, tremor, palpitations, anxiety, increased appetite, sweating, diarrhea, menstrual irregularities, and insomnia. Complications can include atrial fibrillation, bone loss, muscle weakness and, rarely, life‑threatening thyroid storm. Common causes are Graves’ disease, toxic nodules, thyroiditis, or excess medication; evaluation and treatment are recommended.

Tri-iodothyronine (T3) is an active thyroid hormone containing three iodine atoms. It is produced by the thyroid and by peripheral conversion of thyroxine (T4). T3 regulates metabolic rate, heart rate, body temperature, growth and brain development. Blood T3 levels help diagnose thyroid disorders; excess causes hyperthyroidism symptoms and deficiency causes hypothyroidism symptoms.

FT3 stands for free triiodothyronine, the unbound, active form of T3 circulating in blood. It indicates thyroid hormone activity and metabolic rate. FT3 is measured alongside TSH and free T4 to diagnose and monitor hyperthyroidism (typically elevated) or hypothyroidism (typically low). Abnormal FT3 can reflect thyroid disease, medication effects, or altered peripheral conversion of T4 to T3.

Typical adult reference range for free triiodothyronine (FT3) is roughly 2.3–4.2 pg/mL (3.5–6.5 pmol/L). Laboratories and assays differ, so reported “normal” limits may vary slightly (some use about 3.1–6.8 pmol/L). Interpret FT3 alongside TSH and free T4 with clinical context—age, pregnancy, and medications can affect results.

High free T3 (FT3) typically produces hyperthyroid symptoms: unexplained weight loss with increased appetite, rapid or irregular heartbeat and palpitations, tremor, heat intolerance and excessive sweating, anxiety, irritability, nervousness, insomnia, muscle weakness, and increased bowel movements or diarrhea. Women may have lighter or irregular periods; older adults can develop atrial fibrillation and bone loss if untreated.

Early signs of thyroid eye disease often include gritty, dry or irritated eyes, redness and watery discharge, eyelid swelling or retraction causing a staring appearance, and increased sensitivity to light. Patients may notice aching or pressure behind the eyes, puffiness around the orbit, and mild double vision or difficulty moving the eyes. Prompt evaluation is recommended if these symptoms appear.