Tri-Iodothyronine Total (TT3)

Low total triiodothyronine (T3) means there’s reduced circulating T3 hormone. It can reflect hypothyroidism, non-thyroidal illness (euthyroid sick syndrome), medication effects, or altered binding proteins rather than true hormone deficiency. Symptoms may include fatigue, weight gain, cold intolerance, and slow thinking. Interpretation requires correlation with TSH and free T3/T4 levels; consult your clinician for further testing and treatment.

Normal T3 varies by test and lab. Free T3 is commonly about 3.1–6.8 pmol/L (some labs report ≈2.3–4.2 pg/mL). Total T3 typical reference ranges are roughly 80–200 ng/dL. Age, pregnancy and assay methods alter ranges, so interpret results using the lab’s reference interval and your clinician’s advice. If abnormal, clinicians use TSH and free T4 too.

High T3 causes hyperthyroid effects: rapid heartbeat, palpitations, anxiety, tremor, weight loss despite increased appetite, heat intolerance, sweating, insomnia, diarrhea, menstrual changes and muscle weakness. It speeds metabolism, raising risks of bone loss/osteoporosis and heart complications (arrhythmias, heart failure). Causes include Graves’ disease, toxic nodules, thyroiditis or excess hormone; evaluation and treatment prevent complications.

A doctor orders a T3 test to evaluate thyroid activity, especially when hyperthyroidism is suspected or when symptoms persist despite normal T4/TSH. It helps diagnose T3 toxicosis, monitor thyroid replacement or antithyroid therapy, and assess unexplained weight changes, palpitations, fatigue, mood or hair loss. It can also aid evaluation of pituitary disorders and severe illness causing low T3 levels.

Low T3 causes symptoms of hypothyroidism: fatigue, weight gain, cold intolerance, constipation, dry skin, hair loss, muscle weakness, slowed thinking, depression, and menstrual changes. It can slow heart rate and metabolism, raise cholesterol, and impair concentration. Severe untreated deficiency can lead to myxedema (a life‑threatening hypometabolic state), infertility, and increased cardiovascular risk; treatment usually reverses symptoms.

Yes—severe or prolonged stress can lower circulating T3. Illness, psychological stress and high cortisol can reduce conversion of T4 to active T3 (the “low T3” or non-thyroidal illness pattern), causing low T3 despite a normally functioning thyroid. This is often reversible when stressors resolve, but persistent symptoms or abnormal labs warrant medical evaluation.