T3 (Triiodothyronine)
T3 is the active form of thyroid hormone, responsible for regulating metabolism, heart rate, body temperature, and energy levels. Only about 20% of T3 is produced directly by the thyroid — the remaining 80% is converted from T4 in tissues like the liver and kidneys. T3 is 3–5 times more potent than T4.
Reference Ranges
Male
80 – 200
ng/dL
Female
80 – 200
ng/dL
Child
100 – 260
ng/dL
Ages 1–12
Reference ranges may vary by laboratory. Always compare with the range printed on your lab report.
What Low Levels Mean
Low T3 occurs in hypothyroidism and in 'sick euthyroid syndrome' — a temporary drop during severe illness, starvation, or major surgery. The body reduces T3 to conserve energy. Low T3 is often the last thyroid marker to change in hypothyroidism.
What High Levels Mean
High T3 is the hallmark of hyperthyroidism, especially in the early stages or in T3 thyrotoxicosis (where T3 is elevated but T4 is normal). Symptoms include rapid heartbeat, weight loss, nervousness, tremors, and heat intolerance.
Frequently Asked Questions
What is the difference between T3 and free T3?
Total T3 includes both protein-bound T3 (inactive, in transit) and free T3 (unbound, active). Free T3 is a more accurate measure of available thyroid hormone. Pregnancy, birth control pills, and liver disease can alter protein levels and affect total T3 without changing free T3.
Can T3 be high with normal TSH?
In rare cases, yes — this is called T3 thyrotoxicosis, where T3 is selectively elevated. It can occur early in Graves' disease or with toxic thyroid nodules. If symptoms of hyperthyroidism are present with normal TSH, T3 should be specifically tested.
Is T3 supplementation ever needed?
Most hypothyroid patients do well on T4 (levothyroxine) alone, which the body converts to T3. Some patients with persistent symptoms despite normal TSH may benefit from combination T4/T3 therapy, but this is controversial and not routinely recommended.
This information is for educational purposes only and should not replace professional medical advice. Always consult your doctor for interpretation of your test results.
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