Free T3 (Triiodothyronine)
Free T3 is the unbound, biologically active thyroid hormone that sets the body's resting metabolic rate; most of it is made outside the thyroid by converting T4, and it falls with severe dieting even when the thyroid itself is normal.
Plain English
Patient-Friendly Explanation
The thyroid gland makes two hormones, thyroxine, known as T4, and triiodothyronine, known as T3. T4 is produced in much larger amounts but does very little on its own; think of it as a storage form. The active hormone is T3, and most of the body's T3 is not made in the thyroid at all. It is created in the liver, kidneys and muscles by enzymes that strip one iodine atom off T4. Because these enzymes need selenium to work, the conversion step is the vulnerable part of the chain. Most T4 and T3 travelling in the blood is attached to carrier proteins, and only a small unbound fraction - the free T3 measured in a blood test - is available to enter cells and act. T3 is what sets the speed of your metabolism. It binds receptors inside cells and switches on the genes that control how much oxygen cells consume, how much energy the sodium-potassium pumps use, and how much heat the body produces. This is why the thyroid so often comes up in a weight conversation. It also explains a pattern that surprises people on strict diets: when calories are cut hard, the body deliberately lowers its conversion of T4 to T3 to conserve energy. In one careful inpatient study, halving calorie intake lowered serum T3 by around 28 per cent within seven days, with no change at all in TSH or free T4. Low-carbohydrate diets do something similar. A low free T3 reading therefore does not automatically mean thyroid disease. If the thyroid-stimulating hormone is normal, a low free T3 usually reflects illness, fasting or a very low-calorie diet, and it corrects when eating normalises - not with thyroid tablets.
Medical
Clinical Definition
Thyroid hormone is secreted predominantly as the pro-hormone T4 (approximately 90 per cent of output) with only about 10 per cent as T3; roughly 80 per cent of circulating T3 is generated peripherally by outer-ring deiodination of T4 catalysed by the selenoproteins DIO1 (liver, kidney, thyroid) and DIO2 (brain, pituitary, skeletal muscle, brown adipose tissue). More than 99 per cent of circulating hormone is bound to thyroxine-binding globulin, albumin and transthyretin; free T3 represents approximately 0.3 per cent of total T3 and is the fraction available for receptor binding. T3 binds nuclear thyroid hormone receptors TR-alpha and TR-beta with roughly five- to ten-fold the affinity of T4, driving transcription of genes governing mitochondrial biogenesis, uncoupling protein expression, sarcoplasmic reticulum calcium ATPase and sodium-potassium ATPase activity, and thereby setting basal and resting energy expenditure. Adaptive suppression of the axis during energy deficit is well described: a 50 per cent reduction in caloric intake lowered serum T3 by approximately 28 per cent within seven days without altering TSH or free T4, an effect mediated in part by falling leptin and reduced hypothalamic TRH drive, and isocaloric low-carbohydrate diets reduce T3 by 15-25 per cent. Reverse T3, an inactive isomer, rises in the same states. Typical reference intervals for free T3 are approximately 2.3-4.2 pg/mL, although assays vary. The American Thyroid Association does not recommend routine free T3 measurement in the monitoring of treated hypothyroidism; it adds information in persistent symptoms with a normal TSH, post-thyroidectomy patients, and suspected impaired conversion. A low free T3 with a normal TSH should prompt consideration of non-thyroidal illness, starvation or extreme dieting rather than thyroid replacement, since supraphysiological thyroid hormone accelerates bone loss and cardiac work without producing useful weight loss.
Key Insight
Why It Matters
If your free T3 is low while your thyroid-stimulating hormone is normal, the usual explanation is that the body has turned its metabolism down in response to eating too little - it is an adaptation, not a deficiency, and it is one reason crash diets backfire.
Sources
Reviewed against standard medical references.
Medically reviewed by
Family Physician & Registered Acupuncturist, Accord Medical Clinic. MBBS (NUS), Graduate Diploma in Family Medicine (NUS), Graduate Diploma in Acupuncture (Singapore College of TCM).
Last reviewed: October 2026
This page is general health education, not medical advice. Whether any treatment is appropriate for you is a decision made in consultation after assessment.
