TSH suppressed, Free T4 normal, and the patient is symptomatic. That's the classic miss. About one in twenty hyperthyroid cases is T3-predominant, and you only catch it if someone bothers to order Free T3.
T3 is the active hormone. It binds receptors with about four times the affinity of T4 and actually drives metabolism inside cells. Most of it (around 80%) doesn't even come from the thyroid directly. It's made downstream when the liver, kidneys, and other tissues strip an iodine off T4. So Free T3 reflects two things at once: thyroid output and the body's conversion machinery.
It's not a screening test. TSH and Free T4 carry the routine workup. Free T3 earns its place when hyperthyroidism is suspected (especially T3 toxicosis), and when monitoring patients on liothyronine or combination T4/T3 therapy.
What Free T3 measures
The body keeps T3 production on a short leash:
- Direct thyroid release: about 20% of circulating T3.
- Peripheral conversion: about 80%, via deiodinase enzymes that strip an iodine off T4.
- Reverse T3 (rT3): a metabolically inactive byproduct that climbs in illness, fasting, and cortisol excess.
"Free" T3 is the unbound fraction available to enter cells, same logic as Free T4. Total T3 gets distorted by binding-protein status (pregnancy, estrogen, illness) and rarely earns its keep in modern thyroid evaluation.
Free T3 is more variable than Free T4. It moves with illness, fasting, drug interactions, and time of day. Read it in context.
Free T3 reference range
| Grupo demográfico | Bajo | Alto | Unidad |
|---|---|---|---|
| Adults — standard range | 2.3 | 4.2 | pg/mL |
| Adults — SI units | 3.5 | 6.5 | pmol/L |
| Pregnancy — first trimester | 2.4 | 4.2 | pg/mL |
| Pregnancy — third trimester | 2 | 3.5 | pg/mL |
| Children 1–18 years | 2.5 | 5 | pg/mL |
| Hyperthyroid (overt) | 4.3 | 20 | pg/mL |
| Hypothyroid / non-thyroidal illness | 0 | 2.2 | pg/mL |
Most labs report 2.3–4.2 pg/mL (3.5–6.5 pmol/L) for adults. Cutoffs vary by assay.
- 2.3–4.2 pg/mL: standard adult range.
- Below 2.3 pg/mL: low. Often non-thyroidal illness ("low T3 syndrome") in isolation. Backs up overt hypothyroidism when paired with low Free T4 and high TSH.
- Above 4.2 pg/mL: high. Confirms hyperthyroidism when TSH is suppressed. T3 toxicosis = high Free T3 with normal Free T4 and suppressed TSH. Roughly 5% of hyperthyroidism.
- Pregnancy: Free T3 may drift slightly lower as gestation progresses. Use trimester-specific ranges if available.
Order it for a reason. Suspected hyperthyroidism, suppressed TSH with normal Free T4, or monitoring on liothyronine. A clearly normal TSH usually makes Free T3 unnecessary.
What high Free T3 means
Almost always hyperthyroidism. The differential mirrors high Free T4, with a few patterns specific to T3:
- Graves' disease: autoimmune hyperthyroidism. Free T3 often rises earlier and proportionally more than Free T4.
- T3 toxicosis: high Free T3, normal Free T4, suppressed TSH. Seen in early Graves', autonomous nodules, and excess T3 supplementation. Easy to miss without ordering Free T3.
- Toxic nodule or multinodular goiter: autonomous hormone production.
- Liothyronine (T3) over-replacement: combination T4/T3 patients can show high Free T3 even with appropriate TSH if the dose was taken before the draw. T3 peaks 2–4 hours after an oral dose.
- Thyroiditis: early-phase release of preformed hormone.
- Iodine-induced hyperthyroidism: amiodarone, iodinated contrast.
Symptoms: palpitations, tremor, heat intolerance, weight loss, anxiety, frequent stools, insomnia. Atrial fibrillation in older adults is more strongly tied to T3 excess than T4 excess.
What low Free T3 means
Wider differential than low Free T4 because illness so readily suppresses peripheral T4-to-T3 conversion.
- Non-thyroidal illness syndrome ("low T3 syndrome"): by far the most common cause of isolated low Free T3. Critical illness, sepsis, starvation, recent surgery, or severe psychiatric illness shunts T4 conversion away from T3 toward inactive reverse T3. TSH is usually normal or low. Treat the underlying illness, not the lab.
- Overt hypothyroidism: low Free T3 with low Free T4 and high TSH.
- Caloric restriction or anorexia: adaptive T3 suppression.
- Beta-blockers: high-dose propranolol inhibits T4-to-T3 conversion.
- Glucocorticoid excess: endogenous Cushing or high-dose steroids.
- Selenium deficiency: deiodinases are selenoproteins. Rare in selenium-replete populations.
Treating "low T3 syndrome" with thyroid hormone in critically ill patients does not improve outcomes and may cause harm. The low T3 is adaptive, not a disease.
Reading Free T3 in context
Before interpreting, consider:
- Time of last T3 dose: liothyronine peaks 2–4 hours after dosing. Draw before the daily dose.
- Acute illness: non-thyroidal illness drops Free T3. Avoid routine thyroid testing in hospitalized patients unless you really suspect thyroid disease.
- Biotin supplements: interfere with the assay, often falsely elevating Free T3. Stop for 48 hours.
- Recent fasting or very-low-calorie dieting: suppresses T3 conversion.
- Heparin: can falsely elevate Free T3 the same way it does Free T4.
- Beta-blockers: high-dose propranolol modestly lowers Free T3 by inhibiting peripheral conversion.
Free T3 swings more day-to-day than Free T4. Repeat a single borderline value under standardized conditions before acting.
Rastrea este biomarcador a lo largo del tiempo en AskAnything.health — sube tus resultados de laboratorio y ve las tendencias de un vistazo.
When to act on Free T3
- Suppressed TSH with normal Free T4: order Free T3 to look for T3 toxicosis. About 5% of hyperthyroidism is T3-predominant.
- Hyperthyroid symptoms with high-normal Free T4: Free T3 may catch early Graves' before Free T4 crosses the cutoff.
- Patients on liothyronine (Cytomel) or combination T4/T3 therapy: check Free T3 (ideally pre-dose). Over-replacement is common and raises AF and bone-loss risk.
- Isolated low Free T3 in a hospitalized patient: almost always non-thyroidal illness. Don't reflexively start thyroid hormone.
- New atrial fibrillation with normal TSH and Free T4: check Free T3. T3 toxicosis can be the missed diagnosis.
Esta información es solo con fines educativos y no sustituye el consejo médico profesional. Siempre consulta a tu proveedor de salud sobre tus resultados de laboratorio.
Tests that complete the picture
- TSH: the screening test. Free T3 is rarely useful with a clearly normal TSH.
- Free T4: paired with Free T3 to localize the hyperthyroid pattern (T4-dominant vs T3-dominant).
- TSH receptor antibodies (TRAb): confirm Graves' disease.
- Anti-TPO antibodies: confirm Hashimoto thyroiditis.
- Reverse T3 (rT3): sometimes used to characterize non-thyroidal illness, but rarely changes management.
- Radioiodine uptake or thyroid ultrasound: separates Graves' from thyroiditis from autonomous nodule.