Biomarker comparison
T3 vs T4: what the difference is, free versus total, and which to test with TSH
Short answer
The thyroid makes mostly T4 (thyroxine) and a little T3 (triiodothyronine); the tissues then convert T4 into T3, the form that actually acts on cells — three to five times more potent, with a half-life of a day against T4's week. More than 99% of both circulate bound to proteins, so the 'free' fraction is what matters and what is measured; total T4 and total T3 move with the binding proteins (estrogen, pregnancy, liver disease) rather than with thyroid function. The standard pair is TSH plus free T4. Free T3 adds information in hyperthyroidism, where it can be the only hormone raised, and is the least useful test in hypothyroidism, where it is maintained until late.
Medically reviewed by Youmna DiStefano, MD · Last reviewed November 2026
Free T3 vs Free T4 side by side
| Free T3 | Free T4 | |
|---|---|---|
| What it is | Triiodothyronine — the active thyroid hormone; about 80% is made in tissues from T4 | Thyroxine — the thyroid's main product and the circulating reservoir |
| Normal range (adults) | age 18+: 2 – 4.4 pg/mL | age 18+: 0.82 – 1.77 ng/dL |
| Share of thyroid output | About 20% | About 80% |
| Potency and half-life | Three to five times more potent; half-life about one day | A prohormone; half-life about seven days (why levothyroxine is dosed once daily and takes six weeks to settle) |
| Free vs total | About 0.3% free; total T3 rises and falls with binding proteins | About 0.03% free; total T4 doubles in pregnancy or on estrogen without any change in function |
| Best use | Confirming and grading hyperthyroidism (T3 toxicosis); monitoring liothyronine or desiccated thyroid | With TSH, the standard test for both under- and over-activity; monitoring in pituitary disease where TSH is unreliable |
| Low in | Severe non-thyroidal illness (low T3 syndrome), fasting, some medications — usually without thyroid disease; late hypothyroidism | Hypothyroidism (with a high TSH); pituitary disease (with a low or normal TSH) |
| High in | Hyperthyroidism — Graves' disease and toxic nodules, often before T4 rises; liothyronine or desiccated-thyroid excess | Hyperthyroidism; levothyroxine excess; biotin interference and some binding-protein variants (falsely) |
Reference ranges are the reviewed catalog ranges for adults; your laboratory's printed range may differ slightly.
T4 is the reservoir, T3 the active hormone
The thyroid secretes thyroxine, T4, with four iodine atoms. Enzymes in the liver, kidney, muscle and brain remove one iodine to make T3, the form that binds the nuclear receptor and sets metabolic rate; a different enzyme removes a different iodine to make reverse T3, which is inactive. About 80% of the T3 in the blood was made this way rather than secreted. Because T4 lasts about a week and T3 about a day, T4 acts as a buffer: a stable free T4 keeps tissue T3 steady even though T3 itself turns over quickly.
Free vs total: why 'free' is the one to measure
Thyroxine-binding globulin, transthyretin and albumin carry more than 99.9% of T4 and 99.7% of T3; only the unbound fraction enters cells. Anything that changes the carrier proteins changes the totals without changing thyroid function: estrogen (the pill, hormone therapy, pregnancy) raises TBG and the total T4 and T3 by 50–100%, while androgens, nephrotic syndrome, liver disease and high-dose steroids lower them. Free T4 and free T3 assays estimate the unbound hormone and are far less affected, which is why the American Thyroid Association guidelines use free T4 and why a total T4 or total T3 is rarely ordered now.
Which tests to order with TSH
TSH is the most sensitive single test because the pituitary amplifies small changes in free T4; the thyroid hormones are added to confirm and to grade:
- Suspected hypothyroidism — TSH and free T4. Free T3 adds nothing: it is kept normal until the disease is advanced.
- Suspected hyperthyroidism — TSH, free T4 and free T3. In early Graves' disease or a toxic nodule the T3 can be high while T4 is still normal (T3 toxicosis).
- On levothyroxine — TSH, with free T4 if the dose is being adjusted or TSH is unreliable; wait six to eight weeks after a dose change, and draw before the morning dose.
- On liothyronine (T3) or desiccated thyroid — TSH and free T4; a free T3 drawn within a few hours of a dose reads falsely high.
- Pituitary or hypothalamic disease — free T4 is the guide, because TSH can be normal or low even when the thyroid is under-driven.
Low T3 syndrome and reverse T3
During any significant illness, fasting or calorie restriction, the body converts less T4 to T3 and more to reverse T3; the free T3 falls while TSH and free T4 stay normal. This is an adaptive response, not thyroid disease — it resolves with recovery and is not treated with thyroid hormone. Reverse T3 is a research measure of that process; it has no established role in deciding whether someone needs thyroid treatment, and guidelines do not recommend it.
How do you test Free T3 and Free T4?
Prices are cash-pay; every order adds one $6 lab processing fee. Physician authorization is included; no insurance or referral needed.
| Test or panel | Type | Price | Fasting | Results |
|---|---|---|---|---|
| Essential Thyroid Panel | Panel · 3 tests | $49.99 | No | 1–3 business days |
| Hashimoto's Thyroid Panel | Panel · 3 tests | $97.99 | No | 3–5 business days |
| Comprehensive Thyroid Test (TSH, Free T4, Free T3, Reverse T3, TPO) | Panel · 4 tests | $109.99 | No | 3–7 business days |
| Free T3 Test (Triiodothyronine) | Test | $25.99 | No | 1–3 business days |
| Free T4 Test (Thyroxine) | Test | $20.99 | No | 1–3 business days |
Common questions
Is T3 or T4 more important?
T3 does the work in the cells, but T4 is what the thyroid makes and what the tissues convert to T3 as needed, so free T4 is the more informative blood test of thyroid output and the one guidelines pair with TSH. Free T3 matters most in hyperthyroidism, where it can be the only hormone raised.
Why is my T4 normal but T3 low?
Usually because the body has turned down the conversion of T4 to T3 — during an illness, after surgery, with fasting or a very low-calorie diet, or on medications such as high-dose steroids, amiodarone or propranolol. With a normal TSH this 'low T3 syndrome' is an adaptation rather than thyroid disease and is not treated; it recovers when the cause does.
Should I test free T3 or total T3?
Free T3, if a T3 is needed at all. Total T3 rises and falls with the binding proteins — higher on estrogen or in pregnancy, lower with androgens or liver disease — and does not reflect thyroid function. Free T3 is useful when hyperthyroidism is suspected or being treated; in suspected hypothyroidism neither T3 test adds to TSH and free T4.
What is reverse T3, and do I need it?
Reverse T3 is the inactive product made when T4 loses a different iodine atom. It rises during illness, fasting and with some drugs, mirroring the fall in T3. It is not recommended in thyroid guidelines for diagnosing or managing hypothyroidism, and a high reverse T3 is not a reason to start or change thyroid hormone.
Sources
- Jonklaas et al., Guidelines for the Treatment of Hypothyroidism: American Thyroid Association Task Force (Thyroid 2014)
- Ross et al., 2016 American Thyroid Association Guidelines for Diagnosis and Management of Hyperthyroidism and Other Causes of Thyrotoxicosis (Thyroid 2016)
- MedlinePlus (NIH) — Thyroid Tests
- Mayo Clinic Labs
- MedlinePlus (NIH) — Triiodothyronine (T3) Tests
- MedlinePlus (NIH) — Thyroxine (T4) Test
Educational content, not a diagnosis. Lab results are one input; a clinician interprets them with your history and exam.