Typical US rangevsTypical SI range

T4 and T3 are one hormone at two stages. The thyroid releases T4, and your liver, kidneys, muscle and brain strip one iodine atom off it to make T3, the form that switches genes on inside cells. That conversion is why T3 stays normal in an early underactive thyroid and why the standard thyroid pill is T4. A third number, TSH (thyroid-stimulating hormone), is the pituitary gland's signal telling the thyroid to work harder: it goes up when the thyroid is underactive and down when it is overactive.

What is the difference between T3 and T4?

T4 carries four iodine atoms and T3 carries three. The thyroid releases mostly T4, roughly 14 parts T4 to 1 part T3 ², and about 80% of the T3 circulating in your blood is made outside the gland by enzymes called deiodinases, which remove one iodine from T4 ⁴. T3 is the active form because it binds the switch inside cells (the thyroid receptor) about ten times more tightly than T4 does ⁷. T4 has a half-life of about 7 days and T3 about 1 day, so a change in a T4 dose takes around 6 weeks to settle into a new steady level ².

More than 99% of both hormones travel bound to carrier proteins (thyroxine-binding globulin, transthyretin and albumin). Only about 0.03% of T4 and about 0.3% of T3 float free, and the free portion is the part tissues can use ⁸. The typical adult ranges below are a guide only; every lab sets its own, so read your result against the range printed on your report. Pregnancy has its own trimester-specific ranges, and an abnormal thyroid result in pregnancy should be reviewed by your clinician soon, not at a routine visit ².

How T4 becomes T3, and what reverse T3 is

Conversion slows when the body is under strain. Acute illness, surgery, fasting and calorie restriction cut T4-to-T3 conversion. In serious illness this low-T3 state is common enough to have a name, non-thyroidal illness syndrome; it usually resolves with recovery, and giving thyroid hormone has not been shown to help ⁵. Some medicines slow conversion too, including amiodarone, iodine-based X-ray contrast, steroid medicines and high doses of propranolol ³⁴.

Selenium is built into each deiodinase, so a true selenium deficiency can slow conversion ⁴. That is uncommon on a varied diet, and the AACE/ATA guideline does not recommend selenium as a treatment for an underactive thyroid ¹. Reverse T3 is measurable, but the ATA treatment guideline does not recommend it for routine use, because it does not change decisions for most people ².

Which test tells you more: T3 or T4?

The answer depends on which way the thyroid might be going. For a first screen, TSH comes first and free T4 second; TSH vs free T4 covers how that pair works, and how to read your thyroid panel goes through every line. For a suspected underactive thyroid, TSH rises first and free T4 is the next useful number, because a failing gland pushes the body to convert more T4 into T3, so T3 stays normal until hypothyroidism is advanced. The AACE/ATA guideline is direct on the point:

Serum T3 measurement, whether total or free, should not be used to diagnose hypothyroidism.Garber JR et al., AACE/ATA Clinical Practice Guidelines for Hypothyroidism in Adults, Thyroid, 2012 ¹

For a suspected overactive thyroid, T3 becomes essential. The 2016 ATA hyperthyroidism guideline advises measuring total T3 alongside free T4, because early Graves' disease (an autoimmune condition where antibodies switch the thyroid on) and toxic nodules (lumps that make hormone on their own) can raise T3 while T4 is still normal, a pattern called T3 toxicosis ³. A total T3 to total T4 ratio above 20 (ng/µg) points toward Graves' disease or a toxic nodule rather than painless thyroiditis, a temporary inflammation that leaks stored hormone ³. If T3 is high with a low TSH, ask your clinician whether a TSH receptor antibody or TSI test should be added to check for Graves' disease.

People taking levothyroxine are usually followed with TSH and free T4, with T3 added when symptoms persist. People taking liothyronine or desiccated thyroid need to note when they took their last dose, because T3 peaks within a few hours and can push a morning result high ².

Free or total: which T3 and T4 did your lab measure?

Total T4 and total T3 count both the bound and the free hormone, so they move whenever carrier proteins move. Estrogen in oral contraceptives or oral HRT, pregnancy and some liver disease raise thyroxine-binding globulin, which lifts total T4 and total T3 while free hormone stays about the same ¹⁸. That is why free T4 is the preferred T4 test. T3 runs the other way: the standard lab method for free T3 is less accurate than the one for free T4, so total T3 is often the more reliable T3 measurement ³⁸. Older panels may report a T3 uptake or a free thyroxine index, both indirect estimates of free hormone.

What your T3 and T4 results mean together

Read T3 and T4 together with TSH. The table lists the common patterns, the most frequent explanation for each and a sensible next question, drawn from the ATA hyperthyroidism guideline ³ and the review of illness-related low T3 ⁵. Ranges vary by lab, and only your clinician can tie a pattern to you.

As hypothyroidism vs hyperthyroidism puts it: "TSH moves opposite to thyroid hormone, so a high TSH points to an underactive thyroid and a low TSH points to an overactive one." The full pages on hypothyroidism and hyperthyroidism cover symptoms and causes.

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T4 vs T3 as a medicine: levothyroxine, liothyronine and combinations

Levothyroxine is synthetic T4, and the ATA treatment guideline makes it the standard for hypothyroidism: its week-long half-life gives steady levels from one daily dose, and the body converts it to T3 as each tissue needs. The same guideline found the evidence did not support routine T4 plus T3 combinations ². Liothyronine is synthetic T3; it peaks within a few hours and is largely gone within a day, so levels rise and fall through the day ². Desiccated thyroid, made from pig glands, carries about 4 parts T4 to 1 part T3, against roughly 14 to 1 from a human thyroid, so it delivers far more T3 ².

A minority of people on levothyroxine keep a normal TSH but run a lower free T3 and still feel unwell. A 2021 joint consensus from the American, British and European thyroid associations notes that some people who stay symptomatic despite a normal TSH on levothyroxine may be offered a carefully monitored trial of T4 plus T3, while the evidence for routine use is still limited ⁶. Any change in a thyroid medicine is a decision for the prescriber, and blood levels are usually rechecked 4 to 8 weeks after a change, once the new dose has settled ¹².

The standard treatment, with the evidence

Strongest evidenceLevothyroxineSynthetic T4; guideline first choice for hypothyroidism because it gives steady levels and lets tissues make their own T3.

Normal T3 and T4 but still feel off?

Normal TSH, T4 and T3 make a thyroid cause of tiredness less likely, but they do not rule out every thyroid problem: TPO antibodies (antibodies against thyroid peroxidase, an enzyme the gland needs to make hormone) can be positive for years before the numbers move, which is the early stage of Hashimoto's disease. Low iron, low vitamin B12, poor sleep, depression and perimenopause all produce a similar heavy tiredness. If you feel unwell with normal numbers, ask your clinician whether a TPO antibody test and ferritin make sense. Thyroid normal but not yourself walks through the other causes, and the fatigue page covers what to check next.

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