What is free T3?
Your thyroid makes two hormones. By far the most is T4, thyroxine, a storage form that does little. The active hormone is T3, and you make most of it not in your thyroid but in your liver, kidneys and muscles, by removing one iodine atom from T4. T3 switches on the metabolism in almost every cell: your warmth, your heart rate, your energy use, your bowels, your mood. Most T3 travels bound to protein and is inactive for the time being. Free T3 is the small loose part that can enter the cells, and that is what this test measures. What is special is that your body regulates that conversion from T4 to T3 itself, independently of your thyroid. With illness, fever, a crash diet or eating too little for a long time, the body turns the conversion down and breaks T3 down faster: free T3 falls, to save energy, while TSH and free T4 simply stay normal. That is not thyroid disease but an adaptation, and it disappears as soon as you eat normally again or recover. Your report shows free T3 in pmol/L. Normal is about 3.5 to 6.1 pmol/L from age 20 to 50 where the labs overlap, and from 50 the upper limit drops a fraction. With a healthy thyroid the value is stable from day to day.
Why is free T3 relevant?
For the question of whether your thyroid works well, free T3 is not the place to start. TSH is the thermostat and moves first, free T4 shows what the thyroid releases, and those two together tell almost everything. Free T3 rarely adds anything new to that, which is why it is not part of the ordinary thyroid work-up. Anyone who does have it on the report reads it as a supplement. What free T3 does show well is how your body responds to your lifestyle. On a strict diet, in a long period of eating too little, with overtraining or an illness, free T3 falls first, sometimes below the band, while TSH and free T4 stay normal. That is why on a crash diet you get cold, slow down and the weight stops dropping after a few weeks: your body turns the metabolism down. It is not your thyroid faltering, and a thyroid pill is then the wrong answer; more food and rest are the right one. A low free T3 with a normal TSH in anyone who trains, diets or is ill is therefore more often a sign of too little than of an underactive thyroid. The only situation in which free T3 adds something to TSH and free T4 is an overactive thyroid that starts with T3: free T3 raised, TSH suppressed, free T4 still normal. That occurs in early Graves' disease or an overactive nodule, and it is the reason the test exists. Anyone taking a thyroid preparation with T3 in it, or a supplement with biotin, also sees free T3 go up.
Free T3 reference ranges
What counts beside the number:
- TSH and free T4 from the same draw, because free T3 only means something in that combination
- How you have eaten in recent weeks, and whether you were ill or trained hard
- What you take: levothyroxine, a preparation with T3, or biotin
- Symptoms of too fast, such as palpitations, tremor, weight loss and heat, or of too slow
- Your own lab's range, because the edges differ
The band above is where the labs overlap from age 20 to 50; after that the upper limit drops a fraction, and teenagers sit higher. More important than the band is the combination: a free T3 just below the limit with a normal TSH is usually your body saving energy, not an underactive thyroid. For the question of whether your thyroid works well, TSH and free T4 come first.
What Dutch labs actually use
For free T3 the labs sit close together but draw the edges slightly differently, and one of them lets the band drop a little from age 50. From age 20 to 50 they overlap between 3.5 and 6.1 pmol/L. For reading that weighs less than the combination with TSH and free T4, because a free T3 just outside the band rarely says anything about your thyroid with a normal TSH.
3.5 – 6.1pmol/L
Between 2.9 and 3.5 and between 6.1 and 6.80 it depends on the lab, because each sets its limits on its own method and population. Above 6.80 every lab calls it abnormal.
every lab calls 3.5 – 6.1 pmol/L normal. Between 2.9 and 3.5 and between 6.1 and 6.80 it depends on the lab.
Every age, per lab
| Laboratory | Group | Reference range |
|---|---|---|
| Unilabs(Saltro, Medlon, SHO, Atalmedial)2026 | 13–20 yrs | 4.7–7.2 pmol/L |
| From 20 yrs | 3.5–6.5 pmol/L | |
| Star-shl2026 | 12–20 yrs | 3.93–7.70 pmol/L |
| Adults | 3.10–6.80 pmol/L | |
| Certe2026 | 11–20 yrs | 3.93–7.7 pmol/L |
| From 20 yrs | 2.9–6.1 pmol/L | |
| Clinical Diagnostics2026 | Men 15–19 yrs | 3.8–6.6 pmol/L |
| Women 15–19 yrs | 3.9–6.3 pmol/L | |
| Men 19–50 yrs | 3.2–6.7 pmol/L | |
| Women 19–50 yrs | 3.2–6.7 pmol/L | |
| Men from 50 yrs | 2.7–6.0 pmol/L | |
| Women from 50 yrs | 2.7–6.0 pmol/L |
Each band as the lab publishes it, retrieved in 2026, unless marked otherwise.
Free T3 high or low: what it means
A low free T3 usually means your body has turned down the conversion from T4 to T3. In order of how often it occurs: illness, a crash diet or eating too little for a long time, an underactive thyroid that already shows in TSH and free T4, too low a dose of levothyroxine in anyone taking it, and rarely medicines that suppress production. If your TSH is normal, a low free T3 is almost never a thyroid problem, and the question is how you have eaten and lived in recent weeks. A high free T3 means more active hormone in circulation. With a suppressed TSH that is an overactive thyroid, and if free T4 is still normal, one that starts with T3. With a normal TSH it is usually what you take: a thyroid preparation with T3, or biotin, which disturbs the assay and makes free T3 read falsely high. Stop biotin a few days before the draw. What you do about it: with a low free T3 and a normal TSH the answer is food, rest and patience, not a thyroid pill; the value recovers within weeks once your body notices enough is coming in. With a high free T3 and a suppressed TSH, investigation of the thyroid belongs, because an overactive thyroid strains the heart and the bones. Always read free T3 beside TSH and free T4, and never alone. Mention what you take, and compare over time at the same lab, because the ranges differ at the edges.
What lowers it
Usually the body turning down the conversion from T4 to T3, and only then the thyroid itself.
| Cause | How often |
|---|---|
| An underactive thyroidIn Hashimoto's or after treatment of the thyroid, free T3 falls with it, but TSH and free T4 show it earlier and more clearly | Often |
| Too low a dose of levothyroxineAnyone taking thyroid hormone and getting too little sits low in free T4 and free T3 and high in TSH | Often |
| Illness, a crash diet or eating too littleThe body turns the conversion from T4 to T3 down to save energy; TSH stays normal and the value recovers once you eat again or get better | Sometimes |
| Medication that suppresses productionLithium, amiodarone and high doses of prednisone suppress production or conversion; the value falls without thyroid disease | Rare |
What raises it
More active hormone in circulation, or an assay that is disturbed.
| Cause | How often |
|---|---|
| Too high a dose of thyroid hormoneA preparation with T3 in it, or too much levothyroxine, lifts free T3 directly; the dose then needs to come down | Often |
| An overactive thyroidIn Graves' disease or an overactive nodule; if the thyroid starts with T3, free T3 is raised while free T4 is still normal | Sometimes |
| Use of biotinThis supplement disturbs the assay and makes free T3 read falsely high and TSH falsely low; stop it a few days before the draw | Sometimes |
How does a free T3 blood test work?
- Fasting
- Not needed. Stop a supplement with biotin a few days beforehand, and take levothyroxine after the draw.
- The result
- A single number in pmol/L, which you read beside TSH and free T4: suppressed TSH with high free T3 is too fast, normal TSH with low free T3 is usually your lifestyle.
- Repeating
- After a few weeks of eating normally if the value was low with a normal TSH; with an overactive thyroid according to the treatment plan.
- Where to draw
- You can draw at 250+ locations near you, no referral needed. See the locations
Frequently asked questions
What is free T3?
The active thyroid hormone. Your thyroid mainly makes T4, a storage form; your liver, kidneys and muscles remove one iodine atom from it and make T3, the hormone that switches on the metabolism in almost every cell. Most T3 travels bound to protein; free T3 is the small loose part that can enter the cells, and that is what the test measures.
What is a normal free T3 level?
About 3.5 to 6.1 pmol/L from age 20 to 50 where the labs overlap; after that the upper limit drops a fraction. More important than the band is the combination with TSH and free T4: with a normal TSH a free T3 just outside the band is rarely a thyroid problem.
What does a low free T3 mean?
With a normal TSH usually that your body has turned down the conversion from T4 to T3 to save energy: with illness, a crash diet, eating too little for a long time or overtraining. That is an adaptation, not thyroid disease, and the value recovers once you eat normally again or get better. With a high TSH a low free T3 belongs to an underactive thyroid or too low a dose of levothyroxine, and then TSH and free T4 already show it.
What does a high free T3 mean?
More active hormone in circulation. With a suppressed TSH that is an overactive thyroid, and if free T4 is still normal, one that starts with T3: that is the one situation in which free T3 adds something. With a normal TSH it is usually what you take, a preparation with T3 in it, or biotin, which disturbs the assay and makes the value read falsely high.
Why does free T3 fall on a diet?
Because when you eat too little your body turns the metabolism down, and T3 is the dial it uses to do that. It turns the conversion from T4 to T3 down and breaks T3 down faster, while TSH and free T4 stay normal. That is how you get cold, slow down and the weight stops dropping after a few weeks. The answer is not a thyroid pill but enough food, especially enough carbohydrate and protein, and rest; within weeks the value recovers.
Why do TSH and free T4 come first?
Because TSH is the thermostat and moves first, and free T4 shows what the thyroid releases. Together those two tell almost everything about your thyroid, and free T3 rarely adds anything to that, not during treatment with levothyroxine either. That is why free T3 is not part of the ordinary thyroid work-up, and if it is there, you read it as a supplement.
Can a supplement affect my free T3?
Yes. Biotin, vitamin B8, is in many supplements for hair and nails and disturbs the measurement of thyroid hormones: free T3 and free T4 read falsely high and TSH falsely low, without anything being wrong with your thyroid. Stop it a few days before the draw and mention the use. A preparation with T3 in it genuinely lifts the value.
Do I need to fast for a free T3 test?
No. What does count is what you take: stop biotin a few days beforehand and take levothyroxine after the draw. Only request free T3 beside TSH and free T4, because on its own it says nothing.
Which tests include free T3?
Free T3 is part of Advanced, the broader panel. It is not orderable yet.
Related biomarkers
Read on
Sources
- 1.Unilabs, test catalogue (Dutch): free T3. 2026. bepalingenklapper.nl
- 2.Star-shl, test catalogue (Dutch): free T3. 2026. star-shl.nl
- 3.Certe, test catalogue (Dutch): free T3. 2026. bepalingenwijzer.certe.nl
- 4.Clinical Diagnostics, lab guide (Dutch): free T3. 2026. clinicaldiagnostics.nl
- 5.NHG guideline on thyroid disorders (M31), Dutch College of General Practitioners. 2025. richtlijnen.nhg.org
Educational information only, not medical advice. Consult a healthcare professional for clinical decisions.
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Free T3 is part of Advanced, the broader panel. It is not orderable yet.
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