Free Triiodothyronine (FT3) Unit Converter
Free Triiodothyronine (FT3) Unit Converter
Convert free T3 between pg/mL, pmol/L and ng/L, and see why the commonest low FT3 is non-thyroidal illness rather than hypothyroidism.
Free Triiodothyronine (FT3) converter
Mass ⇄ molarFree T3 3.1 pg/mL
Formula and conversion factors
pg/mL = pmol/L ÷ 1.53617
ng/L = pg/mL (numerically identical)
- 1.53617
- derived from the molecular mass of triiodothyronine, 650.97 Da
- ng/L
- numerically identical to pg/mL; no conversion is needed between them
- peripheral origin
- most circulating T3 comes from deiodination of T4 in liver, kidney and muscle rather than from thyroid secretion
- deiodinase shift
- illness, starvation, corticosteroids, amiodarone and propranolol divert T4 away from T3 towards reverse T3, lowering free T3 in a euthyroid patient
Worked example
Free T3 3.1 pg/mL
3.1 × 1.53617 = 4.76 pmol/L
= 3.10 ng/L
Within the adult reference interval of 2.3–4.2 pg/mL
Free T3 across the three units
| pg/mL | pmol/L | ng/L |
|---|---|---|
| 1.50 | 2.30 | 1.50 |
| 2.30 — typical lower limit | 3.53 | 2.30 |
| 3.10 | 4.76 | 3.10 |
| 4.20 — typical upper limit | 6.45 | 4.20 |
| 8.00 | 12.29 | 8.00 |
Reading free T3 in context
| Pattern | TSH | Free T4 | Free T3 |
|---|---|---|---|
| Non-thyroidal illness (low-T3 or sick-euthyroid) | Normal or low | Normal, sometimes low | Low |
| Primary hypothyroidism | Raised | Low | Low or still normal |
| T3 toxicosis | Suppressed | Normal | Raised |
| Graves’ thyrotoxicosis | Suppressed | Raised | Raised, often disproportionately |
Why a low free T3 usually is not hypothyroidism
Free T3 is reported in pg/mL in the United States and in pmol/L elsewhere, with a factor of 1.53617 between them derived from the molecular mass of triiodothyronine, 650.97 daltons. A free T3 of 3.1 pg/mL is 4.76 pmol/L. The third unit shown, ng/L, is numerically identical to pg/mL and needs no arithmetic at all, since one nanogram per litre is one picogram per millilitre.
The physiology explains most of the interpretive difficulty. Only a minority of circulating T3 is secreted by the thyroid; the bulk is produced by deiodination of T4 in liver, kidney and skeletal muscle. A free T3 therefore reflects peripheral conversion as much as thyroid output, and it can move for reasons that have nothing to do with the gland. Illness, starvation, corticosteroids, propranolol and amiodarone all divert T4 away from T3 and towards the inactive reverse T3.
That is why the commonest low free T3 in hospital practice is the low-T3 or sick-euthyroid pattern of non-thyroidal illness rather than hypothyroidism. It appears early in any serious acute illness, usually with a normal or slightly low TSH and a free T4 that is normal until the illness is severe. It is an adaptive response, not thyroid failure, and treating it with thyroid hormone has not been shown to help. The safest course in an acutely unwell patient is to avoid thyroid function testing unless thyroid disease is genuinely suspected, and to repeat the tests after recovery if abnormal results have already been obtained.
Free T3 is a poor screening test on its own and should not be requested as one; TSH, with free T4 where needed, does that job. Where it earns its place is in suspected T3 toxicosis, in which TSH is suppressed and free T4 remains normal so that the diagnosis is missed without it, in assessing the severity of thyrotoxicosis, and in monitoring patients taking liothyronine. In that last group timing matters, because liothyronine produces a peak within a few hours and a sample taken soon after a dose can look thyrotoxic.
Frequently asked questions
How do I convert free T3 from pg/mL to pmol/L?
Multiply by 1.53617, a factor derived from the molecular mass of triiodothyronine, 650.97 Da. A free T3 of 3.1 pg/mL is 4.76 pmol/L. To go the other way, divide by 1.53617.
Is pg/mL the same as ng/L for free T3?
Yes, they are numerically identical, because one nanogram per litre is one picogram per millilitre. No conversion is needed between them.
What does a low free T3 with a normal TSH mean?
In an unwell patient it usually reflects non-thyroidal illness, the low-T3 or sick-euthyroid pattern, in which peripheral conversion of T4 to T3 falls. It is an adaptive response rather than hypothyroidism and should not be treated with thyroid hormone.
Should free T3 be used to screen for thyroid disease?
No. TSH, with free T4 where indicated, is the screening approach. Free T3 moves with illness, drugs and nutrition, so on its own it generates more confusion than information.
When is free T3 genuinely useful?
It is useful in suspected T3 toxicosis, where TSH is suppressed and free T4 is normal so the diagnosis would otherwise be missed. It also helps in judging the severity of thyrotoxicosis and in monitoring patients on liothyronine. In that last group the sample should not be taken soon after a dose, because liothyronine peaks within a few hours.
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References
- Ross DS, Burch HB, Cooper DS, et al. 2016 American Thyroid Association guidelines for diagnosis and management of hyperthyroidism and other causes of thyrotoxicosis. Thyroid. 2016;26(10):1343–1421.
- Jonklaas J, Bianco AC, Bauer AJ, et al. Guidelines for the treatment of hypothyroidism. Thyroid. 2014;24(12):1670–1751.
- Thienpont LM, Van Uytfanghe K, Beastall G, et al. Report of the IFCC Working Group for Standardization of Thyroid Function Tests; part 2: free thyroxine and free triiodothyronine. Clin Chem. 2010;56(6):912–920.