Total Triiodothyronine (Total T3) Unit Converter
Total Triiodothyronine (Total T3) Unit Converter
Convert total T3 between ng/dL, pg/mL, ng/mL and nmol/L — with the factor-of-ten trap between ng/dL and pg/mL, and why a low total T3 in an unwell patient is illness rather than thyroid disease.
Total Triiodothyronine (Total T3) converter
Mass ⇄ molarTotal T3 120 ng/dL
The conversion, and the unit ladder around it
ng/dL = nmol/L ÷ 0.0153615
because 0.0153615 = 1 ng/dL (10⁻⁸ g/L) ÷ 650.98 g/mol, the molecular weight of triiodothyronine
- MW 650.98
- triiodothyronine, C₁₅H₁₂I₃NO₄. Three iodines rather than thyroxine's four, so it is the lighter molecule and the less abundant one — about a fifth of circulating T3 comes from the thyroid and the rest from deiodination of T4 in peripheral tissue
- ng/dL → pg/mL is × 10
- the commonest conversion error on this analyte. A total T3 of 120 ng/dL is 1,200 pg/mL. Reading a pg/mL figure as though it were ng/dL inflates it ten-fold, and the reverse divides a genuine result by ten
- ng/mL is the ng/dL figure ÷ 100
- so 120 ng/dL is 1.2 ng/mL. A total T3 quoted as "1.2" is therefore ng/mL, not ng/dL — and it is not a free T3 either
- not the same as a free T3
- free T3 is reported in pg/mL at single figures, around 2.3–4.2 pg/mL on a typical assay. The same patient's total T3 in pg/mL is roughly four hundred times larger. A total T3 read against a free T3 interval reads as wildly thyrotoxic, and a free T3 read against a total T3 interval as profoundly hypothyroid
- what is being measured
- bound plus free hormone. Around 99.7% of T3 is protein-bound, mostly to thyroxine-binding globulin, so a total T3 moves with the carriers in the same way a total T4 does
Worked example
Total T3 120 ng/dL
120 ng/dL = 1,200 pg/mL = 1.20 ng/mL — the same concentration written three ways
120 × 0.0153615 = 1.84 nmol/L
1.84 nmol/L is mid-interval: Mayo's adult 80–200 ng/dL is 1.23–3.07 nmol/L
Do it wrongly and the scale of the error is obvious. Treat the 1,200 pg/mL as though it were ng/dL and you get 18.4 nmol/L, a figure no patient has ever had
And the other confusion: a free T3 of 3.1 pg/mL on the same sample is a normal free T3, while 3.1 read as a total T3 in ng/dL would be almost undetectable. The units decide which analyte you are looking at
Why a total T3 falls, and why most of the reasons are not thyroid disease
| Setting | Mechanism | TSH and free T4 |
|---|---|---|
| Any significant acute illness — sepsis, myocardial infarction, trauma, major surgery, critical care | Type 1 deiodinase activity falls and reverse T3 rises within a day or two, so less T4 is converted to T3 | TSH normal or low-normal, free T4 normal or low. This is the low-T3 syndrome of non-thyroidal illness, and it is not treated with thyroid hormone |
| Starvation, anorexia nervosa, very-low-calorie dieting | The same fall in peripheral conversion, as an adaptive reduction in energy expenditure | TSH normal |
| High-dose glucocorticoids, propranolol, amiodarone, propylthiouracil, iodinated contrast | Inhibition of T4-to-T3 conversion | Variable; amiodarone raises total T4 and lowers total T3 in most patients taking it |
| Low binding-protein states — androgens, nephrotic syndrome, cirrhosis, malnutrition | Less carrier, so less bound hormone measured | Normal |
| Hypothyroidism | Reduced production, but T3 is preserved at the expense of T4 until the disease is advanced | TSH raised — which is the finding that makes the diagnosis, not the T3 |
The unit ladder, and where results get misread
| Unit | A total T3 of 120 ng/dL is | Who uses it |
|---|---|---|
| ng/dL | 120 | North American reports — the conventional total T3 unit |
| nmol/L | 1.84 | Most laboratories outside North America |
| pg/mL | 1,200 | Rarely used for total T3, and the source of the ten-fold error, because pg/mL is the conventional unit for free T3 |
| ng/mL | 1.20 | Occasionally seen; a bare “1.2” on a report is this |
A hormone made mostly outside the thyroid, and a level that falls with illness
Total triiodothyronine is reported in nanograms per decilitre or nanomoles per litre, and the factor between them comes from T3’s molecular weight of 650.98: one nanogram per decilitre is 0.0153615 nanomoles per litre, so a total T3 of 120 ng/dL is 1.84 nmol/L. Two unit traps sit on this analyte. Nanograms per decilitre and picograms per millilitre differ by a factor of ten, so 120 ng/dL is 1,200 pg/mL; and picograms per millilitre is the conventional unit for free T3, which circulates at single figures. A number in pg/mL therefore needs checking against the analyte name before it is converted at all — the same patient’s total and free T3 differ by a factor of several hundred.
Like a total T4, a total T3 measures bound plus free hormone, and about 99.7% of it is bound. It therefore rises when thyroxine-binding globulin rises — pregnancy, oestrogen, the combined oral contraceptive — and falls when the carriers fall, in androgen use, nephrotic syndrome, cirrhosis and malnutrition, with no change in thyroid status and no change in TSH. That alone makes it a poor test of the thyroid, and it is why free hormone assays replaced total ones.
The particular problem with T3, though, belongs to the hormone rather than to its carriers. Only about a fifth of circulating T3 comes from the thyroid; the rest is made in peripheral tissue by deiodination of T4. Type 1 deiodinase activity falls within a day or two of any serious illness, major surgery, trauma or starvation, and reverse T3 rises in its place, so the total T3 drops while TSH stays normal. This is the low-T3 pattern of non-thyroidal illness. It tracks the severity of the illness rather than the state of the gland, it recovers as the patient recovers, and it is not treated with thyroid hormone. Glucocorticoids, propranolol, amiodarone, propylthiouracil and iodinated contrast produce the same fall pharmacologically. The practical consequence is that thyroid function tested during acute illness mostly produces results that have to be explained rather than acted on.
Total T3 is not a screening test. In hypothyroidism it is preserved until the disease is advanced, because the failing gland and the peripheral deiodinases both favour T3 production, so a normal T3 with a raised TSH is the usual picture in early thyroid failure. Where it does earn its place is at the other end: in suspected thyrotoxicosis with a suppressed TSH and a normal T4, a raised T3 identifies T3 toxicosis, and in a patient taking liothyronine it documents exposure — though the level then depends heavily on how long before the sample the dose was taken.
Frequently asked questions
How do you convert total T3 from ng/dL to nmol/L?
Multiply by 0.0153615, which is one nanogram per decilitre divided by T3’s molecular weight of 650.98 g/mol. A total T3 of 120 ng/dL is 1.84 nmol/L, and a report of 2.0 nmol/L is 130 ng/dL. Divide by the same factor to go back.
What is the difference between ng/dL and pg/mL for T3?
A factor of ten: 120 ng/dL is 1,200 pg/mL. The reason it matters more than an ordinary unit conversion is that pg/mL is the conventional unit for free T3, which sits at single figures — around 2.3 to 4.2 pg/mL on a typical assay. So a small number in pg/mL is almost certainly a free T3 rather than a total T3, and converting it as though it were a total T3 produces nonsense. Check the analyte name first.
Why is my total T3 low when my TSH is normal?
Most often because you are or have recently been unwell. Only about a fifth of circulating T3 comes from the thyroid; the rest is converted from T4 in peripheral tissue, and that conversion falls within a day or two of any significant illness, surgery, trauma or period of starvation, with reverse T3 rising in its place. This is the low-T3 syndrome of non-thyroidal illness. It reflects the severity of the illness, recovers with it, and is not treated with thyroid hormone. Glucocorticoids, propranolol, amiodarone and propylthiouracil do the same thing, and low binding-protein states lower the total as well.
Should total T3 be used to diagnose hypothyroidism?
No. T3 production is defended until thyroid failure is advanced, so the total T3 is typically still normal when the TSH is already raised and the free T4 falling. TSH is the first-line test, with free T4 alongside it. A total T3 adds most at the opposite end — a suppressed TSH with a normal T4 and a raised T3 is T3 toxicosis.
Does pregnancy raise total T3?
Yes. Oestrogen raises thyroxine-binding globulin two- to three-fold, and because around 99.7% of T3 is protein-bound the total rises with it while the free hormone and TSH remain appropriate. It is the same phenomenon that raises total T4 in pregnancy and it is not thyrotoxicosis. The ATA publish a numerical adaptation for total T4 but not for total T3, so in pregnancy a free T3 or, better, TSH with free T4 against trimester-specific intervals is what should be used.
Related calculators
References
- Mayo Clinic Laboratories. Test ID: T3 — T3 (Triiodothyronine), Total, Serum. Electrochemiluminescence immunoassay. Reference values: adult ≥20 years 80–200 ng/dL; 11–19 years 91–218 ng/dL; 1–5 years 92–248 ng/dL.
- Warner MH, Beckett GJ. Mechanisms behind the non-thyroidal illness syndrome: an update. J Endocrinol. 2010;205(1):1–13. doi:10.1677/JOE-09-0412
- Baloch Z, Carayon P, Conte-Devolx B, et al. Laboratory medicine practice guidelines. Laboratory support for the diagnosis and monitoring of thyroid disease. Thyroid. 2003;13(1):3–126.
- Alexander EK, Pearce EN, Brent GA, et al. 2017 Guidelines of the American Thyroid Association for the Diagnosis and Management of Thyroid Disease During Pregnancy and the Postpartum. Thyroid. 2017;27(3):315–389.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.