Total Thyroxine (Total T4) Unit Converter
Total Thyroxine (Total T4) Unit Converter
Convert total T4 between µg/dL, µg/L and nmol/L — and read the result knowing that a total thyroxine follows the concentration of the proteins carrying it, which is why pregnancy and the oral contraceptive raise it in patients who are not thyrotoxic.
Total Thyroxine (Total T4) converter
Mass ⇄ molarTotal T4 8.0 µg/dL
The conversion, and the molecular weight behind it
µg/dL = nmol/L ÷ 12.8722
because 12.8722 = 1 µg/dL (10⁻⁵ g/L) ÷ 776.87 g/mol, the molecular weight of thyroxine
- MW 776.87
- thyroxine, C₁₅H₁₁I₄NO₄. Four iodine atoms make it a heavy molecule for a hormone, which is why the mass-to-molar factor is as large as it is
- µg/L = ng/mL
- identical concentrations, and both are ten times the µg/dL figure. A total T4 of 8 µg/dL is 80 µg/L. North American reports use µg/dL, most of the rest of the world uses nmol/L
- the same factor as free T4
- free T4 is the same molecule measured three orders of magnitude lower, so ng/dL → pmol/L takes the identical 12.8722. That is a convenience and a trap: the two results are not interchangeable, and a free T4 of 1.2 ng/dL has nothing to do with a total T4 of 1.2 µg/dL
- what is being measured
- about 99.97% of circulating thyroxine is bound — roughly 70% to thyroxine-binding globulin, the remainder to transthyretin and albumin. A total T4 is therefore overwhelmingly a measurement of occupied binding sites, and only about three parts in ten thousand of it is the hormone that acts on tissue
Worked example
Total T4 8.0 µg/dL
8.0 µg/dL = 80.0 µg/L = 80.0 ng/mL — the same concentration, shifted one decimal place
8.0 × 12.8722 = 103.0 nmol/L (102.98 before rounding)
103 nmol/L sits comfortably inside Mayo's adult band of 4.5–11.7 µg/dL, which is 57.9–150.6 nmol/L
Now change one thing about the patient and nothing about the thyroid. At 28 weeks' gestation the same 103 nmol/L is read against roughly 6.8–17.6 µg/dL (87–226 nmol/L), because oestrogen has raised thyroxine-binding globulin two- to three-fold — the value that was mid-normal is now near the bottom of the pregnancy interval
And a UK report of 140 nmol/L converts back to 140 ÷ 12.8722 = 10.9 µg/dL, inside the non-pregnant adult interval
What moves a total T4 without touching thyroid status
| Cause | Binding proteins | Total T4 | Free T4 and TSH |
|---|---|---|---|
| Pregnancy — oestrogen-driven TBG rise, plus reduced clearance | TBG up two- to three-fold by mid-gestation | Raised, often well above the non-pregnant interval | Free T4 normal, TSH normal or slightly low in the first trimester |
| Combined oral contraceptive, oestrogen replacement, tamoxifen | TBG up | Raised | Normal |
| Inherited TBG excess (X-linked) | TBG up, lifelong | Raised, lifelong | Normal — the patient has no thyroid disease and never had |
| Androgens, anabolic steroids, high-dose glucocorticoids | TBG down | Lowered | Normal |
| Nephrotic syndrome, protein-losing enteropathy, cirrhosis, malnutrition | TBG and albumin lost or unmade | Lowered | Normal, or free T4 low-normal |
| Severe systemic illness | Binding falls and binding inhibitors accumulate | Lowered, sometimes markedly | TSH usually normal; this is non-thyroidal illness, not thyroid failure |
| Familial dysalbuminaemic hyperthyroxinaemia | A variant albumin binds T4 avidly | Raised | Normal, and some free T4 assays are also spuriously raised |
| Phenytoin, carbamazepine, high-dose salicylates, furosemide | Hormone displaced from its carriers and cleared faster | Lowered | Free T4 normal or low-normal with a normal TSH |
Total T4, free T4 and the free thyroxine index
| Measurement | What it reports | Where it still earns its place |
|---|---|---|
| Total T4 | Bound plus free hormone — in practice, the size of the binding-protein pool and how full it is | Cheap, robust, and unaffected by some of the interferences that trouble free T4 immunoassays. Still the standard in many laboratories outside North America and Europe |
| Free T4 | The small unbound fraction that enters cells | The routine test. Interpreted with TSH; intervals are assay-specific and trimester-specific in pregnancy |
| Free thyroxine index — total T4 × T3 uptake or T4 ÷ TBG | Total T4 corrected for binding capacity | Exists precisely to rescue a total T4 from a binding-protein problem, and remains useful where free T4 assay interference is suspected |
| TSH | Pituitary sensing of thyroid hormone action over weeks | The first-line test in nearly every setting, and the one binding proteins do not disturb |
Mayo Clinic Laboratories reference values, total T4 by ECLIA
| Group | µg/dL | nmol/L |
|---|---|---|
| Adult, 20 years and over | 4.5–11.7 | 57.9–150.6 |
| 11–19 years | 5.9–13.2 | 75.9–169.9 |
| 1–5 years | 6.0–14.7 | 77.2–189.2 |
| Pregnancy, 2nd–3rd trimester | 6.8–17.6 (adult × 1.5) | 87–226 |
A measurement of the carriers, not of the cargo
Total thyroxine is reported in micrograms per decilitre or in nanomoles per litre, and the bridge between them is thyroxine’s molecular weight of 776.87: one microgram per decilitre is 12.8722 nanomoles per litre. Micrograms per litre and nanograms per millilitre are the same as each other and ten times the µg/dL figure, so a total T4 of 8 µg/dL is 80 µg/L and 103 nmol/L. The same factor converts free T4 from ng/dL to pmol/L, because it is the same molecule three orders of magnitude down — which makes the arithmetic easy and makes confusing the two results easy as well.
The substance of this page is what the number represents. Roughly 99.97% of the thyroxine in blood is bound to protein: about seven parts in ten to thyroxine-binding globulin, the rest to transthyretin and albumin. A total T4 therefore measures the carriers and how full they are, and only about three parts in ten thousand of what it reports is the free hormone that enters cells and does the work. Change the concentration of the carriers and the total changes with it, while the free hormone — which the pituitary defends — does not move at all.
That is not a theoretical caveat; it is most of the clinical trouble this measurement causes. Pregnancy raises thyroxine-binding globulin two- to three-fold through oestrogen-driven sialylation and slower clearance, so a total T4 rises well above the non-pregnant interval in a woman who is perfectly euthyroid. The combined oral contraceptive, oestrogen replacement and tamoxifen do the same thing more modestly. In the other direction, androgens and anabolic steroids, high-dose glucocorticoids, nephrotic syndrome and protein-losing enteropathy, cirrhosis, malnutrition and any severe systemic illness lower the binding-protein concentration and take the total T4 down with it. Inherited thyroxine-binding globulin excess or deficiency produces a lifelong abnormal total T4 in a person who has never had thyroid disease. None of these patients is thyrotoxic or hypothyroid, and none of them needs treating on the strength of a total T4.
This is the whole reason free hormone assays displaced total ones in routine practice, and the reason the free thyroxine index was invented before them — it exists to correct a total T4 for binding capacity. Where a total T4 is what you have, interpret it with TSH: the pituitary responds to free hormone and is not deceived by the carrier proteins, so an abnormal total T4 alongside a normal TSH should prompt a thought about binding long before it prompts a prescription. Where the two cannot be reconciled, a free T4, a free thyroxine index or a TBG measurement is what settles it.
Frequently asked questions
How do you convert total T4 from µg/dL to nmol/L?
Multiply by 12.8722, which is one microgram per decilitre divided by thyroxine’s molecular weight of 776.87 g/mol. A total T4 of 8.0 µg/dL is 103.0 nmol/L. Divide by the same factor to go the other way, so a report of 140 nmol/L is 10.9 µg/dL. Micrograms per litre and nanograms per millilitre are identical to each other and ten times the µg/dL number.
Why is my total T4 high in pregnancy or on the contraceptive pill?
Because oestrogen raises thyroxine-binding globulin, the protein that carries most of the thyroxine in blood. More carrier means more bound hormone and therefore a higher total T4, while the free hormone the tissues see stays the same and TSH stays normal. It is not thyrotoxicosis and it needs no treatment. Where a total T4 is used in the second and third trimesters, the ATA suggest reading it against the non-pregnant interval multiplied by 1.5; a free T4 with a trimester-specific interval, or a TSH, is generally preferable.
Is total T4 or free T4 the better test?
Free T4, in nearly every routine setting, because it reports the fraction that enters cells and is not moved by the concentration of the binding proteins. Total T4 remains cheap and robust and is still standard in many laboratories, and it is occasionally more reliable than a free T4 when an immunoassay interference is suspected — which is also when the free thyroxine index, a total T4 corrected for binding capacity, becomes useful. Either way, the result is interpreted with TSH rather than on its own.
What lowers a total T4 in someone with a normal thyroid?
Anything that lowers the binding proteins or displaces hormone from them: androgens and anabolic steroids, high-dose glucocorticoids, nephrotic syndrome and protein-losing enteropathy, cirrhosis, malnutrition, severe systemic illness, inherited thyroxine-binding globulin deficiency, and drugs such as phenytoin, carbamazepine, furosemide and high-dose salicylates. In all of them the free T4 and the TSH are normal or near-normal, which is the clue that the thyroid is not the problem.
Can a normal total T4 occur in someone with thyroid disease?
Yes, and in two directions. A hypothyroid patient with a high binding-protein concentration can have a total T4 pulled up into the reference interval, and a thyrotoxic patient with a low one can have it pulled down into it. In early or subclinical disease TSH moves before either hormone does. A total T4 is never a screening test on its own.
Related calculators
References
- Mayo Clinic Laboratories. Test ID: T4 — T4 (Thyroxine), Total Only, Serum. Electrochemiluminescence immunoassay. Reference values: adult ≥20 years 4.5–11.7 mcg/dL; 11–19 years 5.9–13.2 mcg/dL; 1–5 years 6.0–14.7 mcg/dL.
- 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. doi:10.1089/thy.2016.0457 — adaptation of the non-pregnant total T4 reference interval by a factor of 1.5 in the second and third trimesters.
- 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.
- Refetoff S. Inherited thyroxine-binding globulin abnormalities in man. Endocr Rev. 1989;10(3):275–293.
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.