Total T4 & Free T4 Unit Converter (µg/dL, ng/dL ↔ nmol/L, pmol/L)

Total T4 and Free T4: Why the Same Blood Draw Produces Two Very Different-Looking Numbers

Picture this. You had bloodwork done at home, and your Total T4 came back as 7.2 µg/dL. A few months later you’re overseas, get a repeat panel, and the result reads 93 nmol/L.

Different number, completely different scale, and if you don’t know these are the same unit family in disguise, it looks like your thyroid did something dramatic between the two tests. It didn’t.

The same thing happens with Free T4: a US lab might hand you 1.2 ng/dL while a UK or Australian lab reports 15.4 pmol/L for the exact same hormone level. Both conversions use one shared factor, and this page converts either one instantly.

Why Does T4 Look So Different Between the Two Unit Systems?

µg/dL, ng/dL, nmol/L and pmol/L aren’t just relabeled versions of each other the way U/L and IU/L sometimes are. The mass units (µg/dL, ng/dL) measure weight per volume, while the molar units (nmol/L, pmol/L) measure particle count per volume, and the bridge between the two families is thyroxine’s molecular weight (about 776.87 g/mol). Because T4 is a fairly large, heavy molecule, one microgram of it contains a relatively small number of actual molecules, which is why the molar figure ends up roughly thirteen times bigger than the mass figure for the same underlying amount of hormone — whether you’re looking at Total T4 or Free T4.

The Conversion Factor (Same Number for Total T4 and Free T4)

For Total T4, the standard clinical conversion factor is:

nmol/L = µg/dL × 12.87

And in reverse:

µg/dL = nmol/L ÷ 12.87

Because ng/dL and pmol/L are each exactly 1,000 times smaller than µg/dL and nmol/L respectively, the same 12.87 factor carries straight over to Free T4:

pmol/L = ng/dL × 12.87   |   ng/dL = pmol/L ÷ 12.87

That 12.87 isn’t an arbitrary rounding — it comes directly from T4’s molecular weight and the unit definitions involved, and it’s confirmed against Mayo Clinic Laboratories’ own SI unit conversion tables, which list the identical factor for both the Total T4 and Free T4 rows. You don’t need to derive it yourself — just plug your number into the calculator above, or use the formula directly.

A Real Result, Converted Both Ways

Back to that 7.2 µg/dL Total T4 result from earlier. Here’s what it actually is in nmol/L:

7.2 µg/dL × 12.87 = 92.7 nmol/L

That’s close enough to the 93 nmol/L from the second test to call it a match, well within the kind of variation you’d expect from two different lab machines on two different days. Nothing dramatic happened.

The units were just telling the same story in two different languages. The same logic applies if you’re comparing a Free T4 result of, say, 1.2 ng/dL against an overseas report reading 15.4 pmol/L (1.2 × 12.87 = 15.44) — again, a match.

Before You Convert: Check Which T4 Test You’re Actually Holding

This is the mistake that trips people up more than the math itself. Thyroid panels report several different things that all sound similar: Total T4, Free T4, Total T3, Free T3. The good news is that Total T4 and Free T4 share the same 12.87 conversion factor, so if you mix them up between those two specific tests, the number itself still comes out right — you’ll just be reading it against the wrong reference range, since Total T4 and Free T4 have very different normal ranges (see the tables below).

Where you do need to stop is if your report says T3 anywhere — Total T3 and Free T3 use a completely different molecular weight and a different factor entirely, and running a T3 number through this calculator will give you a confidently wrong answer. Check for the word “T4” specifically before converting.

Free T4: The Other Common Thyroxine Test

Total T4 measures every molecule of thyroxine in your blood, including the roughly 99.97% that’s bound to carrier proteins and biologically inactive at that moment. Free T4 measures only the small unbound fraction that’s actually available for your cells to use, which is why doctors often consider it the more clinically meaningful number, especially when binding-protein levels are affected by pregnancy, liver disease, or certain medications.

In the US, Free T4 is usually reported in ng/dL; most other countries report it in pmol/L. Despite measuring a different fraction of the hormone, Free T4 is chemically still thyroxine, so it shares Total T4’s molecular weight and, therefore, the same 12.87 conversion factor — a point confirmed directly by Mayo Clinic Laboratories’ SI conversion reference and echoed in published clinical chemistry literature.

A typical Free T4 result:

1.4 ng/dL × 12.87 = 18.0 pmol/L

Total T4 vs Free T4: What’s Actually Different

Quick Reference: Common Total T4 Values (µg/dL ↔ nmol/L)

Quick Reference: Common Free T4 Values (ng/dL ↔ pmol/L)

Typical Adult Reference Ranges

Reference ranges vary by lab, age, assay method, and pregnancy status, so treat the figures below as orientation rather than a diagnosis. Always compare against the range printed on your own report.

Source: Mayo Clinic Laboratories reference intervals; other labs commonly cite a slightly wider Free T4 range of about 0.8–1.8 ng/dL (≈10–25 pmol/L) depending on assay.

Why the US Still Uses µg/dL and ng/dL

Most US labs report T4 in µg/dL (Total) and ng/dL (Free) because that’s what clinical chemistry has used domestically for decades, and switching would mean re-anchoring reference ranges and provider habits across an enormous number of labs. Most of the rest of the world moved to the SI molar units — nmol/L and pmol/L — because they’re consistent across every hormone and compound in the periodic table, not just thyroid tests.

Both systems describe the exact same thyroxine molecule. They’re just counting it two different ways.

En Español: Cómo Convertir T4 Total y T4 Libre

The Short Version

If your T4 numbers look wildly different between two reports, check the units before you worry about the trend. Multiply µg/dL (Total T4) or ng/dL (Free T4) by 12.87 to get nmol/L or pmol/L, or divide the molar value by 12.87 to get back to the mass unit — the factor is identical for both tests. Just make sure your report actually says “T4” and not “T3,” since T3 uses a different molecular weight and a different factor altogether.

Want to skip the mental math? The Total T4 & Free T4 Unit Converter above handles both directions and both test types instantly, so you can compare two reports side by side without a calculator.