Understanding pmol/L and pg/mL
Thyroid lab results can be confusing when one report shows Free T3 in pmol/L and another uses pg/mL. Both units measure how much Free T3 is in the bloodstream, but they describe it differently.
pmol/L, or picomoles per liter, is an SI unit that counts molecules per volume. It is common in Europe, Canada, Australia, and many international lab reports.
pg/mL, or picograms per milliliter, is a mass-based unit common in the United States and many conventional thyroid panels.
Since one unit counts molecules and the other measures mass, conversion requires the molecular weight of Free T3. If your report shows Total T3 in ng/dL or nmol/L, use the Total T3 Unit Conversion Calculator instead.
How to Convert Free T3 from pmol/L to pg/mL
Free T3 conversion uses the molecular weight of triiodothyronine: 650.98 g/mol. Once that value is known, the math is straightforward.
pg/mL = pmol/L x 650.98 / 1000 pmol/L = pg/mL x 1000 / 650.98 A useful quick factor is 0.651. Multiply a Free T3 value in pmol/L by about 0.651 to estimate pg/mL.
Step-by-Step Example
If your Free T3 result is 5.0 pmol/L, multiply 5.0 by 650.98, then divide by 1000.
5.0 x 650.98 / 1000 = 3.25 pg/mL Going the other direction, 3.25 pg/mL x 1000 / 650.98 equals about 5.0 pmol/L.
pmol/L vs pg/mL: Key Differences
| Feature | pmol/L | pg/mL |
|---|---|---|
| Unit type | SI unit | Conventional unit |
| Measures | Molecular count | Mass |
| Common in | Europe, Canada, Australia | United States |
| Typical adult Free T3 range | 3.1-6.8 pmol/L | 2.0-4.4 pg/mL |
| Conversion factor | x 0.651 to pg/mL | x 1.536 to pmol/L |
Neither unit is better. They are two ways to describe the same Free T3 concentration.
When to Use Free T3 Unit Conversion
Unit conversion is useful when comparing thyroid panels from different countries, reviewing old and new lab reports after a lab vendor change, reading international research, or preparing for telemedicine and second-opinion consultations.
It is also helpful for students and clinicians learning how molar and mass-based thyroid hormone units relate to each other.
Free T3 Conversion Table
Use this quick-reference chart for typical Free T3 measurements. Values use molecular weight 650.98 g/mol.
| pmol/L | pg/mL | Clinical interpretation |
|---|---|---|
| 2.0 | 1.30 | Below typical range |
| 3.0 | 1.95 | Low-normal border |
| 3.5 | 2.28 | Low-normal |
| 4.0 | 2.60 | Normal |
| 4.5 | 2.93 | Normal |
| 5.0 | 3.25 | Normal |
| 5.5 | 3.58 | Normal |
| 6.0 | 3.91 | Normal |
| 6.5 | 4.23 | High-normal |
| 7.0 | 4.56 | Above typical range |
| 8.0 | 5.21 | Elevated |
| 10.0 | 6.51 | Significantly elevated |
| 12.0 | 7.81 | Hyperthyroid range |
| 15.0 | 9.76 | Severely elevated |
Reference ranges vary by lab, assay method, age, pregnancy status, and clinical context. Always compare your result with the range on your own report.
Common Thyroid Hormones and Molecular Weights
The calculator includes Free T3 and related hormones. Each substance needs the correct molecular weight for accurate conversion.
| Substance | Molecular weight | Primary clinical use |
|---|---|---|
| Free T3 (Triiodothyronine) | 650.98 g/mol | Thyroid function |
| Free T4 (Thyroxine) | 776.87 g/mol | Thyroid function |
| Total T3 | 650.98 g/mol | Thyroid function |
| Total T4 | 776.87 g/mol | Thyroid function |
| Reverse T3 (rT3) | 650.98 g/mol | Non-thyroidal illness context |
| Estradiol | 272.38 g/mol | Reproductive health |
| Testosterone | 288.42 g/mol | Hormone assessment |
| Progesterone | 314.46 g/mol | Reproductive health |
| Cortisol | 362.46 g/mol | Adrenal function |
| 25-OH Vitamin D | 400.64 g/mol | Vitamin D status |
| Aldosterone | 360.44 g/mol | Electrolyte balance |
| Melatonin | 232.28 g/mol | Sleep and circadian rhythm |
Practical Examples in Thyroid Medicine
Free T3 in Hyperthyroidism
A Free T3 result of 11.5 pmol/L converts to 7.49 pg/mL. That is above many typical adult reference ranges and may be clinically significant when paired with symptoms and other thyroid tests such as TSH and Free T4.
Comparing Thyroid Reports Across Labs
If one lab reports Free T3 in pmol/L and another reports pg/mL, converting both into one unit helps you compare trends without mistaking unit changes for biological changes. For broader thyroid and sex hormone unit pairs, the Hormone Unit Conversion Calculator can handle multiple analytes from one page.
Tips for Accurate Free T3 Unit Conversion
- Use the correct molecular weight: Free T3 is 650.98 g/mol.
- Match the reference range to the unit: 3.5 pg/mL and 3.5 pmol/L do not mean the same thing.
- Check decimals carefully: a misplaced decimal can change interpretation.
- Do not mix free and total hormone interpretation: they share molecular weight but measure different fractions.
- Use clinical context: medication, illness, timing, and assay method all matter.
Frequently Asked Questions
What is the conversion factor for Free T3 from pmol/L to pg/mL?
The factor is approximately 0.651. Multiply pmol/L by 0.651 to estimate pg/mL.
Why do labs report Free T3 in different units?
Regional convention is the main reason. Many US labs use pg/mL, while many international labs use SI units such as pmol/L.
Is Free T3 the same molecular weight as Total T3?
Yes. Both refer to triiodothyronine with molecular weight 650.98 g/mol. The difference is whether the test measures free hormone or total hormone.
What is a normal Free T3 level?
Typical adult ranges are about 3.1-6.8 pmol/L or 2.0-4.4 pg/mL, but your lab's own reference range is the one to use.
Can I use the formula for other hormones?
Yes, but only if you use the correct molecular weight for that hormone. The calculator includes several related hormones for this reason.
Conclusion
Converting Free T3 between pmol/L and pg/mL is simple when you know the molecular weight. Use the calculator above for instant results, then compare the converted value with the reference range printed on your lab report.
Accurate unit conversion helps bridge international lab standards and everyday clinical practice. If your thyroid panel also includes TSH, remember that the TSH Unit Conversion Calculator uses a direct 1:1 relationship instead of a molecular-weight formula. Interpretation should always come from a qualified healthcare provider.