# Free Thyroxine (FT4)

> FT4 (free thyroxine) measures the unbound fraction of T4. Reference range, units (ng/dL, pmol/L, pg/mL), pg/mL to pmol/L conversion, and how the test works.

*Source: [https://www.health3.app/biomarkers/ft4](https://www.health3.app/biomarkers/ft4)*

### On This Page

- What it measures
- Measurement units
- Reference ranges
- Health impact
- Related biomarkers
- Scientific references

## What is Free Thyroxine (FT4)?

FT4 is the unbound (free) form of thyroxine in the blood. Because it is free, it can enter cells and be used, and is converted to T3 (the most active thyroid hormone). FT4 helps set the body''s ""energy speed,"" affecting heart rate, temperature, metabolism, mood, and thinking. FT4 and TSH move in opposite directions in most thyroid problems (high FT4 → low TSH; low FT4 → high TSH).

**Low FT4**

Low FT4 with high TSH is a pattern a clinician may associate with hypothyroidism; with low/normal TSH it may point to a pituitary/hypothalamic issue (central hypothyroidism). Symptoms include fatigue, cold intolerance, weight gain, constipation, and mental fatigue.

**High FT4**

High FT4 with low TSH is a pattern a clinician may associate with hyperthyroidism, which often brings fast heartbeat, shakiness, heat intolerance, and weight loss.

**Factors that support healthy FT4 levels**

- **Adequate iodine** intake is supportive.[Zimmermann, 2009]
- Healthy **iron** and **selenium** status supports thyroid hormone production and conversion.[Zimmermann & Kohrle, 2002][Bianco, 2006]
- **Biotin** is worth mentioning to a clinician before testing (it can make FT4 look falsely high on some assays).[Zhang, 2020][Ylli, 2021]
- Some drugs (e.g., **amiodarone**) alter T4↔T3 conversion and can raise FT4 while lowering T3; monitoring is advised.[Harjai, 1997][Lazarus, 2009]

## Measurement Units

Free Thyroxine (FT4) can be measured in: ng/100mL, ng/dL, ng/L, ng/mL, ng%, pg/mL, pmol/L

US labs typically report FT4 in ng/dL, while UK and most European labs use pmol/L (some report pg/mL). The conversion factors: 1 ng/dL equals 12.87 pmol/L, and 1 pg/mL equals 1.287 pmol/L (derived from thyroxine's molecular weight of 776.87 g/mol).

| ng/dL | pg/mL | pmol/L |
| --- | --- | --- |
| 0.8 | 8 | 10.3 |
| 1.0 | 10 | 12.9 |
| 1.2 | 12 | 15.4 |
| 1.5 | 15 | 19.3 |
| 1.8 | 18 | 23.2 |

## Reference Ranges by Age and Gender

Reference ranges represent typical values for healthy individuals. A healthcare provider must interpret individual results.

| Age Range | Gender | Unit | Optimal | Normal | Source |
| --- | --- | --- | --- | --- | --- |
| 18 - 87 | All genders | ng/dL | - | 0.8 - 2.7 | Rifai, 2023 |

## Related Biomarkers

- [**Thyroid-Stimulating Hormone (TSH)**](https://www.health3.app/biomarkers/tsh)

 Higher FT4/FT3 lower TSH via feedback; lower FT4/FT3 raise TSH.[Hershman, 2023]
- [**Vitamin B7 (Biotin)**](https://www.health3.app/biomarkers/vitaminb7b)

 Can falsely **increase FT4** results in some tests.[Zhang, 2020][Ylli, 2021]
- **Total Thyroxine (TT4)** (Coming Soon)

 TT4 must be interpreted with FT4 for a full picture of thyroid status.[Garber, 2012][Koulouri, 2013]
- [**Free Triiodothyronine (FT3)**](https://www.health3.app/biomarkers/ft3)

 Read together to tell if the thyroid is under‑ or over‑active.[Garber, 2012]
- **Total Triiodothyronine (TT3)** (Coming Soon)

 Must be interpreted together for a full picture of thyroid status.[Garber, 2012]

## Related tools

- [**Thyroid Panel Reference Tool**](https://www.health3.app/tools/thyroid-panel-interpreter)
- [**Blood Test Unit Converter**](https://www.health3.app/tools/blood-test-unit-converter)

## Academic References

1. Garber JR. Clinical practice guidelines for hypothyroidism in adults (2012). *Endocr Pract*. [View Source](https://pubmed.ncbi.nlm.nih.gov/23246686/)
2. Hage MP and Azar ST. The link between thyroid function and depression (2012). *J Thyroid Res*. [View Source](https://pubmed.ncbi.nlm.nih.gov/22220285/)
3. Bauer M. The thyroid-brain interaction in thyroid disorders and mood disorders (2008). *J Neuroendocrinol*. [View Source](https://pubmed.ncbi.nlm.nih.gov/18673409/)
4. Rifai N.. Tietz Textbook of Laboratory Medicine (2023). *Elsevier*.
5. Duyff RF. Neuromuscular findings in thyroid dysfunction (2000). *J Neurol Neurosurg Psychiatry*. [View Source](https://pubmed.ncbi.nlm.nih.gov/10811699/)
6. Mullur R, Liu YY, and Brent GA. Thyroid hormone regulation of metabolism (2014). *Physiol Rev*. [View Source](https://pubmed.ncbi.nlm.nih.gov/24692351/)
7. Zimmermann MB. Iodine deficiency (2009). *Endocr Rev*. [View Source](https://pubmed.ncbi.nlm.nih.gov/19460960/)
8. Green ME and Bernet VJ. Thyroid dysfunction and sleep disorders (2021). *Front Endocrinol (Lausanne)*. [View Source](https://pubmed.ncbi.nlm.nih.gov/34504473/)
9. Hershman JM. Thyroid Function Tests (2023). *Clinical Resource*.
10. Bassett JHD and Williams GR. Role of thyroid hormones in skeletal development and bone maintenance (2016). *Endocr Rev*. [View Source](https://pubmed.ncbi.nlm.nih.gov/26862888/)
11. Zhang Y. Assessment of biotin interference in thyroid function tests (2020). *Medicine (Baltimore)*. [View Source](https://pubmed.ncbi.nlm.nih.gov/32118725/)
12. Zimmermann MB and Köhrle J. The impact of iron and selenium deficiencies on iodine and thyroid metabolism (2002). *Thyroid*. [View Source](https://pubmed.ncbi.nlm.nih.gov/12487769/)
13. de Nayer P. Sex hormone-binding protein in hyperthyroxinemic patients (1986). *J Clin Endocrinol Metab*. [View Source](https://pubmed.ncbi.nlm.nih.gov/3084540/)
14. Koulouri O. How to interpret thyroid function tests (binding effects) (2013). *Clin Med (Lond)*. [View Source](https://pubmed.ncbi.nlm.nih.gov/23760704/)
15. Samuels MH. Psychiatric and cognitive manifestations of hypothyroidism (2014). *Curr Opin Endocrinol Diabetes Obes*. [View Source](https://pubmed.ncbi.nlm.nih.gov/25122491/)
16. Ford HC. Serum levels of free and bound testosterone in hyperthyroidism (1992). *Clin Endocrinol (Oxf)*. [View Source](https://pubmed.ncbi.nlm.nih.gov/1568351/)
17. Ylli D. Biotin Interference in Assays for Thyroid Hormones, Thyrotropin and Thyroglobulin (2021). *Thyroid*. [View Source](https://pubmed.ncbi.nlm.nih.gov/34042535/)
18. Ross DS. 2016 American Thyroid Association guidelines (2016). *Thyroid*. [View Source](https://pubmed.ncbi.nlm.nih.gov/27521067/)
19. Zimmermann MB and Köhrle J. The impact of iron and selenium deficiencies on iodine and thyroid metabolism (2002). *Thyroid*. [View Source](https://pubmed.ncbi.nlm.nih.gov/12487769/)
20. Dumoulin SC. Opposite effects of thyroid hormones on binding proteins for steroid hormones (1995). *Eur J Endocrinol*. [View Source](https://pubmed.ncbi.nlm.nih.gov/7749500/)
21. Bianco AC. Deiodinases and thyroid hormone action (2006). *Thyroid*.
22. Zhang Y. Assessment of biotin interference in thyroid function tests (2020). *Medicine (Baltimore)*. [View Source](https://pubmed.ncbi.nlm.nih.gov/32118725/)
23. Kjaergaard AD. Thyroid function, sex hormones and sexual function: a Mendelian randomization study (2021). *Eur J Epidemiol*. [View Source](https://pubmed.ncbi.nlm.nih.gov/33548002/)
24. Ylli D. Biotin Interference in Assays for Thyroid Hormones, Thyrotropin and Thyroglobulin (2021). *Thyroid*. [View Source](https://pubmed.ncbi.nlm.nih.gov/34042535/)
25. Harjai KJ and Licata AA. Effects of amiodarone on thyroid (1997). *Ann Intern Med*. [View Source](https://pubmed.ncbi.nlm.nih.gov/8992925/)
26. Lazarus JH. Lithium and thyroid: clinical aspects (2009). *Best Pract Res Clin Endocrinol Metab*. [View Source](https://pubmed.ncbi.nlm.nih.gov/19942149/)

### ⚠️ Important Medical Information

This reference page is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment.

Reference ranges vary between laboratories. Always review your lab results with a qualified healthcare provider.