# Free Triiodothyronine (FT3)

> FT3 is the active thyroid hormone available to your cells. Your body makes some T3 directly but also converts T4→T3 inside tissues. FT3 is the active form 

*Source: [https://www.health3.app/biomarkers/ft3](https://www.health3.app/biomarkers/ft3)*

### On This Page

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

## What is Free Triiodothyronine (FT3)?

FT3 is the **active** thyroid hormone available to the body's cells. The body makes some T3 directly but also converts T4→T3 inside tissues. FT3 is the active form of thyroid hormones. FT3 influences energy use, heart rhythm, temperature, mood, and thinking. In serious non‑thyroid illness, FT3 often falls (""low T3 syndrome"") even if the thyroid gland is not the problem.

**Low FT3**

Low FT3 with high TSH and low FT4 is a pattern that a clinician may associate with hypothyroidism. Low FT3 with normal/low TSH and near‑normal FT4 in severe illness usually reflects **non‑thyroidal illness**. Low FT3 can be linked with fatigue, slowed thinking, and low mood (especially when overall thyroid function is low).

**High FT3**

High FT3 with low TSH is a pattern a clinician may associate with hyperthyroidism, which can involve palpitations, heat intolerance, tremor, and anxiety.

**Factors that support healthy FT3 levels**

- **Adequate iodine** and **iron** support the making of thyroid hormones.[Zimmermann, 2009][Zimmermann & Kohrle, 2002]
- **Selenium** intake supports the enzymes that convert T4→T3 in tissues.[Zimmermann & Kohrle, 2002]
- **Biotin** use is worth reporting to a clinician; it can **falsely raise FT3** on some tests.[Zhang, 2020][Ylli, 2021]
- **Amiodarone** can block T4→T3 conversion (FT3 may fall while FT4 is high); monitoring is advised.[Harjai, 1997]

## Measurement Units

Free Triiodothyronine (FT3) can be measured in: ng/dL, ng/L, pg/100mL, pg/dL, pg/mL, pg%, pmol/L

## 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 | pg/dL | - | 210 - 440 | 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]
- **Total Thyroxine (TT4)** (Coming Soon)

 TT4 must be interpreted with FT3 for a full picture of thyroid status.[Garber, 2012][Koulouri, 2013]
- [**Free Thyroxine (FT4)**](https://www.health3.app/biomarkers/ft4)

 Read together to tell if the thyroid is under‑ or over‑active.[Garber, 2012]
- [**Vitamin B7 (Biotin)**](https://www.health3.app/biomarkers/vitaminb7b)

 May cause **falsely high FT3** on some tests.[Zhang, 2020][Ylli, 2021]
- **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. de Nayer P. Sex hormone-binding protein in hyperthyroxinemic patients (1986). *J Clin Endocrinol Metab*. [View Source](https://pubmed.ncbi.nlm.nih.gov/3084540/)
2. Garber JR. Clinical practice guidelines for hypothyroidism in adults (2012). *Endocr Pract*. [View Source](https://pubmed.ncbi.nlm.nih.gov/23246686/)
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. Ross DS. 2016 American Thyroid Association guidelines (2016). *Thyroid*. [View Source](https://pubmed.ncbi.nlm.nih.gov/27521067/)
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. Hershman JM. Thyroid Function Tests (2023). *Clinical Resource*.
9. 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/)
10. Zhang Y. Assessment of biotin interference in thyroid function tests (2020). *Medicine (Baltimore)*. [View Source](https://pubmed.ncbi.nlm.nih.gov/32118725/)
11. 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/)
12. Ford HC. Serum levels of free and bound testosterone in hyperthyroidism (1992). *Clin Endocrinol (Oxf)*. [View Source](https://pubmed.ncbi.nlm.nih.gov/1568351/)
13. Ylli D. Biotin Interference in Assays for Thyroid Hormones, Thyrotropin and Thyroglobulin (2021). *Thyroid*. [View Source](https://pubmed.ncbi.nlm.nih.gov/34042535/)
14. Green ME and Bernet VJ. Thyroid dysfunction and sleep disorders (2021). *Front Endocrinol (Lausanne)*. [View Source](https://pubmed.ncbi.nlm.nih.gov/34504473/)
15. Ross DS. 2016 American Thyroid Association guidelines (2016). *Thyroid*. [View Source](https://pubmed.ncbi.nlm.nih.gov/27521067/)
16. Koulouri O. How to interpret thyroid function tests (binding effects) (2013). *Clin Med (Lond)*. [View Source](https://pubmed.ncbi.nlm.nih.gov/23760704/)
17. 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/)
18. 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/)
19. Zhang Y. Assessment of biotin interference in thyroid function tests (2020). *Medicine (Baltimore)*. [View Source](https://pubmed.ncbi.nlm.nih.gov/32118725/)
20. 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/)
21. Ylli D. Biotin Interference in Assays for Thyroid Hormones, Thyrotropin and Thyroglobulin (2021). *Thyroid*. [View Source](https://pubmed.ncbi.nlm.nih.gov/34042535/)
22. Harjai KJ and Licata AA. Effects of amiodarone on thyroid (1997). *Ann Intern Med*. [View Source](https://pubmed.ncbi.nlm.nih.gov/8992925/)

### ⚠️ 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.