Free Triiodothyronine (FT3) icon

Free Triiodothyronine (FT3)

Content Source: Health3 AG
Data Compiled From: Clinical laboratory standards & peer-reviewed research
Quality Assurance: Verified against multiple authoritative sources
Academic Sources: de, 1986; Garber, 2012; Bauer, 2008; Rifai, 2023; Ross, 2016; Mullur, 2014; Zimmermann, 2009; Hershman, 2023; Bassett, 2016; Zhang, 2020; Zimmermann, 2002; Ford, 1992; Ylli, 2021; Green, 2021; Ross, 2016; Koulouri, 2013; Zimmermann, 2002; Dumoulin, 1995; Zhang, 2020; Kjaergaard, 2021; Ylli, 2021; Harjai, 1997
Last Verified: 2025-12-02
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What is 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 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 supports 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 suggests hyperthyroidism and may cause palpitations, heat intolerance, tremor, and anxiety.

To maintain healthy FT3 levels

  • Ensure adequate iodine and iron (for making thyroid hormones).[Zimmermann, 2009][Zimmermann & Kohrle, 2002]
  • Ensure selenium intake to support the enzymes that convert T4→T3 in tissues.[Zimmermann & Kohrle, 2002]
  • Report biotin use; 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. Your healthcare provider must interpret your specific results.

Age Range Gender Unit Optimal Normal Source
18 - 87 All genders pg/dL - 210 - 440 Rifai, 2023

Health Impact

Muscle Function​

FT3 is the most active thyroid hormone in muscle cells. Too little relates to low endurance and strength, while too much causes tremor and weakness in hyperthyroid states.[Mullur, 2014][Ross, 2016]

Mood Regulation​

As the active hormone in the brain, low FT3 can contribute to low mood and slowed thinking. High FT3 leads to nervousness and agitation—returning FT3 to normal typically helps.[Bauer, 2008][Ross, 2016]

Sleep Quality​

High FT3 often aligns with racing thoughts and trouble sleeping. Low FT3 can cause daytime sleepiness and low alertness when overall thyroid function is low.[Ross, 2016][Green, 2021]

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Academic References

  1. de Nayer P. Sex hormone-binding protein in hyperthyroxinemic patients (1986). J Clin Endocrinol Metab. View Source
  2. Garber JR. Clinical practice guidelines for hypothyroidism in adults (2012). Endocr Pract. View Source
  3. Bauer M. The thyroid-brain interaction in thyroid disorders and mood disorders (2008). J Neuroendocrinol. View Source
  4. Rifai N.. Tietz Textbook of Laboratory Medicine (2023). Elsevier.
  5. Ross DS. 2016 American Thyroid Association guidelines (2016). Thyroid. View Source
  6. Mullur R, Liu YY, and Brent GA. Thyroid hormone regulation of metabolism (2014). Physiol Rev. View Source
  7. Zimmermann MB. Iodine deficiency (2009). Endocr Rev. View Source
  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
  10. Zhang Y. Assessment of biotin interference in thyroid function tests (2020). Medicine (Baltimore). View Source
  11. Zimmermann MB and Köhrle J. The impact of iron and selenium deficiencies on iodine and thyroid metabolism (2002). Thyroid. View Source
  12. Ford HC. Serum levels of free and bound testosterone in hyperthyroidism (1992). Clin Endocrinol (Oxf). View Source
  13. Ylli D. Biotin Interference in Assays for Thyroid Hormones, Thyrotropin and Thyroglobulin (2021). Thyroid. View Source
  14. Green ME and Bernet VJ. Thyroid dysfunction and sleep disorders (2021). Front Endocrinol (Lausanne). View Source
  15. Ross DS. 2016 American Thyroid Association guidelines (2016). Thyroid. View Source
  16. Koulouri O. How to interpret thyroid function tests (binding effects) (2013). Clin Med (Lond). View Source
  17. Zimmermann MB and Köhrle J. The impact of iron and selenium deficiencies on iodine and thyroid metabolism (2002). Thyroid. View Source
  18. Dumoulin SC. Opposite effects of thyroid hormones on binding proteins for steroid hormones (1995). Eur J Endocrinol. View Source
  19. Zhang Y. Assessment of biotin interference in thyroid function tests (2020). Medicine (Baltimore). View Source
  20. Kjaergaard AD. Thyroid function, sex hormones and sexual function: a Mendelian randomization study (2021). Eur J Epidemiol. View Source
  21. Ylli D. Biotin Interference in Assays for Thyroid Hormones, Thyrotropin and Thyroglobulin (2021). Thyroid. View Source
  22. Harjai KJ and Licata AA. Effects of amiodarone on thyroid (1997). Ann Intern Med. View Source

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