Quick answer: T3 (triiodothyronine) is the active thyroid hormone that drives metabolism, energy, heart rate and body temperature. Most T3 is made from T4 in tissues such as the liver. Total T3 includes bound and free hormone. High T3 is most useful for evaluating hyperthyroidism, while low T3 can reflect hypothyroidism, illness, calorie restriction or poor conversion.
Key takeaways
T3 is the active thyroid hormone that drives metabolism, energy, heart rate and body temperature.
Most T3 is made from T4 in tissues such as the liver, and total T3 includes both bound and free hormone.
High T3 helps evaluate an overactive thyroid, while low T3 can reflect illness, calorie restriction or poor conversion.
What is T3?
T3 is the most active thyroid hormone. The thyroid makes some T3 directly, but most is converted from T4 in the liver, gut, muscle and other tissues.
Why T3 matters
T3 matters for metabolism and energy, heart rate and body temperature, brain function and mood, and digestion and gut motility.
Understanding your result
Reference ranges vary by lab, age and sex. Your provider interprets your result against the lab’s range and your overall picture, rather than chasing a single number. Functional labs are optional. They can be helpful in more complex cases.
What low T3 can mean
A low T3 can mean hypothyroidism, or serious illness, fasting or very low-calorie diets. It can also reflect low selenium, zinc or iron, or some medications.
What high T3 can mean
A high T3 can mean hyperthyroidism, including Graves’ disease, or taking too much thyroid medication. Pregnancy or estrogen can also raise total T3.
What affects T3
T3 is affected by T4-to-T3 conversion, calorie and protein intake, stress and illness, and binding proteins changed by estrogen.
Ways to support healthy levels
Eat enough protein and calories, since very low-calorie dieting can slow thyroid hormone activation. Get iodine from food such as seafood, dairy and iodized salt, but avoid high-dose iodine supplements unless prescribed.
Selenium (Brazil nuts, seafood), zinc and iron support thyroid hormone production and conversion. Manage stress and sleep, and take thyroid medication as prescribed and retest as your doctor advises.
What does the research show about T3?
In a study of 573 consecutive heart patients followed for one year, the 173 patients with low free T3 had a death rate of 14.4%, compared with 3% in those with normal levels, and free T3 was the strongest predictor of death (hazard ratio 3.58). Low T3 in this setting may mark how sick someone is rather than cause the outcome. (Iervasi et al., 2003)
Researchers studied blood and tissue samples from 80 intensive care patients. Compared with healthy people, these patients had lower T3 and a lower T3-to-reverse-T3 ratio, along with lower activity of the liver enzyme that activates thyroid hormone and higher activity of the enzyme that inactivates it. Because the samples came from people who had died in intensive care, the findings describe severe illness, not everyday life. (Peeters et al., 2003)
In 8 healthy men who fasted for 30 hours, average T3 fell from 2.30 to 1.84 nmol/L and reverse T3 rose, and an 800-calorie meal did not restore T3 that evening. The study was very small and short, but it shows that T3 can shift within a day or two of reduced eating. (Hugues et al., 1984)
These findings help explain why a low T3 result is interpreted in light of recent illness, calorie intake and other thyroid tests.
T3 and healthy aging
Thyroid hormones set the pace of metabolism in nearly every cell and directly influence mitochondria, the energy factories of the cell. Thyroid function and autoimmunity both change with age, so keeping an eye on thyroid health supports energy, heart health and healthy weight over time.
Related thyroid tests: TSH, free T4, free T3, reverse T3, TPO antibodies, thyroglobulin antibodies.
When to see a naturopathic doctor
It may be time to see a naturopathic doctor if you have fatigue, weight change, feeling cold or hot, hair loss or constipation, or anxiety, palpitations or tremor. The same is true if you have irregular periods or trouble conceiving, a family history of thyroid disease, or abnormal thyroid results you want explained.
See a medical provider promptly for a racing or irregular heartbeat, severe confusion or drowsiness, a rapidly growing neck lump, or trouble swallowing or breathing.
T3 at a glance
| Topic | What the article explains |
|---|---|
| Why it matters | Metabolism and energy; heart rate and body temperature; brain function and mood; and digestion and gut motility |
| What can affect it | T4-to-T3 conversion; calorie and protein intake; stress and illness; and binding proteins changed by estrogen |
| Supportive habits discussed | Eat enough protein and calories; very low-calorie dieting can slow thyroid hormone activation; get iodine from food such as seafood, dairy and iodized salt, but avoid high-dose iodine supplements unless prescribed; selenium (Brazil nuts, seafood), zinc and iron support thyroid hormone production and conversion; manage stress and sleep; and take thyroid medication as prescribed and retest as your doctor advises |
Questions to bring to your clinician
These questions can help you get the most out of your next visit.
Could calorie restriction or a recent illness explain my T3 result?
How should I read my T3 alongside the rest of my labs and my symptoms?
How often would it make sense to recheck my T3 in my situation?
Which daily habits or medications could be influencing my T3 before we retest?
Frequently asked questions
What is the difference between T3 and free T3?
Total T3 includes bound and unbound hormone; free T3 is the unbound, active portion.
What does low T3 mean?
It can reflect hypothyroidism, illness, under-eating or poor conversion from T4.
What does high T3 mean?
It often points to hyperthyroidism or too much thyroid medication.
Should T3 be tested on its own?
No. It is interpreted with TSH and free T4.
Sources
- Bianco AC, et al. Paradigms of Dynamic Control of Thyroid Hormone Signaling. Endocr Rev 2019. PMID: 31033998. https://pubmed.ncbi.nlm.nih.gov/31033998/
- Hugues JN, et al. Rapid adaptations of serum thyrotrophin, triiodothyronine and reverse triiodothyronine levels to short-term starvation and refeeding. Acta Endocrinol (Copenh) 1984. PMID: 6695550. https://pubmed.ncbi.nlm.nih.gov/6695550/
- Iervasi G, et al. Low-T3 syndrome: a strong prognostic predictor of death in patients with heart disease. Circulation 2003. PMID: 12578873. https://pubmed.ncbi.nlm.nih.gov/12578873/
- Leung AM, et al. Consequences of excess iodine. Nat Rev Endocrinol 2014. PMID: 24342882. https://pubmed.ncbi.nlm.nih.gov/24342882/
- Peeters RP, et al. Reduced activation and increased inactivation of thyroid hormone in tissues of critically ill patients. J Clin Endocrinol Metab 2003. PMID: 12843166. https://pubmed.ncbi.nlm.nih.gov/12843166/
- Van den Berghe G. Non-thyroidal illness in the ICU: a syndrome with different faces. Thyroid 2014. PMID: 24845024. https://pubmed.ncbi.nlm.nih.gov/24845024/
- Ventura M, et al. Selenium and Thyroid Disease: From Pathophysiology to Treatment. Int J Endocrinol 2017. PMID: 28255299. https://pubmed.ncbi.nlm.nih.gov/28255299/
- MedlinePlus: Triiodothyronine (T3) Tests
- MedlinePlus: Thyroid Tests
This article is for educational purposes only and is not medical advice. It does not replace an individual evaluation. Talk with your healthcare provider before starting supplements or changing medications, especially if you are pregnant, nursing, or managing a health condition.
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