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.
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. This marker is part of the Healthspan Assessment panel available to my 1-on-1 patients.
Why T3 matters
- Metabolism and energy
- Heart rate and body temperature
- Brain function and mood
- 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, and in my practice they are available only to patients working 1-on-1 with me.
What low T3 can mean
- Hypothyroidism
- Serious illness, fasting or very low-calorie diets
- Low selenium, zinc or iron
- Some medications
What high T3 can mean
- Hyperthyroidism, including Graves’ disease
- Taking too much thyroid medication
- Pregnancy or estrogen (raises total T3)
What affects T3
- T4-to-T3 conversion
- Calorie and protein intake
- Stress and illness
- Binding proteins changed by estrogen
Ways to support healthy levels
- 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
- Take thyroid medication as prescribed and retest as your doctor advises
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
- Fatigue, weight change, feeling cold or hot, hair loss or constipation
- Anxiety, palpitations or tremor
- Irregular periods or trouble conceiving
- A family history of thyroid disease
- 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.
Want help making sense of your labs?
A free 15-minute consultation with Dr. Kneessi. No cost, no obligation. In-office in Odenton, MD or by telehealth.
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://pmc.ncbi.nlm.nih.gov/articles/PMC6596318/
- Van den Berghe G. Non-thyroidal illness in the ICU: a syndrome with different faces. Thyroid 2014. PMID: 24845024. https://pmc.ncbi.nlm.nih.gov/articles/PMC4195234/
- Ventura M, Melo M, Carrilho F. Selenium and thyroid disease: from pathophysiology to treatment. Int J Endocrinol 2017. PMID: 28255299. https://pmc.ncbi.nlm.nih.gov/articles/PMC5307254/
- Leung AM, Braverman LE. Consequences of excess iodine. Nat Rev Endocrinol 2014. PMID: 24342882. https://pmc.ncbi.nlm.nih.gov/articles/PMC3976240/
- 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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