Quick answer: T4 (thyroxine) is the main hormone made by the thyroid gland. It acts as a storage form that tissues convert into active T3. Total T4 includes hormone bound to proteins and the small free portion. Low T4 can signal hypothyroidism and high T4 hyperthyroidism, but free T4 and TSH usually give the clearest picture.
Key takeaways
T4, or thyroxine, is the main hormone the thyroid releases, and tissues convert it into active T3.
Total T4 includes hormone bound to proteins as well as the small free fraction.
Because pregnancy, estrogen and binding proteins can shift total T4, free T4 and TSH usually give a clearer picture.
What is T4?
T4 is the primary hormone released by the thyroid. Most circulates bound to proteins; a small free portion enters cells and is converted to T3.
Why T4 matters
T4 provides a steady supply for T3 production and supports metabolism and energy. It also matters for growth and development, and for heart, brain and gut function.
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 T4 can mean
A low T4 can mean hypothyroidism, including Hashimoto’s, or pituitary problems. It can also reflect low binding proteins (liver disease, some medications).
What high T4 can mean
A high T4 can mean hyperthyroidism, including Graves’ disease, or too much thyroid medication. Pregnancy or estrogen can also raise it, because they raise binding proteins.
What affects T4
T4 is affected by binding proteins, iodine intake, medications and biotin supplements (can interfere with tests), and pregnancy.
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 T4?
The NHANES III survey measured TSH and total T4 in a nationally representative sample of 17,353 Americans aged 12 and older. Average total T4 in people without thyroid disease was 112.3 nmol/L, and it was slightly higher in Mexican Americans (116.3 nmol/L) than in white (110.0 nmol/L) or Black participants (109.4 nmol/L). The study was a single snapshot, so it describes typical values rather than what different T4 levels mean for health over time. (Hollowell et al., 2002)
A pooled analysis of 11 cohorts with 30,085 participants found that, among people with normal thyroid function, those with free T4 in the highest quarter had a 45% higher risk of developing atrial fibrillation than those in the lowest quarter (hazard ratio 1.45). This analysis used free T4 rather than total T4, and total T4 is also affected by binding proteins such as those raised by estrogen or pregnancy, so the results cannot be transferred directly. (Baumgartner et al., 2017)
The Leiden 85-Plus Study followed 599 people from age 85 to 89 and found that higher free T4 was linked with a 16% higher death rate per standard-deviation increase (hazard ratio 1.16), while thyroid results were not linked with disability or memory. The cohort was small and limited to the very old. (Gussekloo et al., 2004)
Most modern research uses free T4 rather than total T4, so total T4 is usually interpreted together with free T4 and TSH.
T4 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.
T4 at a glance
| Topic | What the article explains |
|---|---|
| Why it matters | Steady supply for T3 production; metabolism and energy; growth and development; and heart, brain and gut function |
| What can affect it | Binding proteins; iodine intake; medications and biotin supplements (can interfere with tests); and pregnancy |
| 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.
Would free T4 be more useful than total T4 in my situation?
How should I read my T4 alongside the rest of my labs and my symptoms?
How often would it make sense to recheck my T4 in my situation?
Which daily habits or medications could be influencing my T4 before we retest?
Frequently asked questions
What is the difference between T4 and free T4?
Total T4 includes bound hormone; free T4 is the active, unbound portion.
Can biotin affect thyroid tests?
Yes. High-dose biotin can interfere. Tell your provider and ask when to pause it.
What does high T4 mean?
It can reflect hyperthyroidism, medication or higher binding proteins.
Is T4 the same as levothyroxine?
Levothyroxine is a synthetic form of T4.
Sources
- Baumgartner C, et al. Thyroid Function Within the Normal Range, Subclinical Hypothyroidism, and the Risk of Atrial Fibrillation. Circulation 2017. PMID: 29061566. https://pubmed.ncbi.nlm.nih.gov/29061566/
- Bianco AC, et al. Paradigms of Dynamic Control of Thyroid Hormone Signaling. Endocr Rev 2019. PMID: 31033998. https://pubmed.ncbi.nlm.nih.gov/31033998/
- Gussekloo J, et al. Thyroid status, disability and cognitive function, and survival in old age. JAMA 2004. PMID: 15572717. https://pubmed.ncbi.nlm.nih.gov/15572717/
- Hollowell JG, et al. Serum TSH, T(4), and thyroid antibodies in the United States population (1988 to 1994): National Health and Nutrition Examination Survey (NHANES III). J Clin Endocrinol Metab 2002. PMID: 11836274. https://pubmed.ncbi.nlm.nih.gov/11836274/
- Leung AM, et al. Consequences of excess iodine. Nat Rev Endocrinol 2014. PMID: 24342882. https://pubmed.ncbi.nlm.nih.gov/24342882/
- Li D, et al. Association of Biotin Ingestion With Performance of Hormone and Nonhormone Assays in Healthy Adults. JAMA 2017. PMID: 28973622. https://pubmed.ncbi.nlm.nih.gov/28973622/
- 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: Thyroxine (T4) Test
- 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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