Thyroid Health: Key Biomarkers and What Research Shows
The thyroid gland regulates metabolism, energy, and dozens of physiological processes. Understanding your thyroid biomarkers—TSH, Free T3, Free T4, and selenium—can help you have more informed conversations with your doctor.
Why Thyroid Function Matters
The thyroid is a small butterfly-shaped gland in the front of the neck, yet its influence extends to virtually every cell in the body. It produces hormones that govern metabolic rate, body temperature, heart rate, digestion, and neurological development. When thyroid hormone production is too high or too low, the downstream effects can be wide-ranging—from fatigue and weight changes to cardiovascular and cognitive symptoms.[1]
Blood tests are the standard method for evaluating thyroid function. A panel typically includes TSH as the primary screening marker, with Free T4 and Free T3 providing additional detail when TSH falls outside the normal range.[1] Selenium, a trace mineral essential for thyroid hormone metabolism, is increasingly recognized as a relevant nutritional biomarker.[3]
Key Thyroid Biomarkers at a Glance
| Biomarker | Typical Reference Range | What It Measures | Learn More |
|---|---|---|---|
| TSH | 0.4–4.0 mIU/L | Pituitary signal to the thyroid; most sensitive screening marker | TSH guide |
| Free T4 | 0.8–1.8 ng/dL | Main hormone secreted by the thyroid; precursor to active T3 | Free T4 guide |
| Free T3 | 2.3–4.2 pg/mL | Biologically active thyroid hormone; drives cellular metabolism | Free T3 guide |
| Selenium | 70–150 mcg/L (serum) | Trace mineral required for T4-to-T3 conversion and antioxidant defense | Selenium guide |
TSH: The Pituitary's Signal
Thyroid-stimulating hormone (TSH) is secreted by the pituitary gland and acts as a feedback signal to the thyroid. When circulating thyroid hormone levels fall, the pituitary releases more TSH to stimulate production. When levels are high, TSH is suppressed. This inverse relationship makes TSH the most sensitive and widely used first-line test for thyroid disorders.[1]
A TSH above the upper reference limit (commonly 4.0 mIU/L) suggests the pituitary is working harder than normal to stimulate an underperforming thyroid—a pattern consistent with hypothyroidism.[5] A TSH below the lower limit suggests the pituitary has suppressed its output because thyroid hormone levels are too high, pointing toward hyperthyroidism or overtreatment with thyroid medication.
Free T4 and Free T3: The Hormones Themselves
Free T4 (thyroxine) is the principal hormone produced and secreted by the thyroid gland. It is largely inactive until converted to Free T3 (triiodothyronine) in peripheral tissues such as the liver and kidneys. Free T3 is the biologically active form that binds to cellular receptors and drives metabolic effects.[2]
Measuring both Free T4 and Free T3 alongside TSH helps clinicians distinguish between different types of thyroid disorders—for example, primary hypothyroidism (low Free T4, high TSH) versus central hypothyroidism caused by pituitary dysfunction (low Free T4, inappropriately normal or low TSH). Some patients on levothyroxine who still experience symptoms may have suboptimal T4-to-T3 conversion, making Free T3 a clinically relevant marker.[2]
Selenium and Thyroid Health
The thyroid contains the highest concentration of selenium per gram of tissue of any organ in the body.[3] Selenium is a structural component of the deiodinase enzymes responsible for converting T4 into active T3, and of glutathione peroxidase, which protects thyroid cells from oxidative damage generated during hormone synthesis.[3]
Research has explored selenium supplementation in autoimmune thyroid disease, particularly Hashimoto's thyroiditis. A 2010 randomized controlled trial found that selenium supplementation significantly reduced thyroid peroxidase antibody (TPO-Ab) concentrations in patients with autoimmune thyroiditis.[4] While these findings are promising, selenium supplementation is not a standard treatment and should be approached cautiously, as chronic excess intake can cause selenosis. Dietary sources such as Brazil nuts, fish, and whole grains support adequate selenium status for most people.
Interpreting Results in Context
Reference ranges for thyroid tests can vary between laboratories and populations. Pregnant women, for example, have lower TSH reference ranges, especially in the first trimester, due to the cross-reactivity of human chorionic gonadotropin (hCG) with TSH receptors. Age, medications (including biotin supplements), and acute illness can all influence results. A single abnormal value is typically confirmed on repeat testing before a diagnosis is established or treatment initiated.[5]
Key Takeaway: TSH is the primary screening test for thyroid dysfunction. Free T4 and Free T3 add clinical detail, and selenium plays a supporting role in hormone conversion and thyroid immune health. All values should be interpreted together, in the context of symptoms and personal history, by a qualified healthcare provider.
Frequently Asked Questions
Track Your Thyroid Biomarkers With Health3
Scan your lab results, monitor TSH trends over time, and explore how your thyroid biomarkers connect with the Health3 app.
References
- Chaker L, Bianco AC, Jonklaas J, Peeters RP. Hypothyroidism. Lancet. 2017;390(10101):1550-1562. PubMed
- Bianco AC, Kim BW. Deiodinases: implications of the local control of thyroid hormone action. J Clin Invest. 2006;116(10):2571-2579. PubMed
- Rayman MP. Selenium and human health. Lancet. 2012;379(9822):1256-1268. PubMed
- Toulis KA, Anastasilakis AD, Tzellos TG, Goulis DG, Kouvelas D. Selenium supplementation in the treatment of Hashimoto's thyroiditis: a systematic review and a meta-analysis. Thyroid. 2010;20(10):1163-1173. PubMed
- Garber JR, Cobin RH, Gharib H, et al. Clinical practice guidelines for hypothyroidism in adults. Thyroid. 2012;22(12):1200-1235. PubMed
Medical Disclaimer: This page is for informational and educational purposes only and does not constitute medical advice, diagnosis, or treatment. The content is never a substitute for professional medical evaluation. Always consult a qualified healthcare professional before making any changes to your health regimen. Read our full Content Standards & Medical Disclaimer.