If your TSH (thyroid stimulating hormone) is normal but you still feel exhausted, cold, and foggy, you are not imagining things. TSH is a useful screening tool, but it does not tell the whole story of thyroid function. Research points to several nutrient deficiencies and inflammatory conditions that can disrupt thyroid hormone levels — and how the body uses thyroid hormone — even when TSH looks fine on paper. Understanding these patterns can help you have a more informed conversation with your provider.
The Limits of TSH as a Thyroid Marker
TSH reflects what the pituitary gland is signaling to the thyroid, but it is possible for an imbalance to exist in the thyroid hormones that do the actual work in your cells: free T3 (fT3) and free T4 (fT4), despite a normal TSH.
One example is non-thyroidal illness syndrome (NTIS) — a pattern in which T3 is low despite normal or even low TSH. A study by Keşkek and colleagues (2018) investigated this in the context of obesity and found that 9.5% of 219 obese participants had low free T3 despite normal or low TSH. None of the 204 participants in the non-obese control group showed this pattern. The researchers also found that participants with higher levels of high-sensitivity C-reactive protein (hs-CRP), a marker of systemic inflammation, were more likely to have higher body mass index (BMI), higher insulin resistance (measured by HOMA-IR), and lower free T3. The authors mention that reverse T3 would have also been helpful to monitor. Under inflammatory conditions, the body preferentially converts T4 not to active T3 but to reverse T3 (rT3) — a molecule that occupies the same cellular receptors as T3 without producing the same effect. Think of rT3 as a key that fits the lock but does not turn it. When rT3 levels are high, it can effectively block T3 from doing its job at the cellular level, producing symptoms of hypothyroidism even when standard lab values appear normal.
Practical takeaway: If you have symptoms of hypothyroidism but your TSH and free T4 look normal, ask your provider about testing free T3 and reverse T3 together. A high rT3-to-T3 ratio in the context of elevated inflammatory markers may explain why you feel the way you do — and points toward addressing the underlying inflammation rather than simply adjusting thyroid medication.
Selenium, T4 to T3 Conversion, and Inflammation
The thyroid produces mostly T4, which must be converted to the more active T3 in peripheral tissues. This conversion depends on enzymes called deiodinases — and selenium is essential to their function.
A 2024 study by Alehagen et al. found that selenium supplementation improved free T3 levels, but only in those who also had low selenium at baseline. The study also found that higher C-reactive protein (CRP) was associated with lower free T3. The authors conclude that adequate selenium is necessary for the conversion of T4 to T3, and suggest that selenium may be a useful biomarker to assess alongside thyroid hormones — particularly when TSH is elevated and free T3 is low.
Practical takeaway: If your provider is only testing TSH and T4, it may be worth asking about free T3 and selenium levels, especially if you have signs of inflammation or are not feeling well on your current thyroid support.
Iron, Ferritin, and Thyroid Hormone Production
Iron is another nutrient that plays a direct role in thyroid function. The enzyme that produces thyroid hormone — thyroid peroxidase — requires iron to work. When iron is deficient, hormone production can be compromised even when TSH appears normal or only mildly elevated.
A 2024 study by Krishna et al. measured serum ferritin (a marker of iron storage) and total iron binding capacity (TIBC, a marker of iron availability) in participants with elevated TSH but T3 in the normal range. They found that a significant number of these participants had iron deficiency, suggesting inadequate levels of the enzyme needed for thyroid hormone production. The authors recommend testing ferritin and iron regularly in all patients with hypothyroidism, noting that iron deficiency anemia and hypothyroidism are both more common in older adults and can worsen each other if not addressed.
Practical takeaway: Ferritin is not always included in standard thyroid or iron panels. If you have hypothyroidism and are not responding as expected to treatment, asking specifically about ferritin — not just hemoglobin — may be informative.
Vitamin D and Autoimmune Thyroid Disease
Vitamin D deficiency has been associated with a range of autoimmune conditions, and autoimmune hypthyroidism is no exception. A 2024 analysis by Pleić et al. examined the relationship between vitamin D and thyroid function using a research method called Mendelian randomization, which helps establish whether a relationship is likely causal rather than just correlational. They concluded that low serum 25-hydroxyvitamin D (25(OH)D) — the standard marker for vitamin D status — could result in elevated TSH, specifically in the context of autoimmune hypothyroidism such as Hashimoto’s thyroiditis.
Practical takeaway: Vitamin D testing is widely available and inexpensive. If you have Hashimoto’s or autoimmune thyroid disease, knowing your 25(OH)D level and optimizing it may be a meaningful part of your overall thyroid support strategy.
Putting It Together
TSH is a starting point, not a complete picture. If you have symptoms of hypothyroidism — fatigue, cold sensitivity, brain fog, hair loss, weight changes — but your TSH is in the normal range, it is worth exploring:
- Free T3 and free T4 — to see whether thyroid hormone conversion is occurring as expected
- Selenium — particularly if T3 is low or conversion seems impaired
- Ferritin and iron — especially if you have subclinical hypothyroidism or are not responding fully to thyroid medication
- Vitamin D (25(OH)D) — particularly if you have Hashimoto’s or another autoimmune thyroid condition
- Inflammatory markers (hs-CRP) — as chronic inflammation can directly interfere with thyroid hormone conversion, at the same time increasing conversion of T4 to reverse T3 (rT3), which can block T3 receptors, interfering with the normal role of thyroid hormone in your body
These are conversations to have with your healthcare provider, ideally one who is willing to look beyond TSH alone. You deserve a provider who takes your symptoms seriously and investigates the full picture.
References:
Alehagen U, Alexander J, Aaseth JO, Larsson A, & Opstad TB. (2024). Supplementation with selenium and coenzyme Q 10 in an elderly Swedish population low in selenium – positive effects on thyroid hormones, cardiovascular mortality, and quality of life. BMC Medicine, 22(1), 191.
Keşkek, Ş. Ö., Kurşun, Ö., Ortoğlu, G., Bankir, M., Tüzün, Z., & Saler, T. (2018). Obesity without comorbidity may also lead to non-thyroidal illness syndrome. Advances in Clinical and Experimental Medicine: Official Organ Wroclaw Medical University, 27(11), 1515–1520.
Krishna, D. S., Kumari, J. A., Sreedevi, N. N., Khan, S. A., Bhaskar, M. V., Baba, K. S. S. S., & Mohan, I. K. (2024). Iron Deficiency and Hypoferritinaemia in Patients with Subclinical Hypothyroidism: A Retrospective Observational Study. Journal of Clinical & Diagnostic Research, 18(6), 7–11.
Pleić N, Babić Leko M, Gunjača I, & Zemunik T. (2024). Vitamin D and thyroid function: A mendelian randomization study. PloS One, 19(6), e0304253.
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