Thyroid Labs

It is possible for imbalances of thyroid hormones to exist despite a normal TSH.  One example of such an imbalance is non-thyroidal illness syndrome, which was investigated in the context of obesity by Keşkek et al. (2018).  They found that 9.5% of the 219 obese participants in their study had low free T3 despite normal or low TSH, compared to none of the 204 in the non-obese control group.  They also monitored free T4, fasting glucose, serum lipids, creatinine, aspartate aminotransferase (AST), insulin, insulin resistance (HOMA-IR), high-sensitivity C-reactive protein (hs-CRP), erythrocyte sedimentation rate (ESR), and complete blood count in their participants.  Those with high hs-CRP were more likely to have higher BMI, higher HOMA-IR, and also lower free T3.  The authors mention that reverse T3 would have also been helpful to monitor. 

Alehagen et al. (2024) suggest that selenium could be a useful biomarker to assess when TSH is high and free T3 is low, because in their study selenium supplementation improved free T3 levels in participants who also had low selenium levels.  This study also found higher CRP to be associated with lower free T3.  They conclude that adequate selenium is necessary for conversion of T4 to T3.

Krishna et al. (2024) measured serum ferritin and Total Iron Binding Capacity (TIBC) in study participants with subclinical hypothyroidism and found that a significant number of them had iron deficiency, resulting in inadequate levels of the enzyme necessary for thyroid hormone production.  These participants had high TSH but normal T3.  They mention that anemia can be diagnosed by measuring hemoglobin, and that anemia and hypothyroidism are both more common in elderly populations.  They recommend testing ferritin and iron regularly in all patients with hypothyroidism in order to be aware of any iron deficiency that could lead to worsening of the condition. 

Pleić et al. (2024) analyzed studies on vitamin D and thyroid function and concluded that vitamin D deficiency (low serum 25-hydroxyvitamin D [25(OH)D]) could result in high TSH, specifically due to autoimmune hypothyroidism.

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.