Data from a 2025 KiSel-10 sub-study analysis suggest that serum CD5L levels may be an additional biomarker for improved thyroid function. Serum CD5L levels may also be a suitable biomarker for adequate selenium status and for reduced mortality risk.
Combined supplementation of elderly citizens with selenium and Coenzyme Q10 for 48 months significantly increased the level of circulating CD5L. The supplementation was associated with improved health-related quality of life and improved survival odds. Meanwhile, in the study’s placebo group, significantly higher cardiovascular mortality within 10 years was associated with low levels of circulating CD5L. The combined selenium and Coenzyme Q10 supplementation reduced the mortality risk. The active treatment in the KiSel-10 clinical trial was 200 mcg of selenium from selenium-enriched yeast and two times 100 mg of ubiquinone Coenzyme Q10 daily [Alehagen 2025].
The full name for CD5L is Cluster of Differentiation 5. CD5L is a protein that supports numerous activities in the body. Tissue-resident macrophages produce CD5L proteins. Low CD5L levels have been associated with clinical conditions such as atherosclerosis and inflammation [Alehagen 2025].
Selenium and Thyroid Hormones
In this 2025 sub-study, Prof Alehagen et al analyzed CD5L levels in serum samples from 359 KiSel-10 study participants at baseline and after 48 months of intervention. The combined selenium and Coenzyme Q10 supplementation significantly increased the serum levels of CD5L and free thyroid hormone T3. These outcomes indicate that CD5L positively responds to combined selenium and Coenzyme Q10 supplementation. Accordingly, CD5L levels correlate with improved thyroid hormone status and with positive health-related quality of life indices [Alehagen 2025].
In a 2024 sub-study, Prof Alehagen et al reported that combined supplementation with selenium and Coenzyme Q10 for four years had the following beneficial effects on thyroid function [Alehagen 2024]:
- increased free T3 and reverse T3
- decreased free T4
- diminished the increase in TSH levels compared with placebo treatment
The beneficial effects of the combined supplementation on thyroid function extended to improved cardiovascular mortality and improved health-related quality of life. The researchers concluded that considerable numbers of elderly citizens in selenium-poor regions in northern Europe may suffer from suboptimal thyroidal function because of selenium deficiency [Alehagen 2024].
Conclusion: Selenium and Thyroid Function
The data from the KiSel-10 sub-studies seem to show that selenium supplementation can improve selenium-dependent deiodinase expression. This, in turn, supports better thyroid hormone activity.
Combined selenium and Coenzyme Q10 supplementation leads to higher circulating CD5L levels. These higher CD5L concentrations are associated with improved thyroid hormone status, with improved health-related quality of life, and with reduced mortality risk.
Sources
Alehagen U, Aaseth JO, Schomburg L, Alexander J et al. Serum CD5L responds positively to selenium and Coenzyme Q10 supplementation with relation to thyroid hormones, mortality, and health-related quality-of-life—a sub-analysis of a double-blind randomized placebo-controlled trial in elderly low in selenium. Antioxidants. 2025;14(3):366.
Alehagen U, Alexander J, Aaseth JO, Larsson A, Opstad TB. Supplementation with selenium and coenzyme Q10 in an elderly Swedish population low in selenium – positive effects on thyroid hormones, cardiovascular mortality, and quality of life. BMC Med. 2024 May 7;22(1):191.
The information presented in this review article is not intended as medical advice. It should not be used as such.
30 April 2025