Higher Resting Metabolism Is Associated With Increased Free Triiodothyronine Among Female Reindeer Herders in Northern Finland

Background
Thyroid hormones (TH) regulate metabolism and are shaped by environmental factors—ambient temperature in particular. Previous work among indigenous and non-indigenous populations in Russia revealed that there are seasonal shifts in TH dynamics such that total and free triiodothyronine (fT3) and free thyroxine (fT4) increase during winter. Elevated TH levels in these populations were positively correlated with the elevated resting metabolic rate (RMR) commonly seen among indigenous cold climate populations.

Methods
Here we examined the relationship between TH levels (fT3, fT4, and thyroid stimulating hormone) and resting metabolism among reindeer herders (N = 35) and office workers (N = 16) from northern Finland in January 2019 and February of 2023. RMR was measured using indirect calorimetry at both time points and a TH analysis was conducted from venous blood samples collected before RMR measurements in 2023 only.

Results
Controlling for fat free mass, female reindeer herders had significantly higher RMRs than male reindeer herders and significantly higher RMRs than predictive equation estimates. Female herders also had significantly higher fT3 and TSH than male herders and female officer workers. Female herders exhibited a significant positive correlation between fT4 and RMR; significant correlations were not found among male herders or female office workers.

Conclusion
This preliminary study demonstrates variation in the relationship between TH levels and resting metabolism among reindeer herders and office workers in Northern Finland. These results highlight potential sex-based differences in TH and metabolism dynamics, particularly among female reindeer herders, that require further research.