Life Lines by Dr. Dolittle

Sponsored by the American Physiological Society

Now we know how life underground reduces metabolism

…at least for mole-rats.

Photo by Benny Mazur via <a href="http://Benny Mazur, CC BY 2.0 Wikimedia Commons

African mole-rats live in dark underground dwellings where oxygen and food are scarce, and humidity and temperatures are high. These animals have low metabolic rates and body temperatures uniquely adapted to their life underground.

Metabolism is controlled in part by thyroid hormones. To understand the role of thyroid hormone in mole-rats, researchers examined the thyroid gland and hormones in naked mole-rats (Heterocephalus glaber) as well as Ansell’s mole-rats (Fukomys anselli). 

Thyroid hormones increase metabolism, which generates heat in the process. As environmental temperatures decline, animals begin to shiver to create additional body heat. Levels of thyroid hormone are regulated by the hypothalamus and pituitary glands in the brain. The hypothalamus secretes thyrotropin releasing hormone (TRH), which travels to the nearby pituitary gland where it causes the release of thyroid stimulating hormone (TSH). Once TSH reaches the thyroid gland, it stimulates the secretion of T3 and T4 thyroid hormones into the blood.

In a new study published in Scientific Reports, researchers compared the structure of the thyroid gland between Ansell’s mole-rats, naked mole-rats, and laboratory mice. Although Ansell’s mole-rats are larger than naked mole rats, their thyroid gland was smaller. Both species of mole-rats had lower circulating T4 concentrations than mice, which matched findings from prior studies. Although levels of T3 were similar in Ansell’s mole-rats and mice, levels in naked mole-rats were double levels observed in these species.  

Even though both mole-rat species had lower T4 hormone and showed signs of iodine deficiency compared to mice, they seemed to have evolved these characteristics independently as the structure and size of the thyroid gland in each species differed. The reason these two mole-rat species have low iodine is a bit of a mystery as their diet of root vegetables contains enough iodine to make thyroid hormone. The hypothalamic-pituitary-thyroid gland axis described above also appears to be ‘broken’ in naked mole-rats as they have lower levels of TSH in the pituitary gland, even though low levels of thyroid hormone would be predicted to increase the secretion of TSH.

Reduced production of T4 appears to be an adaptation to life underground where these animals keep their body temperature rather low. Having low levels of T4 may also help ensure they do not overheat in their subterranean environment.  


P Gerhardt, S Begall, C Fradrich, K Renko, TB Hildebrandt, S Holtze, A Heinrich, A Sahm, X Meci, J Kohrle Comparative analysis of thyroid hormone systems in rodents with subterranean lifestyle. Sci Rep 13:3122, 2023.  

Categories: Environment, Extreme Animals, Hibernation and Hypoxia, Nature's Solutions

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