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Hypothyroidism in hibernating brown bears.

Anne Mette Frøbert¹, Claus G Nielsen², Malene Brohus¹

¹Department of Chemistry and Bioscience, Faculty of Engineering and Science, Aalborg University, Fredrik Bajers Vej 7H, 9220, Aalborg East, Denmark.

Thyroid Research

|February 1, 2023

Summary

Hibernating brown bears exhibit significantly lower thyroid hormone levels, becoming hypothyroid. This metabolic adaptation may offer insights into human hypometabolic states.

Keywords:

Hibernation Hypothyroidism Metabolism Thyroid hormone Thyroxine Thyroxine-binding globulin (TBG)Triiodothyronine Ursus arctos

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Area of Science:

Endocrinology
Comparative Physiology
Mammalian Hibernation

Background:

Brown bears hibernate for up to half a year, reducing energy demands during food and water scarcity.
Thyroid hormones critically regulate basal metabolic rate in mammals, including humans.

Purpose of the Study:

To investigate the role of thyroid hormones in bear hibernation.
To compare serum thyroid hormone levels between hibernating and active states in brown bears.

Main Methods:

Quantified serum total triiodothyronine (T3) and thyroxine (T4) using electrochemiluminescence immunoassay.
Analyzed liver mRNA levels of thyroid hormone-binding proteins (TBG, TTR, albumin) from published RNA sequencing data.

Main Results:

Hibernating bears showed significantly reduced T4 (44%) and T3 (36%) levels compared to active states.
mRNA levels of thyroxine-binding globulin (TBG) and albumin increased significantly during hibernation.
Transthyretin (TTR) mRNA levels remained unchanged between states.

Conclusions:

Hibernating bears are physiologically hypothyroid, mirroring features seen in hypothyroid humans.
Decreased thyroid hormone signaling is a likely key mechanism in bear hibernation physiology.
Bear hibernation offers potential translational insights for inducing hypometabolic states in humans.