12611 The Thyroid Adapts Remarkably to Overnutrition to Mitigate Hypothyroidism
Authors: Rampy Jessica , Torres Manzo Alejandra Paola , Hoffsmith Kendra , Aaron Loberg Matthew , Wang Huiying , Weiss Vivian L , Carrasco Nancy 
Published: October 2024

Disclosure: J. Rampy: None. A. Torres Manzo: None. K. Hoffsmith: None. M.A. Loberg: None. H. Wang: None. V.L. Weiss: None. N. Carrasco: None.

Obesity affects &gt;40% of the U.S. population and is one of today’s biggest public health concerns. Losing weight is challenging, and maintaining weight loss even more so, owing to the many biochemical and hormonal changes that occur in the pathogenesis of obesity, some of which do not reverse after weight loss. Thyroid hormones (THs) have far-reaching effects on whole-body energy balance. Their synthesis and release by the thyroid are finely regulated by central mechanisms to control serum TH levels. THs are also regulated by nutrient sensing, so that their levels modulate energy expenditure to match energy intake. Given the reciprocal and fine-tuned relationship between THs and energy balance, one would expect strong upregulation of TH levels to counteract diet-induced weight gain, but this is not consistently reported in obesity studies. Thus, we hypothesized that overnutrition impairs thyroid function, preventing a sustained increase in TH levels. We tested this hypothesis in male C57Bl/6J mice fed a high-fat diet and sucrose water. Strikingly, overnutrition decreased T4 levels after just 5 days—and within 3 weeks, we observed mild hypothyroidism, with increased thyroid stimulating hormone (TSH) levels and goiter. After 6 weeks, intrathyroidal T4 levels were significantly decreased, whereas the expression of the TH precursor thyroglobulin (TG) was unchanged, despite high TSH stimulation. Furthermore, we found increased expression of several ER stress-related proteins, including BiP, p-eIF2α, and PDI. These results suggest that TG was not upregulated because overnutrition-induced ER stress impaired protein translation. Remarkably, even though T4 levels were low, the thyroid maintained normal T3 levels, likely due to the pronounced histological and vascular changes we observed, which should promote TH availability. Some changes were consistent with the classical growth effects of TSH stimulation, while others were likely due to upregulation of adrenomedullin 2, a TSH-responsive angiogenic vasodilator. Preliminary studies in female mice showed similar results. Notably, we have observed similar histological changes in human thyroids, which strongly correlate with BMI. Collectively, our results, despite the unchanged T3 levels, are consistent with a struggling thyroid and a mildly hypothyroid state. Existing data suggest that low T4 can itself be problematic, particularly in metabolic tissues that generate T3, the more biologically active form, from T4 to levels beyond those of the T3 available from the bloodstream. Reversibility of this state is being investigated in ongoing experiments. The causal role that overnutrition plays in thyroid impairment and decreased TH availability in the pathogenesis of obesity has scarcely begun to be elucidated. Our work sheds new light on how overnutrition changes thyroid function and highlights the remarkable adaptability of the thyroid.

Presentation: 6/1/2024