This project is designed to evaluate the relationship between the intra-cellular thyroid hormone (TH) levels and brown adipose tissue (BAT) activity, transcriptome and cellular composition and correlate with circulating TH levels and other metabolic parameters in adult humans, to better understand the interaction between the thyroid axis and BAT in the process of thermogenesis and energy expenditure. Studies using positron emission tomography/ computed tomography (PET/CT) scanning have shown metabolically active BAT in adult humans, whose activity correlated inversely with body fat. TH plays an important role in brown adipocyte differentiation and controls adaptive thermogenesis in BAT by uncoupling the oxidative phosphorylation. Based on data from animal studies and findings from our prior studies in adult humans, we hypothesize that intra-cellular T3, the active form of TH stimulates BAT activity by directly affecting the transcriptome and cellular composition of BAT. We plan to study 10 hyperthyroid and 10 hypothyroid patients who require thyroid surgery as part of their standard medical care. Perithyroidal BAT and subcutaneous white adipose tissue (WAT) (control) will be collected during the surgery. Thyroid function tests and metabolic parameters, including fasting glucose, insulin, HbA1c, and lipid panel will be measured at the time of the surgery. A study visit will be scheduled in the Clinical Research Center (CRC) within a week prior to the thyroid surgery. The patients will undergo indirect calorimetry to measure resting metabolic rate (RMR) and dual energy xray absorptiometry (DXA) to measure body composition. A PET- CT scan will then be performed after wearing a cooling vest set at 55-61 °F for 2 hours to measure BAT volume and activity. We will measure adipose tissue TH concentration using liquid chromatography–mass spectrometry (LC-MS). We will investigate the relationship between the intra-cellular and circulating thyroid hormone (TH) levels and BAT activity, RMR, body composition as well as other metabolic parameters. We will also perform block and single nucleus RNA-sequencing of the collected adipose tissue samples to understand the impact of the thyroid axis on the transcriptome and cellular composition of human neck BAT and WAT. The current project will help to further elucidate the complex relationship between the thyroid axis and BAT function, transcriptome and cellular composition in the process of thermogenesis in humans. There are no prior published human studies looking into the effect of intra-cellular TH levels on BAT activity. The study results will be used to write a competitive NIH grant (R01) to experimentally test the connection between TH and BAT in adult humans and find new treatment strategies for obesity and diabetes, which are major public health concerns.