BAT dissipates heat energy during adaptive thermogenesis regulated by several complex neuronal mechanisms including thyroid hormones. Twelve hyperthyroid subjects, treated with an anti-thyroid drug intervention were investigated using simultaneous PET-MR imaging. A significant increase in body weight and decrease in resting metabolic rate after treatment were observed. MR fat fraction significantly increased after treatment, whereas there is no significant change in PET-SUV. MR is more sensitive to the thyroid hormone induced changes, and can be utilized to study the BAT activity.