10 patients with thyreotoxicosis (T3 > 3 nmol/L and suppressed TSH) will be investigated at diagnosis and after 3-6 months of euthyroidism induced by antithyroid treatment. The patients will all be over the age of 50 due to the radiation given by the scans (requested by the local Ethical Committee). All will be scanned for active BAT by the integrated FDG PET-CT before and after treatment.BAT is activated by means of crushed ice placed under the feet in individualized intervals during the scan procedure. Indirect calorimetry is done to determine the basal metabolic rate and subcutaneous fat biopsy to examine the effect on subcutaneous white fat and the possible "browning" effect.