The investigators wish to evaluate the effect of thyroid hormones on brown adipose tissue in humans in an observational study on patients with thyrotoxicosis. In the investigators evaluation the investigators will use FDG-PET/CT, indirect calorimetry and fat biopsies in the acute phase of the disease and in the euthyroid phase after treatment.

It is the investigators hypothesis that high levels of circulating T3 might affect amount and function of brown adipose tissue.

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.

Inclusion Criteria:

• men and women with hyperthyroidism (T3 > 3 nmol/l)
• > 50 years old
• women must be postmenopausal

Exclusion Criteria:

• thyrotoxic crises
• severe concurrent sickness that will make it unsafe to postpone treatment, this includes severe kidney disease (creatinin over 160)- and heart disease (NYHA group 3 and 4).
• diabetes
• former and current treatment with interleukin-2
• Patients who are already treated with β-blockers
• severe claustrophobia
• BMI > 30 kg/m2

10 patients with hyperthyroidism, reffered to our hospital from their primary caretakers will be investigated before and after 3 to 5 months in a proven euthyroid state.