Thermogenesis in Hyperthyroidism and Effect of Anti-Adrenergic Therapy

Inclusion Criteria: * Informed Consent as documented by signature * Age 18 to 70 years * Hyperthyroidism: Thyroid-stimulating Hormone (TSH) below 0.2 mU and free T4 at or above 25 pmol/L or free T3 at or above 8 pmol/L. Exclusion Criteria: * Contraindications to propranolol: hypersensitivity or allergy * Therapy with a beta blocker for reasons other than hyperthyroidism (e.g. cardiac arrhythmia or heart failure) * Treatment with amiodarone * Women who are pregnant or breast feeding, * Intention to become pregnant during the course of the study, * History of asthma or chronic obstructive pulmonary disease * Occlusive peripheral artery disease; Raynaud's syndrome. * Other clinically significant concomitant disease states: * Known renal failure (GFR \< 50 ml/min) * Known hepatic dysfunction * known heart failure or unstable angina pectoris * Known or suspected non-compliance, drug or alcohol abuse, * Inability to follow the procedures of the study, e.g. due to language problems, psychological disorders, dementia, etc. of the participant, * Participation in another study with investigational drug within the 30 days preceding and during the present study, * Previous enrolment into the current study, * Enrolment of the investigator, his/her family members, employees and other dependent persons. * ECG-criteria * Resting heart rate below 60 bpm * Complete left bundle branch block * Atrioventricular block (AV-block) grade 2 or 3