The overall objective of the HEAT study is to determine the acute effect of propranolol on energy expenditure in patients suffering from hyperthyroidism and to evaluate the consequences of the change of hyper- to euthyroidism on metabolism during the course of treatment, especially with regard to the reaction to cold ambient temperatures.

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