Subclinical hyperthyroidism is defined as the presence of serum free thyroxine (T4) and triiodothyronine (T3) levels within the reference range and a reduced serum thyrotrophin (TSH) level. Evidence is accumulating that it has important clinical effects. Randomised clinical trials are needed to answer the question whether or not treatment of subclinical hyperthyroidism prevents cardiac problems, especially atrial fibrillation, and preserves bone mineral density. A randomised, Dutch multicentre trial has recently been started. Its goal is to study whether radioiodine treatment prevents the development of atrial fibrillation and prevents decreases in bone mineral density.