From a recent concise review:
Subclinical hyperthyroidism is defined by a low or undetectable serum thyroid-stimulating hormone level, with normal free thyroxine and total or free triiodothyronine levels. It can be caused by increased endogenous production of thyroid hormone (e.g., in Graves disease, toxic nodular goiter, or transient thyroiditis), by administration of thyroid hormone to treat malignant thyroid disease, or by unintentional excessive replacement therapy. The prevalence of subclinical hyperthyroidism in the general population is about 1% to 2%; however, it may be higher in iodine-deficient areas. The rate of progression to overt hyperthyroidism is higher in persons with thyroid-stimulating hormone levels less than 0.1 mIU per L than in persons with low but detectable thyroid-stimulating hormone levels. Subclinical hyperthyroidism is associated with an increased risk of atrial fibrillation and heart failure in older adults, increased cardiovascular and all-cause mortality, and decreased bone mineral density and increased bone fracture risk in postmenopausal women. However, the effectiveness of treatment in preventing these conditions is unclear. A possible association between subclinical hyperthyroidism and quality-of-life parameters and cognition is controversial. The U.S. Preventive Services Task Force found insufficient evidence to assess the balance of benefits and harms of screening for thyroid dysfunction in asymptomatic persons. The American Thyroid Association and the American Association of Clinical Endocrinologists recommend treating patients with thyroid-stimulating hormone levels less than 0.1 mIU per L if they are older than 65 years or have comorbidities such as heart disease or osteoporosis.