The availability of sensitive assays for thyrotropin (TSH) resulted in the identification of patients who have low serum TSH concentrations (<0.5 mU/L) but normal serum free thyroxine (T4) and triiodothyronine (T3) concentrations, a constellation of biochemical findings defined as subclinical hyperthyroidism. The term overt hyperthyroidism refers to patients with elevated levels of free T4, T3, or both, and a subnormal TSH concentration. Both subclinical and overt hyperthyroidism are biochemical definitions since hyperthyroid symptoms are non-specific and may be present in patients with subclinical disease, and absent in those with overt disease, especially the elderly.
Subclinical hyperthyroidism will be discussed here. Overt hyperthyroidism is discussed separately. (See "Diagnosis of hyperthyroidism" and "Overview of the clinical manifestations of hyperthyroidism in adults".)
The causes of subclinical hyperthyroidism are the same as the causes of overt hyperthyroidism and, like overt hyperthyroidism, subclinical hyperthyroidism can be persistent or transient (table 1). (See "Disorders that cause hyperthyroidism".)
Exogenous subclinical hyperthyroidism — As many as 10 million people in the United States, and possibly as many as 200 million people worldwide, are taking thyroid hormone. All are at risk for subclinical hyperthyroidism, whether intentional or unintentional. Among patients taking T4, as many as 25 percent have low serum TSH values [1,2]. (See "Exogenous hyperthyroidism".)
Many of these patients have hypothyroidism, and in them subclinical hyperthyroidism is not the goal of thyroid hormone therapy. However, subclinical hyperthyroidism is the goal of thyroid hormone therapy in patients with thyroid cancer and in some patients with solitary thyroid nodules, multinodular or diffuse goiters, or a history of head and neck irradiation. In these patients, the benefits of TSH suppression are thought to outweigh the risks of subclinical hyperthyroidism. (See "Differentiated thyroid cancer: Overview of management", section on 'Thyroid hormone suppression' and "Thyroid hormone suppressive therapy for thyroid nodules and benign goiter".)