Subclinical hypothyroidism is defined biochemically as a normal serum free thyroxine (T4) concentration in the presence of an elevated serum thyroid-stimulating hormone (TSH) concentration. Some patients with subclinical hypothyroidism may have vague, non-specific symptoms suggestive of hypothyroidism, but attempts to identify patients clinically have not been successful [1,2]. Thus, this disorder can only be diagnosed on the basis of laboratory test results.
This topic will review the diagnosis and management of subclinical hypothyroidism. The clinical manifestations, diagnosis, and management of overt hypothyroidism are reviewed separately. (See "Clinical manifestations of hypothyroidism" and "Diagnosis of and screening for hypothyroidism in nonpregnant adults" and "Laboratory assessment of thyroid function" and "Treatment of hypothyroidism".)
In population-based studies, the prevalence of subclinical hypothyroidism ranges from 4 to 15 percent [1,3-7]. In the United States National Health and Examination Survey (NHANES III), which excluded subjects with known thyroid disease, 4.3 percent of 16,533 people had subclinical hypothyroidism . The prevalence rises with age, is higher in females than males, and is lower in blacks than in whites [5-8]. However, the prevalence is determined by the upper limit of normal for serum thyroid-stimulating hormone (TSH). If the upper limit of normal rises with age, as appears to be the case, then the prevalence may not be as high as has been previously thought. (See 'Diagnosis' below.)
In Europe, where iodine intake is variable, subclinical hypothyroidism is more prevalent in areas of iodine sufficiency. In one study, the prevalence of subclinical hypothyroidism ranged from 4.2 percent in iodine-deficient areas to 23.9 percent in an area of abundant iodine intake, despite a similar prevalence of patients with high serum concentrations of antithyroid peroxidase (anti-TPO) antibodies .
The causes of subclinical hypothyroidism are the same as those of overt hypothyroidism (table 1). (See "Disorders that cause hypothyroidism".)