Subclinical Hyperthyroidism
The causes of subclinical hyperthyroidism are the same as the causes of overt hyperthyroidism. The potential clinical consequences of subclinical hyperthyroidism include progression to overt hyperthyroidism, cardiovascular conditions, bone loss, fractures, and dementia.
In overt hyperthyroidism, serum levels of free thyroxine (T4) and triiodothyronine (T3) or T3alone are elevated, and serum thyrotropin (TSH) levels are suppressed. In subclinical hyperthyroidism, levels of free T4and T3 are normal, TSH levels are suppressed, and thyroid hormone (total T4) levels are usually in the middle to upper range of normal. Levels of free T4and T3 should be promptly assessed in patients with a serum thyrotropin level of less than 0.1 mU/L to rule out overt hyperthyroidism. In the absence of overt disease, it is reasonable to defer further evaluation for 2 to 3 months, at which time repeat testing should be performed. Subnormal serum thyrotropin levels are transient in up to 50% of patients, most often in those with mild disease.
The best predictor of progression from subclinical hyperthyroidism to overt hyperthyroidism is the baseline serum thyrotropin level rather than the cause of the disease. Serum thyrotropin levels in patients with mild subclinical hyperthyroidism frequently normalize during follow-up, whereas patients with thyrotropin levels lower than 0.1 mU/L usually have persistent disease or progression to overt hyperthyroidism. Patients with nodular thyroid disease and subclinical hyperthyroidism are at increased risk for progression to overt hyperthyroidism after exposure to a large iodine load.
Although data are lacking from randomized clinical trials to guide treatment decisions, professional organizations recommend treatment of subclinical hyperthyroidism in persons older than 65 years of age and postmenopausal women, especially when serum thyrotropin levels are less than 0.1 mU/L. Premenopausal women and younger patients should be considered for treatment if serum thyrotropin levels are less than 0.1 mUL and they have symptoms of hyperthyroidism or coexisting conditions such as osteopenia, osteoporosis, or cardiovascular disease. There is no indication for treatment in younger patients who do not have coexisting conditions if the serum thyrotropin level is 0.1 mU/L or higher.
Reference
Biondi B and Cooper BS, Subclinical Hyperthyroidism, N Engl J Med 2018;378:2411-2419
