Clinical Considerations

Subclinical thyroid dysfunction is defined as an abnormal biochemical measurement of thyroid hormones without any specific clinical signs or symptoms of thyroid disease and no history of thyroid dysfunction or therapy. This includes individuals who have mildly elevated TSH and normal thyroxine (T₄) and triiodothyronine (T₃) levels (subclinical hypothyroidism) or low TSH and normal T₄ and T₃ levels (subclinical hyperthyroidism). Individuals with symptoms of thyroid dysfunction, or those with a history of thyroid disease or treatment, are excluded from this definition and are not the subject of these recommendations.

When used to confirm suspected thyroid disease in patients referred to a specialty endocrine clinic, TSH has a high sensitivity (98%) and specificity (92%). When used for screening primary care populations, the positive predictive value of TSH in detecting thyroid disease is low; furthermore, the interpretation of a positive test result is often complicated by an underlying illness or by frailty of the individual. In general, values for serum TSH level below 0.1 mU/L are considered low and values above 6.5 mU/L are considered elevated.

Clinicians should be aware of subtle signs of thyroid dysfunction, particularly among those at high risk. People at higher risk for thyroid dysfunction include the elderly, postpartum women, those with high levels of radiation exposure (>20 mGy), and patients with Down syndrome. Evaluating for symptoms of hypothyroidism is difficult in patients with Down syndrome because some symptoms and signs (for example, slow speech, thick tongue, and slow mentation) are typical findings in both conditions.

Subclinical hyperthyroidism has been associated with atrial fibrillation; dementia; and, less clearly, osteoporosis. However, progression from subclinical to clinical disease in patients without a history of thyroid disease is not clearly established.

Subclinical hypothyroidism is associated with poor obstetric outcomes and poor cognitive development in children. Evidence for dyslipidemia, atherosclerosis, and decreased quality of life in adults with subclinical hypothyroidism in the general population is inconsistent and less convincing.

Recommendations of Other Groups

The American Thyroid Association recommends measuring thyroid function in all adults beginning at age 35 years and every 5 years thereafter, noting that more frequent screening may be appropriate in high-risk or symptomatic individuals (1). The Canadian Task Force on the Periodic Health Examination recommends maintaining a high index of clinical suspicion for nonspecific symptoms consistent with hypothyroidism when examining perimenopausal and postmenopausal women (2). The American College of Physicians recommends screening women older than age 50 years with 1 or more general symptoms that could be caused by thyroid disease (3). The American Association of Clinical Endocrinologists recommends TSH measurement in women of childbearing age before pregnancy or during the first trimester (4). The American College of Obstetricians and Gynecologists recommends that physicians be aware of the symptoms and risk factors for postpartum thyroid dysfunction and evaluate patients when indicated (5). The American Academy of Family Physicians recommends against routine thyroid screening in asymptomatic patients younger than age 60 years (6).

Appendix

Members of the U.S. Preventive Services Task Force are Alfred O. Berg, MD, MPH, Chair, (University of Washington, Seattle, Washington); Janet D. Allan, PhD, RN, CS, Vice-Chair (University of Maryland Baltimore, Baltimore, Maryland); Paul Frame, MD (Tri-County Family Medicine, Cohocton, and University of Rochester, Rochester, New York); Charles J. Homer, MD, MPH (National Initiative for Children's Healthcare Quality, Boston, Massachusetts); Mark S. Johnson, MD, MPH (University of Medicine and Dentistry of New Jersey–New Jersey Medical School, Newark, New Jersey); Jonathan D. Klein, MD, MPH (University of Rochester School of Medicine, Rochester, New York); Tracy A. Lieu, MD, MPH (Harvard Pilgrim Health Care and Harvard Medical School, Boston, Massachusetts); C. Tracy Orleans, PhD (The Robert Wood Johnson Foundation, Princeton, New Jersey); Jeffrey F. Peipert, MD, MPH (Women and Infants' Hospital, Providence, Rhode Island); Nola J. Pender, PhD, RN (University of Michigan, Ann Arbor, Michigan); Albert L. Siu, MD, MSPH (Mount Sinai School of Medicine, New York, New York); Steven M. Teutsch, MD, MPH (Merck & Co., Inc., West Point, Pennsylvania); Carolyn Westhoff, MD, MSc (Columbia University, New York, New York); and Steven H. Woolf, MD, MPH (Virginia Commonwealth University, Fairfax, Virginia). This list includes members of the Task Force at the time this recommendation was finalized. For a list of current Task Force members, go to http://www.ahrq.gov/clinic/uspstfab.htm.