Key Points

• Subclinical thyroid disease is based on TSH levels only marginally outside the normal reference ranges in an asymptomatic patient with normal FT4.

• Screening for subclinical thyroid disease is not recommended by the American Association of Clinical Endocrinologists (AACE), U.S. Preventive Services Task Force (USPSTF), or American Academy of Family Physicians (AAFP).

• The American Thyroid Association (ATA) recommends screening all adults for thyroid disorder beginning at age 35 and then every 5 years.

• Treatment of subclinical hyperthyroid disease is not recommended, although follow-up should occur every 6 months.

• Treatment of subclinical hypothyroid disease is recommended by the American College of Physicians (ACP) for women over 50 with symptoms consistent with hypothyroidism.

Discussion about subclinical thyroid disease has focused on whether it is a real clinical entity. Subclinical thyroid diseases are defined as (1) subclinical hyperthyroidism (or subclinical thyrotoxicosis) with sTSH less than 0.1 mIU/L and normal circulating FT4 and FT3 or (2) subclinical hypothyroidism with sTSH greater than 4.5 mIU/L but less than 10.0 mIU/L with normal circulating FT4. Both entities assume a patient who is asymptomatic or has minimal signs and symptoms (Ross, 2005a, 2005b).

The primary question concerning subclinical thyroid disease is whether early intervention is beneficial and patients should be screened. The only disease state that has been directly related to subclinical thyroid disease is overt hypo-thyroidism. Individuals with subclinical hypothyroidism have a higher incidence of progression to overt hypothyroidism than the general population. Annually, 3% to 5% of patients identified with subclinical hypothyroidism will progress to overt hypothyroidism with sTSH levels over 10.0 mIU/L (Toft and Beckett, 2005). The majority of these represent early CAT (Hashimoto's thyroiditis). Currently, no consensus exists among national organizations as to whether these patients should start therapy during this phase of their disease. Treating those with symptoms seems appropriate, but there is no evidence to support the premise that early treatment alters the disease course or associated comorbidi-ties (hyperlipidemia, hypertension, CAD) (Helfand, 2004).

Studies demonstrate a two to three times higher incidence of atrial fibrillation in patients with subclinical hyperthyroidism compared to individuals with normal sTSH levels (Ross, 2005b). The Framingham data suggest some individuals with subclinical hyperthyroidism are at increased risk of paroxysmal atrial fibrillation (Oiknine and Mooradian, 2006). However, no data support early intervention. Osteoporosis is associated with overt hyperthyroidism, and some speculate that treating subclinical hyperthyroidism may prevent or delay this process. The American Association of Clinical Endocri-nologists (AACE, 2002) recommends treatment of subclinical hyperthyroidism caused by nodular thyroid disease.

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