41: Thyroid, Parathyroid, & Adrenal Gland Disorders

Diseases of the Thyroid Gland Subclinical Hypothyroidism

General Principles in Older Adults

Subclinical hypothyroidism affects a relatively large number of older persons. Although a small percentage of these cases will progress to overt clinical hypothyroidism each year, individuals with high levels of antimicrosomal antibodies are at greater risk of decline in thyroid function. Several studies have observed beneficial effects of thyroxine (T₄) therapy in patients with subclinical hypothyroidism; no studies have specifically addressed this question in older patients.

Clinical Findings

Older patients with subclinical hypothyroidism may present with few or no complaints. Studies have demonstrated increased intestinal transit time, increased intraocular pressure, higher low-density lipoprotein-cholesterol levels, an increased risk for atherosclerosis, reduced cognitive function, and changes in cardiac performance and congestive heart failure. Older women with atherosclerosis, and an even higher percentage of those with a history of myocardial infarction, have a higher incidence of subclinical hypothyroidism. Treatment with L-thyroxine compared with placebo results in an overall improvement in general well-being. In addition, noninvasive indexes of myocardial contractility also improved, as did memory, psychomotor speed, and serum cholesterol levels. Subclinical hypothyroidism progresses to frank hypothyroidism in 5% to 8% of affected persons each year, with higher rates in those with high levels of antimicrosomal antibodies.

Treatment

Although some physicians advocate replacement therapy for all persons with subclinical hypothyroidism, many believe that treatment is best reserved for those individuals with thyroid-stimulating hormone (TSH) levels >10 mU/L or for those with serum TSH levels between 5 and 10 mU/L with coexisting high levels of antimicrosomal antibodies. If treatment is not initiated, careful follow-up is essential because a percentage of these individuals will develop hypothyroidism each year. The goal of treatment, when initiated, is to normalize serum TSH values as long as the dose of thyroid hormone that is required produces no unwanted clinical effects. Most experts recommend targeting a normal TSH range in older patients, although it should be noted that serum TSH concentrations increase with age and 1 study examining individuals with “extreme longevity” noted serum TSH to be significantly higher in centenarians, with 7.5 mU/L considered to be the true upper limit of normal for those age 80 years and older.