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DISEASES OF THE THYROID GLAND

SUBCLINICAL HYPOTHYROIDISM

General Principles

Subclinical hypothyroidism, defined as serum thyroid-stimulating hormone (TSH) level above the upper limit of normal with normal concentrations of free thyroxine (T4), affects a relatively large number of older persons. It is most common in older women, affecting 15% to 20% of women over the age of 75 years, most of whom will demonstrate elevated serum levels of antithyroid antibodies. Older women with serum TSH >6 mU/L and the presence of antimicrosomal and antithyroid peroxidase antibodies will progress to overt clinical hypothyroidism at a rate of 5% to 7% per year. Conversely, older women with a modest increase in serum TSH who are antithyroid antibody negative may undergo spontaneous reversion to a euthyroid state. Several studies have observed beneficial effects of T4 therapy on measures of cardiovascular function and general well-being in patients with subclinical hypothyroidism. However, there remain questions as to the benefits of T4 treatment in patients with minimal clinical symptoms.

Clinical Findings

Older patients with subclinical hypothyroidism may present with few or no complaints. Studies have demonstrated reduced cognitive function, increased intestinal transit time, increased intraocular pressure, higher low-density lipoprotein cholesterol levels, an increased risk for coronary artery and peripheral vascular atherosclerosis, decreased systolic and left ventricular diastolic contractility, and increased risk of congestive heart failure. Subclinical hypothyroidism has been associated with an increased risk of ischemic heart disease, cardiovascular mortality, and all-cause mortality. Older women with atherosclerosis, and an even higher percentage of those with a history of myocardial infarction, have a higher incidence of subclinical hypothyroidism.

Some older patients with severe illness may have transient elevation of serum TSH along with decrease in T4 and T3 due to lowered serum thyroid-binding proteins. Among the illnesses is traumatic brain injury. These findings have been termed sick euthyroid syndrome and can be confused with subclinical hypothyroidism, but the changes in thyroid function tests will usually normalize over several weeks as the patients recover from their underlying illness.

Treatment

Treatment with L-thyroxine compared with placebo results in an overall improvement in general well-being, memory, psychomotor speed, and serum lipid levels. Many studies have documented that L-thyroxine treatment will improve systolic and diastolic ventricular function, left ventricular ejection fraction, and endothelial function while decreasing systolic vascular resistance. L-Thyroxine treatment of subclinical hypothyroidism has recently been shown to be effective in reducing ischemic heart disease events in persons aged 40 to 70 years but not in those over the age of 70 years. Other studies have also shown little benefit of treatment in older persons on cardiovascular events and mortality.

Although some physicians advocate thyroid replacement therapy for all persons with subclinical hypothyroidism, current guidelines recommend that treatment is best reserved for ...

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