Disorders of the thyroid gland usually involve the failure to produce enough thyroid hormone (hypothyroidism) or the production of too much (hyperthyroidism). Approximately 4.6% of the American population has hypothyroidism and 1.3% has hyperthyroidism. Graves disease, an autoimmune disorder, is the most common cause of hyperthyroidism, with other causes including toxic multinodular goiter and toxic adenoma.

Over the last 30 years, numerous clinical studies have shown that thyroid disorders are associated with an increased risk for manifestations of coronary heart disease (CHD) as well as higher rates of cardiac events, cardiovascular disease (CVD)-related death, and all-cause mortality. For those with existing CVD, disorders of the thyroid gland can worsen old cardiac symptoms or cause new ones. These disorders can accelerate the underlying heart problems. A review, published in the August 2010 issue of Nature Reviews: Endocrinology, compared the cardiovascular risks associated with these disorders.

Subclinical Thyroid Disease

Subclinical hyperthyroidism is a mild form of hyperthyroidism that is diagnosed by abnormal blood levels of thyroid hormones, often in the absence of any symptoms. Subclinical hypothyroidism, also called mild thyroid failure, is diagnosed when peripheral thyroid hormone levels are within normal reference laboratory range but serum thyroid-stimulating hormone (TSH) levels are mildly elevated. This condition occurs in 3% to 8% of the general population. Subclinical hypothyroidism, the most prevalent form of thyroid diseases, is more common in females and its prevalence increases with age, affecting up to 20% of women over the age of 60. Of patients with subclinical hypothyroidism, 80% have a serum TSH of less than 10 mIU/L. With subclinical hypothyroidism, there is a high likelihood of progression to clinical hypothyroidism.

Thyroid Disorders & Cardiac Complications

Some of the most serious complications of hyperthyroidism involve the heart. Common cardiovascular symptoms associated with thyroid conditions include a rapid heart rate, angina pectoris, atrial fibrillation, and congestive heart failure. Cardiac symptoms can be seen in anybody with hypothyroidism but are especially likely in an individual who already has underlying heart disease (Figure). Subclinical hyperthyroidism is associated with important cardiovascular risk factors, yet there are conflicting results on cardiovascular mortality.

The possibility that subclinical hypothyroidism is a cardiovascular risk factor has been a subject of debate. To determine the potential association between subclinical thyroid disorders and CVD, Rohit R. Arora, MD, and colleagues recently performed a systematic meta-analysis of studies that followed patients from 4 to 20 years. The study, published in the March 2008 International Journal of Cardiology, evaluated mortality from CHD, CVD, and all other causes. “Subclinical hypothyroidism was associated with a significant risk of CHD at baseline and at follow-up,” says Dr. Arora. “We also found that all-cause mortality was not increased among patients with hypothyroidism. Mortality from cardiovascular causes was significantly higher at follow-up among those with subclinical hyperthyroidism.”

Diagnosis & Treatment of Thyroid Disorders

Physical examinations may reveal thyroid enlargement, tremor, hyperactive reflexes, or an increased heart rate. Systolic blood pressure may also be high. Blood tests should be performed to measure levels of thyroid hormones. “A major challenge for physicians is to rule out underlying coronary disease and determine if a thyroid condition is present,” adds Dr. Arora. Several clinical differences can help differentiate the causes of hyperthyroidism: age at presentation, clinical features and severity of signs and symptoms, and duration and progression of the disease. Associations with other autoimmune conditions are particularly important, including Graves disease, toxic adenoma, and toxic multinodular goiter (Table).

Undiagnosed and untreated thyroid disorders can result in a wide range of severe cardiac conditions, including atrial fibrillation, cardiomyopathy, and congestive heart failure. A large proportion of the United States population unknowingly has laboratory evidence of thyroid disease. “Screening for early detection of thyroid disorders is important and useful,” says Dr. Arora. “Time is of the essence because most consequences are potentially reversible with appropriate treatment.”

Therapeutic options for patients with Graves disease include thyroidectomy and antithyroid drugs, but these treatments are not as effective as radioactive iodine, which has become the treatment of choice. In clinical hypothyroidism, the standard treatment is levothyroxine replacement, which should be tailored to individuals by taking into account patient preference, presence of symptoms, age, and associated medical conditions. “Subclinical thyroid disease often remains undiagnosed, so it’s important to increase awareness among clinicians,” Dr. Arora says. “It’s also critical that systems of care incorporate regular follow-up surveillance by physicians, that patients are thoroughly educated about the condition, and that patients are actively involved in treatment decisions.”