As thyroid function declined, the study found, patients reported more symptoms. But, while there was a positive association between the proportion of symptoms reported and progressive thyroid failure, this distinction was not as clear as would have been expected. In addition, no one symptom was a clear indicator of thyroid failure.

"The link between all stages of hypothyroidism and cardiovascular health, and the vague correlation between symptoms and disease state, points to the need for more widespread thyroid stimulating hormone (TSH) testing and more aggressive treatment, especially for subclinical patients," said E. Chester Ridgway, MD, head of the Division of Endocrinology at the University of Colorado Health Sciences Center.

Study Findings

The study was conducted to determine: the prevalence of abnormal thyroid function; the relationship between thyroid function and lipid levels; and the connection between thyroid failure and the presence of symptoms.

Prevalence

The study found that of the 25,862 participants, 11.7 percent had abnormal serum TSH levels. Evaluating incidence according to over- and underactive thyroid cases, 2,450 patients or 9.5 percent had an underactive thyroid (hypothyroidism) and 570 or 2.2 percent of the population had an overactive thyroid (hyperthyroidism).

"Surprisingly, the prevalence of hypothyroidism was higher than expected," Dr. Ridgway said. "Based on previous data, we suspected five to ten percent of the population had a failing thyroid gland. But these results here showed that hypothyroid prevalence was on the higher end -- closer to 10 percent."

Among patients not taking thyroid medication, 8.9 percent were hypothyroid and 1.1 percent were hyperthyroid. This indicates 9.9 percent of the population had a thyroid abnormality that had most likely gone unrecognized. When extrapolated to account for national demographics, there may be 13 million Americans with an undiagnosed thyroid condition.

The percentage of patients with hypothyroidism was greater for women for each decade of age after age 34.

Thyroid Disease & Cholesterol

A higher proportion of clinically hypothyroid patients had elevated total cholesterol levels as compared to those with normal thyroid function. While it has been known for decades that overt hypothyroidism contributes to elevated cholesterol levels, this is the largest study to show that the cholesterol levels among patients with mildly decreased thyroid function were significantly higher than the cholesterol levels in euthyroid patients.

Average total cholesterol levels for patients with overt hypothyroidism were 251 mg/dL and the average total cholesterol levels for subclinical hypothyroid patients were 224 mg/dL -- both above 200 mg/dL, the marker used to indicate elevated cholesterol levels that warrant medical attention. Because the connection between hypothyroidism and cholesterol is so clear, the National Cholesterol Education Program and the U.S. Food and Drug Administration recommend thyroid testing in patients with high cholesterol levels.

"This study was novel in that it drew a clearer connection between mild or early stages of thyroid failure and its effect on cholesterol levels," Dr. Ridgway said. "It showed that as the thyroid gland fails and less thyroid hormone is produced, blood cholesterol levels rise. This has serious long-term consequences for the patient's health particularly in the area of cardiovascular disease."

Symptoms Scales as Indicators to Thyroid Disease

Overt hypothyroid patients reported a greater percentage of symptoms than did the subclinically hypothyroid group. Both overt and subclinical patients reported more total symptoms than euthyroid individuals. But no one symptom was a predictor of thyroid failure. While there was an increase in the likelihood of thyroid disease as the number of reported symptoms increased, these symptoms are often vague and develop slowly so they go un-noticed.

"Thyroid symptoms are so common and are often mistaken for signs of aging, menopause, depression or stress," said Gay Canaris, MD, assistant professor of internal medicine, University of Nebraska Medical Center. "Since we can't rely upon reported symptoms alone to detect disease, we as physicians should be conducting more thyroid testing."

Study Design

This cross-sectional study evaluated the largest-ever patient population. Participants were solicited from the annual statewide health symposium in Colorado which provides testing for hypertension, colon cancer, glaucoma and skin cancer. In 1995, sensitive tests of thyroid function were added to the panel of blood analyses, and a questionnaire for hypothyroid symptoms was included with the survey. Demographics and thyroid function analyses for 25,862 patients, representing 111 sites, were quantified and reported in this study.

The Thyroid Health Survey included a symptoms questionnaire that evaluated traditional thyroid symptoms and asked the patient to further identify each symptom as "current" (present at the time of the survey) or "changed" (symptom that emerged within the past year). A symptom index was calculated in the manner of Billewicz, et al. The survey also included questions on personal history, family history and demographics.

Serum TSH concentrations were measured by third-generation immunochemiluminescent assay. Normal range was a TSH level between 0.3 and 5.1 mIU/L, subclinical hypothyroidism was characterized by an elevated TSH level (greater than 5.1 mIU/L) and a normal T4, and overt hypothyroidism was evaluated as an elevated TSH level (greater than 10.0 mIU/L) and a decreased T4.

The Critical Role of the Thyroid Gland

The thyroid gland plays a vital role in overall body function during all stages of life. Although relatively small, it produces a hormone that influences every cell, tissue and organ in the body. The thyroid regulates the body's metabolism -- the rate at which the body produces energy from nutrients -- and affects heart rate, energy and mood. If a person has normal thyroid function, they are considered to be euthyroid.

When the thyroid gland is not working properly, it can become either underactive (resulting in hypothyridism) or overactive (resulting in hyperthyroidism). Signs and symptoms of an underactive thyroid include fatigue, depression, forgetfulness, unexplained weight gain, and menstrual irregularities. An overactive thyroid is marked by irritability/nervousness, sleep disturbances, unexplained weight loss, muscle weakness and vision problems. If left untreated, thyroid disease may lead to an increased risk for heart disease, osteoporosis and infertility.

Thyroid disease can strike anyone at any time, but is more common in women. One woman in eight will develop a thyroid disorder during her lifetime. Incidence also increase with age -- by age 60, more than 20 percent of American women will have a thyroid disorder.

Thyroid disease can be diagnosed through a simple blood test called a TSH (third generation thyroid stimulating hormone). Once diagnosed, hypothyroidism can be treated with a synthetic hormone replacement tablet (levothyroxine sodium tablets, USP), taken once-a-day.

SOURCE: University of Colorado