Complete tests for thyroid function include three measurements: for the hormones thyroxine (T4) and triiodothyronine (T3) produced by the thyroid itself, and for thyroid-stimulating hormone (TSH, also called thyrotropin) produced by the pituitary gland to regulate the thyroid. Production of thyroid hormones requires iodine in the diet, found in high amounts in iodized salt, eggs, sea vegetables like kelp, fish and shellfish from ocean waters and unpasteurized dairy products.

Even when blood levels of thyroid hormone are normal, if the level of thyroid-stimulating hormone is low (so-called subclinical hyperthyroidism), serious problems can result. In a Swiss study of 70,298 men and women followed for a decade, among the 2,219 who had subclinical hyperthyroidism, the risk of fractures – especially hip fractures – was significantly elevated.

Likewise, subclinical hypothyroidism (normal levels of thyroid hormone but too much thyroid-stimulating hormone) may raise the risk of heart problems, especially in young and middle-aged adults. Treatment of this condition with the drug levothyroxine may reduce this risk, according to a British study.

Although the United States Preventive Services Task Force has not yet found sufficient evidence to recommend routine thyroid screening of people without obvious symptoms, the American Association of Clinical Endocrinologists believes thyroid levels should be routinely measured in older people, especially women. And the American Thyroid Association recommends screening adults for TSH starting at age 35 and repeating the test every five years.

Experts believe that between 40 percent and 60 percent of people with thyroid disease do not know they have it. Yet, even for subclinical disorders, proper diagnosis and relatively simple treatment of abnormal thyroid levels can result in a much improved quality of life.

People found to have low levels of thyroid hormone can be easily treated with a daily pill of synthetic hormone, levothyroxine (marketed under such names as Synthroid and Levoxyl), starting with a low dose and gradually increasing (or decreasing) the amount as indicated by careful monitoring. Patients often experience diminished symptoms after about two weeks of hormone treatment.

However, diagnosing subclinical hypothyroidism can be tricky. Dr. Robin P. Peeters of Erasmus Medical Center in the Netherlands recently noted in The New England Journal of Medicine that about 75 percent of patients with this condition have test results that suggest only mild thyroid failure, which doctors may not consider serious.