Older people may too often be diagnosed with an underactive thyroid and prescribed thyroid hormones, which can cause new troubles and expenses without improving their lives, researchers say.

A recent case study provides a snapshot of the larger problem, the authors write in JAMA Internal Medicine.

Just 1 percent to 2 percent of people have hypothyroidism, in which their thyroid gland is underactive and requires treatment, coauthor Dr. Juan P. Brito of the Mayo Clinic in Rochester, Minnesota, told Reuters Health. But about 15 percent of people have "subclinical hypothyroidism" - hormone levels that are between the healthy range and the diagnostic cutoff for hypothyroidism and that cause few or no symptoms.

Clinical hypothyroidism can cause constipation, depression, fatigue, dry skin, unexplained weight gain and greater sensitivity to cold.

The authors present the story of a 72-year-old obese man with type 2 diabetes, high blood pressure and heart disease who complained to his doctor of fatigue. The doctor ordered a hormone panel and found a blood thyroid-stimulating hormone (TSH) level of 7.2 microunits per milliliter (mlU/L).

TSH triggers the thyroid to make hormones that control metabolism, so elevated TSH is considered a sign the thyroid might be making too little thyroid hormone. The healthy TSH reference range for adults is usually 0.3 to 5.0 microunits per milliliter.

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The elderly patient was prescribed 75 micrograms daily of levothyroxine, available under brand names like Synthroid and Novothyrox, to treat subclinical hypothyroidism.

A month after starting the medication, the patient came to the emergency room with heart palpitations, labored breathing and chest pain. A new hormone panel found his TSH levels had dropped to 0.1 mlU/L.

Doctors brought his heart rate under control and stopped levothyroxine therapy. Three months later his heart rhythm had returned to normal and his TSH level was 5.6 mlU/mL. That was still slightly above healthy reference range, but considering the pros and cons of treating subclinical hypothyroidism, he and his clinicians decided not to restart levothyroxine,

"There is some evidence linking subclinical hypothyroidism to cardiovascular events," Brito told Reuters Health. "But there's no really good argument or data to support treatment."

Despite that, levothyroxine is the most prescribed drug in the U.S., he said. A 30-day supply of generic levothyroxine costs about $13, according to Healthcare Bluebook.

"This is what happens when we pay too much attention to values and not to the patient," Brito said.

The medication is relatively safe but may push TSH levels so low that it causes side effects, like the ones this elderly patient experienced, he said. Not all subclinical hypothyroidism may require drug treatment, especially for the elderly.

"I don't think it's the spirit of the guideline to treat everyone," Brito said.

For the elderly, the healthy reference range may be higher than the adult range - having a TSH of 5.6 mlU/L may still be healthy, he said.

Healthy ranges may also differ by individual, said Dr. Jennifer Mammen of Johns Hopkins School of Medicine, who was not involved in the case study.

Testing TSH levels more than once in a three to six-month period may also give a clearer picture of whether the thyroid is truly underactive.

"Our recommendation for diagnosis of subclinical hypothyroidism is to check the levels twice with a separation between them," Mammen told Reuters Health by phone.

"The vast majority of people with symptoms (like fatigue) do not have thyroid problems to begin with," she said.