High blood levels of T.S.H. indicate that the thyroid gland is less active than it should be. Usually, doctors order a blood test to measure the hormone when patients complain of symptoms suggestive of a failing thyroid — fatigue, constipation, weight gain, muscle weakness or cold sensitivity.

Often, though, doctors simply order T.S.H. readings as part of routine blood work.

“As people get older and see doctors more often, they’re more likely to get these tests,” said Dr. Caleb Alexander, co-director of the Johns Hopkins Center for Drug Safety and Effectiveness.

True or “overt” hypothyroidism can cause intense symptoms and, if untreated, can lead to heart disease and other threats. People with T.S.H. readings over 10 milli-international units per liter of blood, and who have low levels of other thyroid hormones, generally get a prescription for levothyroxine — and need one.

The questions arise when T.S.H. is only slightly higher than normal. Is that a disease?

A slightly elevated T.S.H. may represent a normal consequence of aging. Or a temporary problem. Patients with these test results rarely develop serious hypothyroidism.

Yet millions of older Americans, diagnosed with subclinical hypothyroidism based on T.S.H. numbers, are taking a medication that now appears pointless.

“We were very aware the practices we adopt weren’t really based on good evidence,” said Dr. David Stott, a geriatrician at the University of Glasgow who led the new study, conducted with 737 adults over age 65 in Scotland, Ireland, the Netherlands and Switzerland.

He and his colleagues found subjects (average age: 74) through laboratory databases indicating that physicians had tested their T.S.H. at least twice. All had higher-than-normal T.S.H. levels and, on questionnaires assessing tiredness and other hypothyroid symptoms, nearly all expressed some of the standard complaints.