Up to 15 percent of adults over age 65 have hypothyroidism—an underactive thyroid. This occurs when the butterfly-shaped thyroid gland at the front of the neck produces too little thyroid hormone (thyroxine, or T4) and can slow a wide variety of essential body processes, including your heart rate and metabolism.

Commonly associated with type 2 diabetes, high blood pressure, and high cholesterol, hypothyroidism is treated with a daily dose of synthetic thyroxine (Levoxyl, Synthroid, and generic).

But experts are divided on when it’s appropriate to prescribe the medication and when it's not necessary. Here's what you should know about hypothyroidism.

A Tricky Diagnosis

Hypothyroidism, often triggered by an autoimmune thyroid disorder, progresses slowly and has vague symptoms—cold intolerance, fatigue, memory problems, weight gain, constipation, dry skin—which are often dismissed as normal aspects of aging.

Doctors diagnose the condition by looking for elevated levels of thyroid-stimulating hormone (TSH) in the blood.

But as with most hormones, TSH levels can fluctuate throughout the day. A second test done on another day, but at about the same time, is needed to confirm any elevation. 

The Tussle Over Treatment

Most experts agree that people who have a TSH of 10 milli-international units per liter (mIU/L) or above require medication to normalize thyroid hormone levels.

The decision to treat borderline levels (usually considered 4.2 to 10 mIU/L) is less clear-cut, notes Consumer Reports’ chief medical adviser, Marvin M. Lipman, M.D.­

It’s often based on factors such as the severity of symptoms, a rising TSH, or the presence of an autoimmune disorder.

But one professional organization considers levels as low as 2.5 mIU/L borderline. “There’s been a steady lowering of the upper TSH threshold for treatment initiation,” says Peter Taylor, M.Sc., a researcher specializing in diabetes and endocrinology at Cardiff University in the United Kingdom. “As a result, there has been a massive increase in people on thyroxine.”

Prescribing thyroxine to people with TSH levels below 10 mIU/L may be inappropriate, he says. And research from Cardiff suggests that older adults are the group most likely to be given thyroxine for such borderline TSH test results.  

“We don’t want to give the elderly thyroid hormone unless they really need it,” says Stephanie Lee, M.D., Ph.D., director of the Thyroid Health Center and the Thyroid Nodule and Cancer Research Group at the Boston University School of Medicine. 

That's because like all medications, thyroxine, currently the most frequently dispensed drug in the U.S., can cause side effects. And some of those effects, such as an irregular or rapid heartbeat and bone thinning, can be especially problematic for older adults.

Getting It Right

If you suspect you have hypothyroidism, talk with your doctor and take the following steps for accurate test results:

  • Avoid biotin supplements for at least three days before. They can affect results.
  • Make sure the lab is testing your TSH, not just your T4. The TSH test is much more sensitive.
  • Reschedule if you have an acute illness, such as a cold or diarrhea, which can suppress TSH and affect results.
  • Borderline result? Your doctor should retest before starting you on meds. Use the same lab if possible.

Editor's Note: This article also appeared in the April 2017 issue of Consumer Reports on Health.