A million Americans may take drugs with hypothyroidism that they don’t need.

As many as 90% of people taking Levothyroxine [synthroid] may have been unnecessarily prescribed medications for hypothyroidism.

About 23 million Americans take levothyroxine, making it one of the most prescribed drugs in the country 1. To test for hypothyroidism (hypothyroidism), clinicians measure levels of thyroid-stimulating hormone (TSH) and free thyroxine (free T4).

However, the new study points to the natural seasonal variations of these hormones that are not taken into account in current laboratory tests. This leads to a huge number of unnecessary levothyroxine prescriptions, and its overuse poses a risk to patients, Dr. Joe El-Khoury, Associate Professor of Experimental Medicine, warns in a letter published in Clinical Chemistry on 3/1.

“The new evidence is of great concern, especially in older people over the age of 80, as they are actively providing patients with unwanted medications that can potentially cause serious side effects,” said El-Khoury.

How hypothyroidism is diagnosed

Patients suffering from hypothyroidism may experience symptoms such as fatigue and depression, and in extreme cases, the condition can be debilitating.

The provider first diagnoses hypothyroidism by measuring TSH levels in the blood. If the level is rising, they will conduct a second test examining the free T4 level. If TSH is high and free T4 is low, the provider diagnoses patients with hypothyroidism. Levothyroxine works with exogenous T4 by supplementing with low hormone levels.

However, some patients have slightly elevated tsh and normal free T4. Providers diagnose this as asymptomatic hypothyroidism and may also be treated with levothyroxine to lower TSH. This habit can do more harm than good. “Post-study studies have shown that in patients who do not need levothyroxine, over-treatment with levothyroxine increases the risk,” says El-Khoury.

A 2021 study involving collaborators from the Yale School of Public Health, the Mayo Clinic, and the University of Arkansas found that of the 2,300 million Americans who are actively taking levothyroxine, about 2,100 million [-90%] are more likely to not need a prescription. In extreme cases, cardiovascular morbidity and death, especially in patients over the age of 80, the finding is alarming, El—Khoury said, because levothyroxine can cause side effects such as anxiety, heat intolerance and diarrhea.

Diagnosing hypothyroidism may depend on the season

According to El-Khoury, the main reason for improper diagnosis is seasonal fluctuations. Usually TSH is higher in winter. This is even when more people may experience symptoms of seasonal affective disorder (SAD), which can have symptoms similar to hypothyroidism. “But if many of those same people had taken the same test a few months later, it would have come back as normal,” says El-Khoury, “because the current means of screening do not account for natural changes, and many patients showing similar signs of hypothyroidism have naturally higher levels of TSH.”

El-Khoury raises awareness about the extent of this problem and encourages providers to retest their patients after at least 3 months before starting levothyroxine treatment, and in addition, he said laboratory organizations, such as the American Thyroid Association and the AACC Academy of American Clinical Chemistry, need to carefully consider and agree on the clinical decision limits of TSH.

The assay used in Yale defines the normal limit of TSH as 0.27-4.2mIU/L.However, recent studies have suggested that levothyroxine has no benefit for patients who start treatment if the level is naturally less than 7.0 Miu/L.”This evidence needs to be reviewed and agreed upon by these medical associations so that changes can be made nationally and possibly internationally,” he says.

For many, raising the limit from 4.0Miu/L to 7.0Miu/L may seem like a big change. However, the new findings emphasize that the scope currently accepted is no longer supported by evidence and that existing literature needs to be reevaluated. “TSH has so high biological variability that it has not been considered for many years,” says El-Khoury. “We need to reconsider what we thought was normal.”

For patients currently taking levothyroxine El-Khoury urges them not to stop taking levothyroxine without consulting a medical professional. “If your TSH value was less than 7mIU/L when you started treatment, ask your doctor if levothyroxine is really suitable for you,” he says.