The endocrine community has long debated whether generic levothyroxin is equivalent to the branded forms in terms of controlling thyroxine blood levels as well as patient symptoms and outcomes. At least as far as cardiovascular impact, a new study indicates they are.

According to an article in Mayo Clinic News Network, “Levothyroxine is the most prescribed medication in the U.S., with more than 23 million prescriptions written annually.” The American Thyroid Association estimates that 20 million Americans have some form of thyroid disease and up to 60% of them are unaware of it. Approximately 80% of all thyroid conditions occur in women, and a decrease in thyroid function is positively correlated with age. 

Levothyroxin addresses hypothyroidism by mimicking thyroxine or T4, one of the hormones produced by the thyroid. Thyroid hormones T3 (triiodothyronine; not included in this study) and T4 (thyroxine) control metabolism throughout the body and affect cholesterol, heart rate, fatigue, control of energy metabolism, mental function, and the function of virtually every organ. 

When the levels of the two thyroid hormones are too low, the body does not clear low-density lipoproteins as efficiently. Low levels also appear to increase intestinal absorption of cholesterol. By increasing blood cholesterol levels, hypothyroidism directly affects cardiovascular health by raising the risk of heart attacks, heart failure, and stroke. As a result, both the American Association of Clinical Endocrinologists and the American Thyroid Association recommend considering levothyroxine to treat even subclinical hypothyroidism in patients under age 65 years who have high cholesterol levels.

"More than 90% of thyroid prescriptions are for levothyroxine, and there has been disagreement as to whether generic levothyroxine and branded thyroxine preparations are equivalent,” said Robert Smallridge, MD, a Mayo Clinic endocrinologist and the study's principal investigator. “These findings suggest that generic and brand levothyroxine therapy are similar as related to cardiovascular events risk."

The Mayo Clinic study analyzed deidentified patient data from an administrative claims database co-founded by Mayo Clinic and OptumLabs Data Warehouse in 2012. The study followed 87,902 patients from around the United States for an average of 1 year. The patients were divided into two cohorts that were balanced for age, gender, diabetes, prior pertinent history, and hypertension, among other factors. The study analyzed hospitalization for four types of cardiovascular events: heart attacks, congestive heart failure, atrial fibrillation, and stroke. 

This study found generic and brand-name levothyroxine to be equivalent in terms of risk for these cardiovascular outcomes over a 1-year period. Additional study with longer term event rates and higher risk patient populations still needs to be conducted to confirm these findings. Dr. Smallridge advised all patients with thyroid issues to maintain close contact with their healthcare provider, seeking an office visit and bloodwork at least annually. 

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