Why Synthroid Is the Most Prescribed Drug in the US

two prescription bottles with pills next to them
Katie Mui
Katie Mui is on the Research Team at GoodRx.
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Why is Synthroid (levothyroxine), a medication that treats thyroid hormone deficiencies—or hypothyroidism, as it’s called—the number one drug in terms of the number of prescriptions written by physicians? 

It’s easy to understand why atorvastatin would be on the list. For years, widespread marketing has flooded with TV commercials and website ads for statins as a means (after changes in diet and exercise) to treat high cholesterol. And high cholesterol leads to heart disease, which is the biggest cause of death in the US. So yeah, there are going to be a lot of prescriptions for that.

But a synthetic hormone for thyroid problems — do that many people have serious issues with their thyroid gland?

In 2016, doctors currently handed out 123 million prescriptions for Synthroid and its generic equivalent levothyroxine, slightly edging out atorvastatin (otherwise known as Lipitor) for the top spot.

Causes of hypothyroidism

Let’s start with some basic biology. Normally, the thyroid gland secretes hormones that help regulate metabolism and adjust how fast the body uses energy. With hypothyroidism, the thyroid gland cannot produce enough hormones naturally, so a drug such as levothyroxine replaces what the body doesn’t make itself.

The most frequent cause of hypothyroidism is an autoimmune disorder known as Hashimoto’s thyroiditis, which can occur in 0.15% of the population each year, and can be up to 10 times more common in women than in men. With Hashimoto’s thyroiditis, the immune system attacks the thyroid gland, creating inflammation and decreasing hormone production. Treatment with levothyroxine corrects the problem. Another possible cause of low amounts of circulating thyroid hormones is thyroid cancer, which affects 1.1% of people in the U.S. sometime during their lifetime, and accounted for 3.8% of new cancer cases in 2014. If a large enough tumor engulfs the thyroid, or the gland itself is surgically removed, the levels of the hormones it produces drop. Patients, then, require medication — such as levothyroxine — to boost the amount of circulating hormones back to normal. Still, that means about 5 million Americans need levothyroxine during their lifetime, not per year. So what else might be going on?

Is the rate of thyroid cancer increasing?

According to the Surveillance, Epidemiology, and End Results (SEER) Program, the overall incidence rate of thyroid cancer is on the rise, which may at least partially explain why there are so many prescriptions for levothyroxine written each month. In fact, according to the SEER report, the number of thyroid cancer cases has been increasing 5.5% per year in the U.S. over the last 10 years.

However, a New York Times article questioned whether the apparent increase of thyroid cancer was real or just an artifact of better detecting the disease, such as through ultrasound imaging or needle biopsy of the thyroid. More cases detected doesn’t necessarily mean more people actually have the disease. But it would help explain why there are so many prescriptions.

So is there detection bias here? Let’s look at the death rates over the same period of time. At first blush, the SEER data on thyroid cancer seem to show that the number of deaths from the disease is flat, fueling critics’ assertions that the increased rate of thyroid cancer detection is due to better diagnostics. But upon closer examination, that isn’t quite the case, mostly because there is a discrepancy in the death rate when you separate the data based on certain factors. (For biostats geeks, more details on the why in the endnote*).

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Quick check: Are people searching for information on thyroid disease online?

The popularity of levothyroxine prescriptions is even more confounding when you look for other signals of demand, such as search activity on Google. Search Google Trends and you’ll see that the volume for thyroid-related searches is relatively low compared to other predominant conditions. “Thyroid” and “hypothyroidism” searches occur much less frequently than, say, searches for “depression” or “Hypertension.” Of course, the demographic most affected by thyroid disorders tends to be older, which, according to a Pew Internet study, means that fewer may be searching online for medical information compared to their younger counterparts. Conversely, there is a much larger age span of people affected by hypertension and major depression that could inflate the search volume for these conditions.

Bottom line: levothyroxine works

So what’s the answer? Why is levothyroxine so popular? Maybe it’s just that it seems to work very well. In survey data from Iodine.com, based on over 1,000 people who are taking (or had previously taken) levothyroxine, about 84% of women said that the drug is worth it for them. This demographic nearly coincides with the most common hypothyroidism patient profile: women with a median age of 50.

So put these together and you have a fairly clear hypothesis: high satisfaction with the treatment combined with growing cancer rates combined with higher diagnosis rates equals a lot of people taking a lot of one hormone drug.

One last wrinkle: Many people have concerns that the generic version of levothyroxine doesn’t work as well as the branded Synthroid version. These concerns are especially acute for people whose insurance may not cover the brand drug – which is can be two or three times more expensive than the generic version. Since these are hormones, the two versions are not entirely interchangeable; there has been much controversy over the issues of “bio-equivalence.” Ultimately, it’s important to work with your doctor to find the best drug that works for you, including affordability.

*Endnote: First, gender: Five years after thyroid cancer diagnosis, survival rates for women increased 4.7% from 1974 to 2001, while death rates in men increased by 2.4% from 1992 to 2000. If the increase in incidence rate of thyroid cancer was simply due to better diagnostic tools, proponents argue, we wouldn’t see this dichotomy between female and male survival rates.

Second, geographical location: Healthcare professionals are seeing an increase in thyroid cancers outside of the U.S., in countries that don’t have access to the advanced diagnostic tools that are commonplace in American hospitals. This supports the hypothesis that there is a legitimate increase in the number of thyroid cancer cases not just in the U.S., but worldwide. Last, 10-year survival data: Unlike the 35-year survival data shared by the Center for Disease Control (CDC), looking at thyroid cancer over 10 years shows that death rates from the disease have increased at 1.2% per year from 2001 to 2010, making thyroid cancer the second fastest growing cause of cancer death (liver cancer tops this category). Most other cancers, by contrast, had a decreasing death rate over that decade.

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