Why Synthroid Is Among the Most Prescribed Medications in the U.S.

Synthroid (levothyroxine) is a commonly prescribed medication that treats hypothyroidism (an underactive thyroid). It’s estimated that 5 out of every 100 people in the U.S. have hypothyroidism. And it’s believed that the same number of people have an underactive thyroid and don’t even know it. 

So it’s no wonder that Synthroid is among the top prescribed medications in the U.S. Every year, healthcare providers write almost 100 million prescriptions for medications like Synthroid.

What does Synthroid do?

Synthroid mimics the action of the natural thyroid hormone — T4. When the thyroid gland is underactive, it doesn’t make the amount of T4 the body needs to function properly.

Synthroid is a synthetic (human-made) version of T4. It’s also called “thyroid hormone replacement medication.” That's because it gives your body the thyroid hormone that a healthy thyroid would normally make on its own. The body can use Synthroid in place of T4 for healthy metabolism and other body functions.

What causes hypothyroidism?

Hypothyroidism can occur for many reasons, but Hashimoto’s thyroiditis is the most common. It’s an autoimmune disorder where your immune system attacks the thyroid gland. This can create inflammation and lower thyroid hormone production.

Other common causes of hypothyroidism include:

• Surgery to remove all or most of the thyroid (thyroidectomy)

• Thyroiditis (thyroid inflammation)

• Medications (like lithium or amiodarone)

• Radiation treatments (to treat cancer or thyroid disease)

• Congenital hypothyroidism (low thyroid hormone from birth)

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Another important cause of low thyroid hormones is thyroid cancer. The effect on your thyroid hormone levels depends on the type of cancer and the size of the abnormal growth. 

Your thyroid hormone levels can drop if the cancer surrounds the thyroid, you have radiation treatment, or you have the gland removed. 

Are higher thyroid cancer rates causing more hypothyroidism?

Thyroid cancer rates have increased dramatically over the last 30 years. Currently, there are about 14 per 100,000 people each year who develop thyroid cancer. In the 1990s, this was closer to 6 per 100,000 people. Fortunately, thyroid cancer rates have remained stable over the last decade. 

There is some debate whether thyroid cancer is on the rise or if technology allows for better detection of the disease. Both scenarios may be true. 

There may be some degree of improved early detection with imaging tests. But some studies show an increase in large thyroid cancers and cancers in children. And in these cases, imaging wasn’t used for detection — it was by physical exam. So it remains unclear why thyroid cancer is getting detected more often.

Both factors may be responsible for the increasing thyroid cancer rates. There is also a link between hypothyroidism and certain thyroid cancers. This may in part explain why providers write so many prescriptions for Synthroid and its generic equivalents each year.

Does levothyroxine really work? 

Many people wonder if levothyroxine — the generic version of Synthroid — works as well as the brand-name medication. This may be an area of particular concern if your insurance does not cover the brand-name version. In some cases, Synthroid can be at least two or three times more expensive than the generic version.

In some people, the two versions are not entirely interchangeable. While they contain the same hormones, there may be small differences in how your body responds to them. That’s not to say that one form is more effective than the other. It just may not be easy to switch between the two without changing the dose.

If you need to make a switch between Synthroid and levothyroxine, your provider may check your blood work. This is to make sure your thyroid hormone levels remain consistent. This may also be necessary if you switch manufacturers of your generic levothyroxine.

For some, consistency is key. So it’s important to work with your provider to find the best medication and dose that will work for you.

Does anything affect how well levothyroxine works?

Yes, there are several factors that can change the way Synthroid works in your body. These include internal and external factors. Common issues are:

• Change in medication: Switching brands can affect your medication, as can switching manufacturers. Make sure you ask the pharmacy who the supplier is for your current medication. This can help make sure there is consistency from month to month.

• Weight changes: Changes in your weight can lead to changes in your dosing. This is true whether you lose or gain weight. If your weight has changed and you’re noticing symptoms, you may need to check your thyroid hormone levels.

• Vitamins and supplements: Certain dietary supplements can decrease the absorption of your medication. This includes iron and calcium. Supplements with iodine, like kelp and seaweed, may increase your thyroid hormone levels.

• Medications: You may need to increase your levothyroxine dose if you take certain medications. These include birth control pills and proton pump inhibitors (like omeprazole). When starting a new medication, ask your provider how it can affect your thyroid medication.

• Pregnancy: Hormonal changes in pregnancy can cause big changes in your thyroid medication dosing. When pregnant, your provider may suggest you see a specialist to manage your thyroid hormone levels. 

• Dietary changes: When and what you eat have a huge effect on how well your body absorbs and uses your thyroid medication. Many foods — like fiber, coffee, and nuts — may make your medication less effective. That’s why you should take your levothyroxine on an empty stomach.

Why trust our experts?

Written by:

Karla Robinson, MD

Karla Robinson, MD, is a medical editor for GoodRx. She is a licensed, board-certified family physician with almost 20 years of experience in health through varied clinical, administrative, and educational roles.

Reviewed by:

Sophie Vergnaud, MD

Sophie Vergnaud, MD, is the Senior Medical Director for GoodRx Health. A pulmonologist and hospitalist, she practiced and taught clinical medicine at hospitals in London for a decade before entering a career in health education and technology.

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