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Synthroid: The Hardest Working Drug Around?

Medically reviewed by L. Anderson, PharmD Last updated on Jan 24, 2019.

Has Your Metabolism Slowed to a Crawl?

Your thyroid gland sits inside the lower front part of your neck below your Adam's apple, and you probably don't even notice it. But it's an important gland. Your metabolism - the rate at which the body produces energy from nutrients and oxygen - is controlled by the thyroid hormone which is made by this gland.

But things can go arwy. When the thyroid doesn't make enough hormone, your metabolism can slow to a crawl and cause a condition known as hypothyroidism -- or underactive thyroid. The good news? It's very treatable.

What Are the Symptoms of Low Thyroid Hormone?

Low thyroid hormone production can lead to many unpleasant and dangerous side effects. Various organs in the body can be affected. Symptoms of low thyroid might include:

  • Weight gain
  • Fatigue, a constant tired feeling
  • Mood swings, confusion or depression
  • Dry skin or coarse hair
  • Constipation
  • Being sensitive to cold temperatures
  • Developing a hoarse voice and slowed speech
  • Numbness in the hands and fingers
  • Headaches

Who's At Risk for Low Thyroid?

If you have hypothyroidism, you are not alone, but you are more likely to be female.

In fact, women are 5 to 8 times more likely than men to have thyroid problems. One out of every eight women will develop a thyroid disorder during her lifetime. People over 50 years of age are at higher risk, too.

Hypothyroidism occurs for various reasons. It could be due to an immune system disorder, an infection, a tumor, or even a medication. Hypothyroidism is a life-long condition that will usually require a thyroid hormone replacement medication like levothyroxine sodium (Synthroid, Levoxyl, and others).

What Are the Brand Names for Levothyroxine?

Levothyroxine sodium (Synthroid), approved in the 1950's, is a veteran drug that replaces the thyroid hormone in those with hypothyroidism. Levothyroxine also helps to decrease the size of an enlarged thyroid gland (known as a goiter) and can be used to treat thyroid cancer.

Other brand names for levothyroxine include Levoxyl, Tirosint, or Unithroid; Levothroid has been discontinued.

Synthroid, from manufacturer AbbVie, is one of the most commonly prescribed brand name drugs in the U.S., and has been for many years.

Although brands and generics exist for levothyroxine, you should stick to the same product -- they are not interchangeable.

When Was Synthroid Approved?

When Synthroid first came into use in 1955, it was not actually "FDA-approved" the way we think of approved drug products today.

Here's what happened: In 1962, the Food and Drug Act was revamped and the approval process became more rigorous. Because Synthroid had been on the market for so many years, it was "grandfathered" into use -- no effectiveness data was needed -- along with about 30 other drugs.

However, over time doctors and patients noticed that different batches of Synthroid produced uneven results. Eventually, Abbott, who then owned Synthroid, submitted an NDA, and Synthroid was officially "FDA-approved" in July 2002.

Is It OK to Use Generic Levothyroxine?

If you take Synthroid, you may be wondering: "Should I should switch to a less expensive generic?

According to FDA, interchangeable generic levothyroxine drugs are as effective as the brands. However, in the past, some doctors and patients voiced concerns about the generic effectiveness, especially when manufacturers were switched from month to month.

In 2007, FDA tightened it potency specifications for all levothyroxine products - brand and generic. And guidelines from the American Thyroid Association confirm that levothyroxine (brand or generics) should remain the treatment of choice for hypothyroidism. Talk to your doctor about switching to the generic if you use the brand, and vice versa. Otherwise, it's best to stay with the same product from month to month.

Other Options for Hypothyroidism

Back in 2017 when the major hurricanes hit Puerto Rico, there was a shortage of levothyroxine for a time, as some of the manufacturing plants for this drug were hit. Levothyroxine is often manufactured in these Puerto Rican plants and the storms caused a disruption in the supply. The FDA no longer lists levothyroxine as a drug in short supply.

However, if you find you cannot fill your prescription for whatever reason, including a shortage, contact your doctor. There are other options that your doctor could prescribe to keep your thyroid levels steady. You may need to be followed with blood tests or get a new prescription stating the new drug and dose.

Other brands used for hypothyroidism, based on availability, might include:

What Does Synthroid Look Like?

To prevent possible fluctuating blood levels of levothyroxine, many healthcare providers recommend that you stay with the same brand or generic manufacturer and don't switch between the two. For example, if you are on Synthroid, be sure you check your prescription before you leave the pharmacy to identify the correct Synthroid tablet, color, strength, and manufacturer. If you only want the brand name product, be sure your doctor writes "dispense as written" on the prescription.

Alternatively, if you prefer generic levothyroxine, be sure to tell your pharmacist, and ask for the same manufacturer each time. You might need to call ahead to double check on this before going to the pharmacy.

Levothyroxine Dosing: It's Important

Managing your underactive thyroid means your dose needs to be precise, day after day. Why is dosing so important with levothyroxine?

These products, including Synthroid, are considered narrow therapeutic index drugs. With a narrow therapeutic index drug, if your dose doesn't stay even, you could experience side effects or your underactive thyroid might get worse.

Your doctor will determine the best dose based on your weight and adjust it as needed. Take your levothyroxine the same time each day -- most people follow a routine and take it on an empty stomach 1/2 to 1 hour before breakfast with a full glass of water.

Your Dose: Keep It In Check

Once you get your dose right, it's important to keep it that way. Most patients with an underactive thyroid will remain on levothyroxine for a lifetime. Don't abruptly stop your medication. Your doctor will monitor your levothyroxine dose by checking your thyroid stimulating hormone (TSH) levels with a blood test. The right dose will help to minimize side effects and keep your thyroid in check.

Common side effects due to excessive doses include:

  • Nervousness
  • Weight loss
  • Fast heart beat
  • Irritable mood
  • Anxiety

Medicines That Can Cause Low Thyroid Hormone

Certain medicines, such as lithium, amiodarone (Cordarone), interferons, and bexarotene (Targretin) can lead to or worsen hypothyroidism.

That's why it's important to give a complete history of all the medications you take to your doctor. That includes prescription drugs, over-the-counter medications, and even vitamins and herbals. It's amazing how many drug interactions can occur, even with "natural" herbal products.

Always have your pharmacist run a drug interaction screen each time you start or stop a medication, too.

Synthroid Drug Interactions Can Affect Absorption

A more common drug interaction you should be on the lookout for? Those that bind with and lower the absorption of levothyroxine. Iron, calcium and aluminum may commonly be found in vitamins, minerals, and antacids, so be careful with over-the-counter (OTC) medicines. Drugs that may bind with Synthroid and lower absorption include:

Can Dogs Get Hypothyroidism?

Humans aren't the only ones that can have low thyroid levels. Our family pets can have this condition, too.

Dogs are most commonly affected by hypothyroidism, but cats and horses can also be affected rarely by the gland disorder.

In dogs, the symptoms are similar to what you see in humans: tiredness, cold intolerance, no interest in playing or exercise, and weight gain.

Luckily for man's best friend, thyroid replacement therapies work well in dogs. But doses are different, so never use a human dose for a pet; always check with your vet.

Join a Group. Be a Contributor.

New to thyroid treatment? Or maybe you are a veteran with years of experience and advice. Either way, consider joining the Thyroid Support Group and the Levothyroxine Support Group.

Thyroid treatment is a lifelong process, so lean upon your healthcare team and those with similar experiences for support. Here, you can voice opinions, raise concerns, and follow along with the up-to-date medical news that tops the headlines. Empower yourself with the knowledge you need to keep your thyroid working well.

Finished: Synthroid: The Hardest Working Drug Around?

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  • AbbVie. Synthroid Package Insert. Accessed 1/31/2018 at
  • FDA Consumer. Thyroid Medications: Q and A with Mary Parks, M.D. Accessed January 24, 2019 at
  • After 46 Years of Sales, Thyroid Drug Needs F.D.A. Approval. New York Times. Accessed January 24, 2019 at
  • Standard Treatment for Underactive Thyroid Gland Still Best: Experts. Accessed January 24, 2019 at
  • Levothyroxine shortage reported by Walmart, causing drug’s price to temporarily increase. Accessed January 24, 2019 at
  • LaFlore A. Walmart Pharmacy increases drug prices, blames shortage on Hurricane Maria.

Further information

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.