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Synthroid: The Hardest Working Drug Around?

Medically reviewed by Leigh Ann Anderson, PharmD. Last updated on Jan 7, 2022.

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.

Brand names for levothyroxine include:

Although brands and generics exist for levothyroxine, you should stick to the same product and not switch around from month to month, without approval from your doctor. The FDA has determined that certain levothyroxine products are interchangeable and can be substituted at the pharmacy. Many products are now available that are quite affordable, and may cost even less than your insurance copay. Ask your pharmacist.

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.
  • Over time, doctors and patients noticed that different batches of Synthroid seemed to produce uneven clinical outcomes or side effects in patients.
  • 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 switch to a less expensive generic?

  • According to FDA, interchangeable generic levothyroxine drugs are as effective as the brands. However, in the past, some patients have voiced concerns about the generic effectiveness, especially when manufacturers were switched from month to month at the pharmacy.
  • In 2007, FDA tightened it potency specifications for all levothyroxine products - brand and generic.
  • Today, medical experts state that either a brand name or generic formulation of levothyroxine is acceptable, but it's best not to switch around.
  • Talk to your doctor first 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. If you do switch (or the pharmacy substitutes with a different manufacturer) a serum TSH level may need to be evaluated by your healthcare provider in 6 weeks to be sure it is still in the therapeutic range.

Other Options for Hypothyroidism

Way 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 shut down. Levothyroxine is often manufactured in these plants and the storms caused a disruption in the supply.

Today, the FDA no longer lists levothyroxine as a drug in short supply. The American Thyroid Association states that levothyroxine should remain the standard of care for treating hypothyroidism.

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

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.
  • You can view Synthroid images and colors here.

Alternatively, if you prefer generic levothyroxine, be sure to tell your doctor and 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 or Synthroid 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 can lead to or worsen hypothyroidism as a possible side effect or drug interaction.

Examples include:

That's why it's important to give a complete history of all the medications you take to your doctor and pharmacist. 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

Here is a more common drug interaction you should be on the lookout for. Those that bind with and lower the absorption of levothyroxine. This will prevent your drug from doing its job.

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 of levothyroxine for a pet. Only give the dose your vet prescribes for your pet.

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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  • Brito JP, Ross JS, Sangaralingham L, et al. Comparative Effectiveness of Generic vs Brand-Name Levothyroxine in Achieving Normal Thyrotropin Levels. JAMA Netw Open. 2020 Sep 1;3(9):e2017645. doi: 10.1001/jamanetworkopen.2020.17645.
  • Benvenga S, CarlĂ© A. Levothyroxine Formulations: Pharmacological and Clinical Implications of Generic Substitution. Adv Ther. 2019;36(Suppl 2):59-71. doi:10.1007/s12325-019-01079-1
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  • AbbVie. Synthroid Package Insert. Revised 7/2020. Accessed January 27, 2021 at
  • FDA Consumer. Thyroid Medications: Q and A with Mary Parks, M.D. Accessed January 7, 2022 at
  • After 46 Years of Sales, Thyroid Drug Needs F.D.A. Approval. New York Times. Accessed January 7, 2022 at
  • LaFlore A. Walmart Pharmacy increases drug prices, blames shortage on Hurricane Maria January 7, 2022 at

Further information

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