levothyroxine and Synthroid

Levothyroxine, or T4 thyroid hormone, is a very old medication. Scientists first extracted this compound from pig thyroid glands more than 100 years ago, but no drug maker marketed the synthetic form of the hormone until the 1950s. When it was introduced under the brand name Synthroid, endocrinologists embraced it. They believed that a synthetic product would vary less from lot to lot than a biological medication. Consequently, once they established a patient on an appropriate dose, they wouldn’t have to keep adjusting it. Brand name Synthroid can be pricey, however, so many people take a generic formulation. Do patients have any problems when they have to switch generic levothyroxine?

How Often Do You Switch Generic Levothyroxine?

Q. I have taken generic levothyroxine for many years after my thyroid was removed. Recently, my pharmacy chain changed the manufacturer. I was taking Mylan and was told it was no longer available in my strength (125 mcg). I was switched to Lannett. Then after only two months, I was switched again to Sandoz.

So I have taken levothyroxine by three different manufacturers in less than six months. I’ve heard it is not good to change manufacturers. Can you tell me if this is a problem? If so, what can I do about it?

What Happens If You Switch Generic Levothyroxine?

A. It doesn’t sound as if you have experienced symptoms of under- or overdosing from these switches. That is the main concern: while generic manufacturers can generally maintain consistency within their lines of levothyroxine, the dose isn’t always comparable between generic drug makers. Nor can doctors simply switch generic levothyroxine in for brand-name Synthroid at the same dose and assume all will be well. They may need to do testing and adjust the dose of the new formulation until symptoms disappear.

If you’ve not experienced problems, you don’t need to do anything. If, however, you have had difficulty with symptoms of inadequate or excess thyroid hormone, you may need to find a pharmacy willing to help. It should be able to supply you with levothyroxine from the same generic manufacturer for a reasonable time.

Symptoms of Inadequate Thyroid Hormone:

Many people with hypothyroidism complain of constant fatigue, muscle pain, dry skin, thinning hair, constipation, brain fog, depression and other problems. When they take an appropriate dose of levothyroxine, such symptoms should fade. (Some individuals feel best if they get some T3 along with the T4.) If they must switch generic levothyroxine formulations as you have had to, the dose may not be comparable. Consequently, they may suffer the same symptoms.

Signs of Excess Thyroid Hormone:

On the other hand, sometimes the opposite happens when patients switch generic levothyroxine. They may be troubled with heart palpitations, tremor, diarrhea, excessive sweating, anxiety, insomnia and high blood pressure, among other problems. Sometimes they notice these symptoms when they switch from one generic formulation to another. The formulations may not be strictly equivalent to each other in the way they affect the body.

Learn More:

You can learn more about the treatment of hypothyroidism, the use of brand-name vs. generic levothyroxine and the controversy over desiccated thyroid hormone in our Guide to Thyroid Hormones. You may also wish to listen to Show 1096: What You Need to Know About Treating Thyroid Disease.

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  1. Patricia
    Melbourne, FL

    I have taken Levothyroxine for over 30 years. Every so often the pharmacy changes manufacturers, but I have never had any problem. I have a blood test twice a year and other than lowering from 1 several years ago to .75, the tests are always right on the button.

  2. Galen

    and what do you do when the insurance company says NO? The insurance company and sometimes I think that the pharmacy decides to put you on a generic to save them money.

  3. Sarah

    I started on Synthroid at the age of 13. That’s 50 years ago! Stayed on the brand name until I was in my 40’s, then switched to levothyroxine to save money. It wasn’t long until I was just dragging around. Fortunately, the PA I saw put me back on Synthroid. What a difference! Fast forward to a couple of years ago, I could tell something wasn’t right so I went to a more naturopathic practitioner who put me on NP thyroid. It took quite a while to find the right dosage and a while to adjust, but things are great now! I even lost a few pounds without really dieting.

  4. B
    North Carolina

    When I first started taking thyroid meds, the pharmacy gave me generic. When I refilled the prescription, the new pills knocked me out. Three days later I went to the pharmacy and asked to see the box they came from. There was a large red label that said, “NEW FORMULA.” Switched to synthroid that day. Having said that, it took 10 years to determine the right dose and add T3 (I had to insist on being tested on that). I now take Synthroid and compounded, extended release T3, and seem to be okay. When I tried Armour, I got an allergic reaction.

    Unless you have thyroid issues, you have no idea of the long term struggles to find:
    1) a doctor who will work with you.
    2) a good pharmacist who will work with you.
    Not easy at all. I have found endocrinologists to be the most stubborn.

  5. judith

    After using Synthroid for about 15 years, I decided to try to a generic version. I felt tired and crabby and constipated for at least two weeks and switched back to Synthroid. In addition to that, the pills were tiny brown things that would dissolve in my mouth before I could swallow them. I called the manufacturer (can’t remember which one) and was told to use more water to take them. They treated me like a baby who doesn’t know how to take meds. I do order mine from the Canadian Pharmacy, and the cost is about $26 for 90 tablets. Even adding the $15 S & H, the cost is less than for 30 tablets in the USA.

  6. Yvonne S.

    I agree with the 2 commenters here. I was put on Levo, when first diagnosed with hypothyroidism. I never felt good on it at any dose. Then a couple of years later, I went to a ND and she put me on Nature Thyroid. What a blessing! I am doing great on it, and she has no trouble carrying it, yet it is unavailable from any pharmacy now. I was getting it at Kroger at first because my insurance paid part. Now I pay for it fully, and I don’t care. Shame on the medical kingdom for playing games with our health!

  7. Yesendi

    I agree with the above comments. I do take Levoxyl because my Dr WILL NOT prescribe NDT. I switched to Levoxyl because Levothyroxine was making me extremely sick due the lactose as one of the “inactive” ingredients. I am lactose and gluten intolerant so knowing the inactive ingredients helps.

  8. JANE S.
    St. Louis, MO


  9. Rebecca

    For years I’d been told not to switch levothyroxine suppliers, so dutifully I always requested Mylan Labs. My otherwise very stable Hashimoto’s then became – for a decade – highly unstable. Finally this winter, weather conditions (we knew we were about to be snowed in without access to our mail) necessitated I get the prescription from my local pharmacy, which only carries Amneal levothyroxine.

    Son of a gun, all my crazy thyroid levels have calmed completely. My husband did some research, and discovered that Mylan outsources levothyroxine to another company, Westminster. Westminster has had many, many problems with inconsistent and impure dosages of its levothyroxine – and yet Mylan keeps selling it! (This is similar to the problems many have with Armour. Yes it’s “natural,” but I know three people who went to the ER with thyroid storm, a result of Armour. Take at your own risk.)

    It’s going to cost me a little more, but I’m staying with Amneal.

  10. Rebecca

    Rather than go into my rather long, frustrating and life-altering history of undiagnosed Hashimotos Disease, I will simply say that I am disappointed to read your comments on synthetic Levothyroxine. When I was diagnosed, Synthroid was prescribed and I took it for 5-6 years in varying doses, all with inadequate results. My mother, who had thyroiditis in the “pre-synthetic” era, took natural hormones – I think Armour – for decades. I also began to wonder about the 100s of 1000s, perhaps millions, of mostly women who are prescribed this Synthroid each year. In my research, I discovered that the introduction of synthetic hormones seemed to be largely an economic decision which benefited the manufacturer. The story goes that natural hormones which had served well for a very long time, were suddenly maligned by doctors. If you have not read the history of Synthroid I urge you to do some research. I now take Armour and have for about 5 years. I feel much better and do not suffer the ill effects of a synthetic medicine that has been foisted upon unsuspecting thyroiditis sufferers for many decades.

  11. Mary Spaulding
    Washington State

    Right from the beginning I was disturbed by your article. It was not Levothyroxine, or T4 thyroid hormone that was a very old medication, but desiccated pig thyroid which contains not only T3 and T4, not to mention T1 and T2. And it is as closely watched for a consistent medication level from batch to batch as any other prescription medication.

    I switched from Armour Thyroid to Synthroid at one period and I didn’t feel as well with it as I did with natural Armour Thyroid.

    I have learned all I need to know from the false shortage of natural thyroid years ago. Big Pharma will do anything to get compliance from the public. But we natural thyroid uses fought back and refused to switch. We had online help on where to get thyroid during this false shortage and even went to Canada to get our natural thyroid where there didn’t seem to be any shortage. I don’t trust Big Pharma and never will they are a bunch of money grubbing pigs. And our FDA doesn’t seem to have any teeth to rein them in. It’s every person for themselves.

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