Thursday, March 17, 2011

Metformin and Thyroid Tests

Every drug we take has many effects. The main effects are usually known by patients as well as physicians. For example, most people know that the drug metformin often causes gastric distress, which can be reduced by starting with a small dose and gradually working up to a therapeutic dose.

But most drugs also have minor effects. Sometimes your doctor is aware of these but doesn't tell you because the incidence of these effects is low and the doctor doesn't want to worry you. This is often true of the muscle weakness and memory problems that statins can cause.

Sometimes even your doctor isn't aware of the minor side effects.

Sometimes no one has yet discovered some side effects. This can be because you have to be on a drug for a certain amount of time before these side effects show up. It can be because no one has noticed the link between a particular drug and some side effect.

Or it can be because drugs can interact with other drugs, and when you're taking a lot of different drugs -- say a diabetes drug, a blood pressure drug, a lipid-lowering drug, an anti-reflux drug, an antidepressant, a beta blocker, an antihistamine, an osteoporosis drug, and an asthma drug -- and you complain of fatigue, it's not immediately clear which one of these drugs or which combination is causing that problem.

One relatively unknown drug-hormone interaction was first reported in 2006.

It seems that metformin suppresses thyroid-stimulating hormone (TSH; also called thyrotropin), the hormone that is generally tested to ascertain your thyroid function.

When your thyroid hormones (called T4 and T3) are too low, your pituitary gland secretes TSH. The TSH then tells the thyroid gland to secrete more T4 and T3.

Thus a high TSH level suggests low thyroid, and a low TSH level suggests high thyroid.

Your doctor often tests the T4 and T3 levels too, but often not. If the TSH is in the normal range, your doctor may assume your thyroid levels are fine and refuse to do more testing.

The normal ranges are controversial. The usual range is said to be about 0.4 to 5 microunits per milliliter. But some people say the cutoff on the high end should be lower, about 2.5. And graphs in endocrinology books show that the average TSH level in people considered to have healthy thyroid control is only 1.1, with very few in the upper ranges.

The new research shows that metformin therapy suppresses TSH levels. Two studies showed that it did this without affecting T4 and T3 levels. A third found that free T4 levels increased as TSH went down.

Most of the T4 and T3 in your blood is bound to proteins. The free (unbound) levels of the hormones are the active hormones, and that's what the free T4 (fT4) and free T3 (fT3) measure.

No one yet understands the mechanism of the TSH reduction by metformin. It's especially puzzling because it doesn't seem to be linked with the thyroid hormone level. And the metformin has no effect on TSH in people who have no thyroid problems.

But what it does mean for you is that if you're on metformin you should be aware of this link. Let's say you're on thyroid medication and then you start taking metformin. Your TSH goes down, and your doctor may worry that your thyroid is now too high and might reduce your dose.

But what if it's just a result of the metformin? Then you'd end up with a thyroid level that was too low.

So if you're on metformin and your TSH test doesn't seem to agree with how you're feeling, discuss this interaction with your doctor and have your T4 and T3 levels tested as well as the TSH. It could be that the lower TSH is caused by the metformin and notw higher thyroid levels.

Does this mean that metformin could interact with other lab tests? It's possible. The metformin-TSH interaction was only noticed in 2006, more than 10 years after the drug first became available.

Does this mean that other drugs could interact with the TSH test? It's possible.

We need to be vigilant about all the drugs we take, and if something seems wrong, we need to try to figure it out. Sometimes the published science reports can't tell us.

Trust your body. You know it better than anyone else. And don't let some doctor tell you that your symptoms are all in your head because there's no evidence for what you're saying. Maybe you're right and the current literature is wrong.


31 comments:

  1. Thanks Gretchen.

    I always learn from your posts.

    You should change that last paragraph to Capitals and bold type; it is too true, sadly.

    Cheers, Alan, T2, Australia

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  2. I went from hypo - to hyper since being on Metformin last year. Doctors in Ohio AND Florida say that Metformin couldn't do that - but thank you - I now see that it CAN. :)

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  5. Desiccated porcine supplement can do wonders for hypothyroidism. Of course, asking your doctor applies in this case.

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  9. My TSH results have also been going from hypo -to hyper since being on Metformin and are now blocked at the lowest level. Doctors in Belgium say that Metformin couldn't do that - but I've found several serious research articles and I now see that it CAN. :)

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  10. Doctors need to learn to stop saying "can't." New research often shows that something *can* happen.

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  11. I have just got up to the full dose of Metformin this week and also taking thyroid medication, Thankyou for the interesting read Gretchen, I will definantly be going back and getting all the tests done, Thankfully I have a doctor who runs regular tests
    Cheers ;-)

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  12. I'm on second round of thyroid tests as I feel terribly tired after my doctor lowered my thyroid dose after going from hypo to hyper thyroid after adding Metformin. She insists that the tests are right, but I feel terrible and am so glad to find this blog. I'm at a loss as to what my next step is now.

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  13. Anonymous, I can't give medical advice to an individual, and even an MD wouldn't do so on the basis of an Internet query.

    You might try another physician.

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  14. This doesn't really make sense to say that metformin suppresses TSH, but that fT4 goes up. It seems more likely that metformin stimulates the thyroid to either produce or release more T4 hormone, which in turn suppresses the TSH.

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    1. Possible. They say they don't understand how this works.

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  15. This is crazy. I have just recently started my metformin and also am having my tsh levels checked yet again! I see my doc tomorrow to discuss what could be going on with the levels. As I was resting in my chair here, it hit me that I have started taking another hormone! I decided to go online to check into it and guess what I find? No wonder my levels are dropping! Now what do I do when I see my doc tomorrow?

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  16. I'd ask his opinion on all this and then decide what you want to do.

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  17. Like this article. I am on Metformin and having trouble with my TSH being low. My T4 and T3 are normal. Had no trouble with the TSH reading low prior to Metformin. I am gaining weight too.

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  18. I have been on Metformin for about a year and my last lab test indicated normal T3 and T4 levels but my TSH was 8.3. My weight and cold sensitivity is consistent with hypothyroidism but I am not on any medication as yet. Waiting to see if the TSH comes down next lab as I started taking PNG recently.

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  19. Oh, wow! I've been on Metformin for 8 years. Nearly five years ago I was told I have an overactive thyroid (although my symptoms said the opposite). I was pushed from pillar to post, first given Carbimzaole, which left me dragging myself around only half-alive, then Levothyroxine which didn't help much, then told that I would have to go back on Carbimazole and subsequently have my thyroid gland destroyed by radio-active iodine. I called the endo a few well-chosen names and went home. I had been told "your symptoms are the daylight, the blood test is the clock." I had previously suspected that their vaunted clock might have a gremlin in it, but they adamantly denied that Metformin could cause a low TSH. When I go for my next review, somebody's head will be on a pole!!!

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  20. I'm mystified by why they would give you a drug to suppress thyroid and then a drug to increase it. But whatever, I hope when you go back you'll get some relief.

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    1. It was because of the fluctuating TSH, here in the UK the TSH is the be-all and end-all. I had, for some years, been on metformin with sitagliptin, then was put back onto plain metformin and it was shortly after then that all the trouble started. I shall be pointing that out - very pointedly!! And thank you for your good wishes. I'll let you know what transpires.

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  21. I have requested to go back on Metformin with Sitagliptin, and have been stalled. I received a phone-call telling me that I would be contacted by a nurse at the Health Centre for a telephone consultation - in two weeks' time! When she phoned I told her all about it and was asked to go for a blood test, again in two weeks' time. This has now been done and I am awaiting the results. However, all the emphasis was on my Type 2 diabetes and they appear to be in total ignorance of the fact that Metformin can cause a suppressed TSH. I'm currently waiting to know what they decide. I did mention that Berberine can be an alternative - the woman had never even heard of it!!!

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  22. Berberine seems similar to metformin, so it's possible it would have the same side effects, but because it's OTC, drug companies won't study it.

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  23. The woman I saw was telling me that she had recently obtained a degree in diabetes, but, in addition to never having heard of Berberine, she had also never been taught that Metformin can suppress TSH. I wish to goodness I could be referred to somebody who has a degree in thyroid matters!

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  24. If possible, you should discuss this with the doctor. I think a lot of them don't keep up with supplements, and re TSH, I found this:

    "The adjusted hazard ratio (HR) for low TSH levels for metformin users vs sulfonylurea users was a significant 1.55 overall. The difference was not significant during the first 90 days (HR, 1.73) but increased significantly after 90 to 180 days of use (HR, 2.3) and returned to nonsignificance after 180 days (HR, 1.29).

    "In contrast, this effect of metformin was not seen in the euthyroid group, with an adjusted hazard ratio for low TSH of 0.97 compared with sulfonylurea use."

    https://www.medscape.com/viewarticle/832394

    So it's possible the effect is temporary. However, it might make sense to seek a new doctor who would be open to discussing this.

    gretchen

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  25. I seldom even get to SEE the doctor at the Health Centre, it's usually some nurse. I've been struggling for 5 years now with hypothyroid symptoms, while being told I'm hyperthyroid. When I found out that Metformin can suppress the TSH it began to make some sense, especially as it started just after my medication was changed. But when the result of my blood test comes through maybe I'll get some sense out of them.

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  26. Well, they readjusted my diabetes medication, adding sitagliptin to the metformin and my TSH is now 'satisfactory'. I believe that I am still hypothyroid, but we will see what the next blood test shows. They fobbed me off with some 'health-care assistant', who proudly told me that she had a degree in diabetes (when I was more concerned about the thyroid issues!) So I asked her what she had learned on her degree course about metformin suppressing the TSH - NOTHING!!

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  27. Ben, I hope this new regimen works for you. Do you *feel* as if you're hypothyroid? It's so frustrating when one is dealing with health care people who aren't familiar with the situation.

    Here are a few more recent articles.

    https://www.medicalnewstoday.com/articles/282854.php

    https://www.medpagetoday.com/endocrinology/diabetes/47788

    https://joe.bioscientifica.com/view/journals/joe/233/1/R43.xml

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    1. I do still have some symptoms - but I feel that I've won the first part of the battle in showing that I am NOT hyperthyroid. I'm hoping that the next lot of blood tests will show that I need a thyroid supplement, and thank you for the links, which I will peruse.

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