Synthroid Overdose: Signs you are Taking Too Much Thyroid Medication

Are you concerned that your dose of Synthroid is too high?

That may be the case, especially if you are experiencing symptoms such as hair loss, heart palpitations, and heat intolerance. 

The good news is that Synthroid overdose can be easily treated by simply reducing your dose. 

This article will outline the major side effects associated with taking too much Synthroid, what to do about it, the long-term (and short-term) consequences of taking too much, and how to determine your optimal dose

Let's jump in: 

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Thyroid Medication And Dosing

Synthroid is one of (if not the most) commonly prescribed medications in the United States

This is important for 2 reasons:

#1. Because it indicates that a very large number of people have hypothyroidism (a discussion for another post). 

And #2. That because so many people are using this medication, there is a high likelihood that some people are being over-dosed (1) and some people are being over-dosed. 

It's hard to quantify exactly how many people are being mismanaged when it comes to their thyroid medication because of a number of variables but we do know that it is quite a lot. 

And this is very important. 

Why?

Because overdosing in thyroid medication is not good for your short-term and long-term health (2).

Your thyroid plays an important role in regulating several systems in your body and in order for these systems to work properly you must be taking the exact right amount of medication. 

Too much and you will experience the symptoms of overdose and too little and you will experience the symptoms of hypothyroidism. 

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Signs You Are Taking Too Much Synthroid

So how do you know if you are taking too much Synthroid?

The best and easiest way to determine is by looking at your symptoms. 

Your body will tell you when something is wrong or off! All you have to do is listen. 

Common symptoms of Synthroid overdose include:

  • Increased hair loss
  • Weight loss (sometimes but not always)
  • Tremors in the extremities
  • Jittery sensation
  • Heart palpitations
  • Anxiety
  • Flushing of the skin
  • Heat intolerance (not able to stand hot environments)

These are the most common negative side effects of taking too much Synthroid and they are usually felt rather quickly and within days of increasing your dose. 

They also can develop slowly over a period of time if your dose is only slightly too high for your body. 

Rare but serious side effects of long-term Synthroid overdose include:

  • Osteoporosis or decreased bone density
  • Heart enlargement
  • Atrial fibrillation
  • Chest pain
  • Shortness of breath
  • *If you experience any of these symptoms you should seek medical care as soon as possible.

This list of rare side effects are far more serious than the ones listed above but they usually take a longer period of time to set in. 

Thyroid lab tests which indicate your dose may be too high include:

If your dose of Synthroid is truly too high then you will also notice changes in your thyroid lab tests as well. 

What Are the Long-Term Dangers of Being Over-Dosed?

Some people prefer to be slightly overdosed when it comes to their thyroid medication. 

Why?

Because your thyroid helps to regulate your metabolism by turning on cellular function. 

The more thyroid hormone you have the higher your metabolism will be, the more fat you will burn at rest, and the more energy you will have. 

While this does sound appealing, it doesn't come without a cost. 

As you rev up the cellular machinery in your body you rev up more than just your fat burning potential. 

Increasing your thyroid dose also puts excess strain both on your bone density and on your heart tissue. 

Using a consistently high dose of Synthroid puts you at an increased risk for developing osteoporosis (3) (decreased bone density) and heart conditions such as atrial fibrillation (4).

The absolute risk is not well defined and there are some of studies which show that suppressing the TSH does NOT cause these conditions (5), but they are still important to consider for each person. 

Even if your risk is increased for developing these conditions, though, doesn't mean that you will ALWAYS experience them. 

It may be that taking a higher dose of Synthroid increases your risk of both osteoporosis and atrial fibrillation, but the risk may only increase from say 1% up to 3-4% per person per year. 

These numbers are made up (because we don't actually know what they are), but these are plausible values based on my own experience and looking at the data. 

How Long Does Synthroid Stay in your System? 

Synthroid, because it is a T4 only thyroid medication, has a half-life of around 5-7 days (6).

What does that mean?

It means that it will take 5 half-lives for this medication to be nearly completely eliminated from your system. 

If you do the math (7 days x 5 half-lives) you find that Synthroid will be in your system for a total of about 35 days. 

By the 35 day mark, you will most likely metabolize out the majority of thyroid medication. 

And by metabolism, I mean that your body will basically use up all of that thyroid hormone for cellular function. 

This is both good and bad. 

Bad in the sense that if you take too much thyroid medication it may remain in your system for a long period of time and good in the sense that if you miss a dose here or there it's probably not going to affect you because you have a significant amount built up in your system. 

But it's important to note that just because it's in your system for 35 days doesn't mean that it will be equally potent throughout all of those days. 

For instance:

Around day 17-18 the concentration of Synthroid will be about half of what you took originally. 

Finding Your Optimal Dose of Synthroid

So how are you supposed to find your optimal dose of Synthroid?

The answer that most people look to is their thyroid lab tests. 

And this is a great starting point, but it's not sufficient by itself. 

Most people, when they look at their thyroid lab tests, focus solely on the TSH as a marker of thyroid function. 

And while the TSH is a potentially great marker, it's not perfect and it shouldn't be the only thing you look at.

More valuable measurements may be found by evaluating your free thyroid hormone concentrations. 

And by free thyroid hormone levels, I am referring to both Free T3 and Free T4

These markers tell you exactly how much thyroid is floating around in your bloodstream and give you an idea if you are absorbing and utilizing the Synthroid you are taking by mouth. 

If you focus only on the TSH you miss the information provided by looking at these levels. 

The TSH only gives you information about your pituitary gland (7) and it doesn't give you information about the absolute levels of free thyroid hormone levels in your body. 

But beyond your free thyroid hormone levels, you also want to look at one other very important factor... 

Your symptoms!

In other words:

How are you feeling? Are you noticing an improvement in the symptoms that brought you to the doctor in the first place? Are you feeling any better? Is your weight normalizing? Is your hair loss slowing down? 

All of these questions are things that you should be thinking about as you alter your dose!

As you alter your dose of Synthroid you should feel BETTER, not worse. 

As you do these things (evaluate both your free thyroid hormone levels and your symptoms) you will find your optimal dose. 

What to do if you are Experiencing Overdose Symptoms

The good news is that overdose symptoms can be easily solved by adjusting your dose of Synthroid. 

I don't recommend that you make any changes to your Synthroid dose without first consulting your doctor, but the answer to solving your issue is usually as easy as reducing your total dose. 

Let's use this as an example:

Say that you are taking 100mcg of Synthroid each day. 

Previously you were taking 75mcg per day and feeling pretty good, but once you started taking the 100mcg dose you started to notice increased hair loss, heart palpitations, a jittery feeling, and some anxiety. 

You also found that your TSH was low and your free T4 was high after thyroid blood testing. 

This is a pretty classic example of Synthroid overdosing. 

So what do you do?

The most obvious answer is to reduce your dose back to where you felt "normal" which would be around 75mcg of Synthroid each day. 

Not all cases are this clear-cut, but this is the logic that you should apply to your situation. 

In reality, you may find that you need slightly more than 75mcg but less than 100mcg and you might find that your ideal dose is really closer to 88mcg per day. 

This can be teased out as you evaluate your lab tests and as you follow your symptoms. 

What if you Accidentally Double or Triple your Normal Dose of Synthroid? 

In most cases, this won't be a huge deal (8).

You may feel jittery or anxious for about a week or so, but the side effect should fade rapidly over a few days. 

If you take an excessively high dose you may be tempted to compensate by reducing your dose the next few days, but this isn't actually a good idea. 

thyroid lab tests after acute ingestion of thyroxine

It's best not to play chemist when it comes to your medications and instead stick to your regular program as if the change in your dosing didn't happen. 

By adding more complexity to your dosing equation (by skipping other doses) you risk causing more side effects down the road which can confound and confuse you further. 

If after taking a high dose you are experiencing symptoms which are troublesome then you can reach out to your Doctor. 

In some cases, they may want you to use a medication known as a beta-blocker which can help reduce the side effects you are experiencing. 

Even if you take a high dose of Synthroid in 1 day, the consequences should not be long-lasting or severe. 

Conclusion

Synthroid is an incredibly common medication which is prescribed for treating hypothyroidism and Hashimoto's thyroiditis. 

Because it is so commonly used, the percentage of people who temporarily take too much medication is quite high!

The good news is that in most cases, the side effects and consequences are not severe and should fade on their own over a period of a few days. 

Synthroid has a half-life of about 5-7 days which means that it is normally eliminated entirely from your system in about 30-35 days

You can treat overdose symptoms usually be simply reducing your dose of thyroid medication. 

Also, you can find your optimal dose of Synthroid by evaluating both your thyroid lab tests and by keeping an eye on your symptoms. 

Now I want to hear from you:

Are you suffering from Synthroid overdose symptoms?

What dose are you currently taking?

Do you feel your dose is too high or too low?

Leave your questions or comments below! 

References (Click to Expand)

Dr. Westin Childs

Dr. Westin Childs is a Doctor of Osteopathic Medicine. He provides well-researched actionable information about hormone-related disorders and formulates supplements to treat these disorders.He is trained in Internal Medicine, Functional Medicine, and Integrative Medicine. His focus is on managing thyroid disorders, weight loss resistance, and other sex hormone imbalances.You can read more about his own personal journey here.

26 thoughts on “Synthroid Overdose: Signs you are Taking Too Much Thyroid Medication”

  1. Hey Dr. Childs,
    I think this is what’s going on in my body. I started out with FT3 mid range, FT4 low, TSH 0.9, elevated RT3-could be due to stress and experiencing neuropathic symptoms (burning arms, tingling). In an attempt to figure out whether my body is in an hypothyroid state, I increased my dose from 75mcg to 98mcg. After two weeks, the burning got worse, hair loss, feeling jittery and shortness of breath. Conclusion, 98mcg is too much. Is 75mcg too little. I don’t know. We’ll see. The neuropathy could be related to the thyroid (in this regard it would be nice, if you could add a blog post with this topic in the future) or not. I know that my labs are not in the optimal range, but I am not sure how to get them there. Maybe it is, as you describe it above, just a matter of a little bit more. Happy Monday!

  2. I experienced some symptoms of hypothyroidism, so we changed my dose from 75mcg to 94mcg. starting out with tsh 0.9, ft4 0.9(low), ft3 2.6 (mid range). after 2 weeks I felt a bit anxious, hair started falling out, shortness of breath. I guess the dose was too high for me even though the blood test showed improvements tsh 0.9, ft3 3.0, ft4 1.5. the optimal dose is probably somewhere in between 75 and 94mcg.

  3. I have been taking 25 mcg of thyroid med for about 6 months. I didn’t catch that the pharmacy refilled my script for 50mcg. My symptoms sent me to my cardiologist and he figured out what was going on from my labs. I had taken the higher dose for only 10 days. I have a pace maker and occasional A-Fib. Don’t know if my existing heart issues increased my symptoms but they were pretty intense. I have been back on 25 mcg for 6 days. Symptoms are diminishing but definitely not gone. How long would you estimate it will take to eliminate the overdose from my system? Can taking an overdose for only 10 days cause this many problems?

    • Hi Bonnie,

      The extra medication should be out of your system within about a month or so! And yes, it just depends on how sensitive you are to the changes in your dose.

  4. Hi Dr. Childs. I have Hashimoto’s. I have been taking levothyroxine 100mcg and liothyronine 5mcg both once daily in the morning. It was definitely making me feel much better but I was still feeling lightheaded all the time. I went to a new endocrinologist and he took me off of my current medication and put me on 90mg NP Thyriod by Acella. Exactly two weeks after being on this medication I was feeling absolutely horrible. Extreme fatigue, confusion, slurred speech, blurred vision, headache, feeling like I was going to faint, increased appetite and very emotional. These were also my symptoms before I was put on thyriod medication in the first place. I called my doctor to explain my symptoms and he told me they weren’t thyroid related symptoms. I then called my old doctor and he told me to take half of my levothyroxine, which made it 50MCG. Within one hour I started feeling better. By evening all symptoms were gone. He then instructed me to go back on my old medication and stop taking the NP Thyroid. Did this medication just not work for me or was it the wrong dose? I was very disappointed that the prescribing doctor dismissed my horrible symptoms. Have you seen this happen before? Why is Hashimoto’s causing me lightheadedness? Thank you for your time.

    • Hi Kristin,

      It sounds like you were not dosed appropriately. Before you switched medications you were taking 100mcg of T4 and 5 mcg of T3, after the switch you were taking 57mcg of T4 and 13mcg of T3. Most doctors are clueless when it comes to switching medications so they often underdose which causes the symptoms you experienced.

  5. Hello Dr.

    I am 31 weeks pregnant. My Levothyroxin dose has been increased from 100 to 125 when I was 8 weeks. After taking 125 for six weeks, I did the blood work and found out that 125 was too much. The Dr. reduced it 112. Since then I’m on 112. Last month the pharmacy told me that they don’t have 112 tablet and suggested to take 1 and half of 75. I started doing that for few days. This morning I found out that I have been taking 1 and half of the 125 for about two weeks. The colors are very similar. Even though I don’t have any symptoms, I am very much worried. Does it harm the baby? I have appointment after two days. What should I do till then?

  6. I am experiencing the symptoms of too much synthroid in my system. I have been having heart palpitations, bad shakiness in my hands and legs it feels like severe anxiety. I have been feeling awful. My previous endocrinologist said that my dose was too high and that previous tests by my primary doctor were very off and she did not notify me. So I have been taking .150 for over a year and these symptoms just hit me really hard. I found a new doctor that said my symptoms were all because of too much synthroid, he has dropped me to .125, even by me stopping it for 2 weeks my levels are getting close to normal range. He wants me to begin taking the new dose Monday.
    He suggested putting me on a beta blocker for my palpitations. My question is how long can these symptoms last its just so uncomfortable and I am thinking about taking the Beta Blocker but am nervous about taking it.

    • Hi Anne. I just learned I have been taking too high of a dose and am having the same systems as you – fast heart beat, anxiety, plus weight loss. My dose is being lowered. I am wondering how long it took before you felt better? Thanks.

  7. Hello there,
    Where do I start, Total TT 3 years ago, a mess since, been on Synthroid and different doses, cannot find correct dose the last dose of Synthroid was 88 a year ago, most doses give me palpitations or extreme tiredness, I tried armor , I gained 10 lbs within 4 months, and started swelling legs and arms. I was on 90 mcg. I am now back on Synthroid, back to feeling crappy, I’m lightheaded and dizzy more often than not, and I’m on a bigger dose than I have ever been I’m on 137. I’m sure it’s too high, considering my last Synthroid was 88. I am not handling the dizzyness lightheadedness well, make me feel like I’m going to pass out, or have a stroke, I just get all tingling. He also prescribed me t3 when he gave me the Synthroid 6 weeks ago, but I haven’t touched it yet. Any suggestions? I sure would appreciate it.

  8. Hi Dr Childs!

    Three years ago, I had mysterious bouts chronic urticaria/idiopathic anaphylaxis (not allergic to anything except mushrooms…) led to discovering my hypothyroidism, and was put on 75mg of levothyroxine. It was like a miracle, and even got rid of my hives and allergy symptoms completely. I got mono in December, and it destroyed my immune system, and I also lost about 10 pounds. I was having all they symptoms of overdose, with these intense and intolerable flushing attacks which made me turn bright red. This was happening every day. My doctor lowered it to 50mg and that worked for~ two weeks, and now I’m back to overdose symptoms, and he refuses to lower it again. I can’t deal with the anxiety, the palpitations, and the flushing. He said there’s nothing he can do, I have to wait another month, that I’ll adjust. I broke down and started crying in the office, which is embarrassing enough- it’s debilitating, and horrible, but is that true? I think he thinks I’m being overly dramatic, but I’m really not! This is the third doctor I’ve had to switch to because they all eventually leave the city, and I thought I finally found a good place. Now I feel alone, frustrated, and not taken seriously. I feel like if I stop for a few days to give me a break from the flushing, I’ll be gambling with random anaphylaxis again. I don’t know what to do or how to convince my dr this is seriously affecting my daily life. I have an appointment with another doctor, but can’t get in to see them until end of April. I feel stuck.

    Thank you!
    -Lisa

  9. I have been suffering from balance problems for 2.5 months. My ENT concluded it is not ear related. I have been on 150/175 Synthroid for about a year and have been taking fairly high doses for 15 years since oral cancer radiation therapy ruined my thyroid. My last test showed a TSH of .138 shows I have been overdosing. I’ve reduced my dose to 125/150. I feel OK but have balance problems when standing. Can my balance problems be caused by overdosing? And is there a time frame for when I may feel normal?

    • Hi Steve,

      It could be related to your thyroid dose, but there’s no way to know for sure unless you adjust your dose appropriately. My guess is that it is probably related to something else, however.

  10. Hi Dr. No, absolutely not! I don’t think there’s any amount 9f Synthroid and Cytomel that would ever make me overdosed. If I forget to take a Synthroid, I feel it within hours and I pay hard the next day. It’s not been as severe since the cytomel was added but it’s still a fast reaction. I don’t believe Synthroid stays in my body 5-7 or 35 days.
    Also, my TSH has been under the range .28 and the next month a 7.8. this was before cytomel. After it was less of a swing like .40 to 1.3 (but I feel hypo symptoms and awful when I get close to 1.0). I feel ok – not good, not great just a tad below normal at . 40 and. 28. A 2.0 really has me feeling bad. I had a surgery last year that delayed thyroid maintenance. I ran out of Synthroid for a week but still had cytomel. Then got a refill to hold me over thru my appointment. So 22 days after that week of missing Synthroid, my TSH is. 04 but free T4 and T3 (not free) are smack dab in the middle of the range. I don’t have any symptoms of hypothyroidism not even close.
    I do have questions what causes a person to be absolutely nothing like the classic “symptoms”. For example, metabolism never increased back to normal, the entire time I was hypothyroid at a 2 TSH, even tho I had significant weight gain, fatigue, light headedness, digestive issues, insomnia, anxiety, hair loss, heat intolerance and cold intolerance both, severe allergies, eczema. What causes that? Lots of those symptoms are tagged as hyperthyroidism but I was significantly hypothyroid. Although the treatment hasn’t affected my metabolism at all, my hair loss resolved itself, I no longer need allergy medication, I no longer get severe bronchitis every spring. Using name brand Synthroid stopped the all over muscle quivering I had accepted. There was a slight improvement in really severe fatigue and over the years it improves. In short measures. But why are we told certain symptoms are hypothyroidism and certain symptoms are hyperthyroidism when that’s not true? It causes “normal” people as well as doctors to make incorrect assumptions.

  11. My 85 year old mother has accidentally been overdosing on Levothyroxin for several weeks now and I just discovered it last night when checking her pill organizer. She was increased from 88 to 100 a few weeks ago but I found several of the 88s randomly stuck in the little compartments…so she’s been overdosing here and there. She has horrible symptoms and we’ve spent a lot of time having tests and seeing different doctors the past several weeks. Yesterday was the worst day. She was shaky, confused, had to be helped to stand and walk. She also had bloodwork yesterday which is going to turn out really bad. I don’t know whether to give her a Levothyroxin this morning or not! I’m thinking it might be best to skip it today since she’s obviously overdosed.

    • Hi Becky,

      The treatment is quite easy if her symptoms are truly due to taking too much medication, but in the elderly and in situations like this, you shouldn’t make any changes to her medication without consulting with her physician.

  12. Hi Doctor! I had papillary cell carcinoma of the thyroid 40 years ago followed by a dose of radioactive iodine. I have taken Synthroid ever since and was advised to keep my levels of TSH at the lowest range possible usually about .3 to .4. As the years have gone by with no problems at and have been well until a few years ago when they lowered my level of .125mcg to 6 days / week. Then a few months ago my endocrinologist told me to take it 5 days/ week. I had to have a blood test for other health issues a few days ago and about fainted when my level of TSH registered at 12.1!!!!!! What now? Any thoughts….. faulty lab test, poor Synthroid quality etc., etc., etc.? I am 78 years old and cannot gain any weight and have lost both muscle mass and should be about 125 lbs not 100 lbs!!!

  13. Hi,

    I was recently put on 25mcg of synthroid three months ago and noticed it working and was feeling great. Almost four weeks ago, I had my IUD removed and since then, all hell has broken loose on my body – aching, jittery, tingling, dizzy, nauseous, flush face, tingly thyroid gland on occasion…. it’s a mixed bag.

    I have a follow up apt with my doctor to go over my three month bloodwork but am curious if it’s possible I may not even need synthroid now with the IUD out and my body trying to regulate?!?

    • Hi Cindy,

      It’s always a possibility. You will need to check your labs to make sure. There’s also going to be some dysregulation of your hormones after you stop taking any sort of birth control (assuming your IUD was secreting hormones) which you may be dealing with as well.

  14. Maybe I have a neurological problem that’s developing, but I think it’s too much Synthroid. I feel normal when I wake up in the morning, I take my Synthroid at the same time every morning at wait an hour until I eat breakfast. By breakfast time my muscles get very stiff and I’m having peripheral neuropathy. As the day wears on my symptoms slowly subside. Have I suddenly become over sensitive to Synthroid? I only have my TSH checked, and it’s always within range. Do you have any suggestions? Thank you!

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