Synthroid Overdose: Signs & Symptoms You’re Taking too Much

Synthroid Overdose: Signs & Symptoms You’re Taking too Much

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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: 

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. Because so many people are using this medication, there is a high likelihood that some people are being overdosed (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. 


Because overdosing on 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 an overdose and too little and you will experience the symptoms of hypothyroidism. 

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 is far more serious than the ones listed above but they usually take a longer period of time to set in. 

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

  • Suppressed TSH – This should be differentiated from a low TSH which. A Suppressed TSH means that your TSH is undetectable. A low TSH does not always increase your risk of developing the negative side effects listed above. 
  • High free T4
  • High free T3

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


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

Some people prefer a slight overdose when it comes to their thyroid medication. 


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 studies that 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 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

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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 that 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. 


Synthroid is an incredibly common medication that 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 by 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! 









signs you are taking too much thyroid medication for your body

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About Dr. Westin Childs

Hey! I'm Westin Childs D.O. (former Osteopathic Physician). I don't practice medicine anymore and instead specialize in helping people like YOU who have thyroid problems, hormone imbalances, and weight loss resistance. I love to write and share what I've learned over the years. I also happen to formulate the best supplements on the market (well, at least in my opinion!) and I'm proud to say that over 80,000+ people have used them over the last 7 years. You can read more about my own personal health journey and why I am so passionate about what I do.

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