Levothyroxine Side Effects: What to Expect Using This Thyroid Med

Levothyroxine Side Effects: Hair Loss, Weight Gain, Weight Loss & More

If you just started levothyroxine and you are wondering about side effects, I know what you are worried about:

  • Is levothyroxine actually working for me?
  • Why do I still feel bad if I’m on thyroid medication?
  • How long will it take for me to start feeling BETTER?
  • Are my new symptoms caused by levothyroxine?
  • Why am I still gaining weight on thyroid medication?
  • Are these symptoms normal?

If you fall into any of these categories don’t worry, because I’m going to go over each one of them. 

If you are looking for the complete list of side effects from levothyroxine there will be a section below, but…

I think you will find more value in reading about what kind of positive side effects you should be expecting, what happens if you don’t experience these side effects and what kind of bad side effects you might be experiencing, and what to do about those…

Starting on new thyroid medication can be tricky but this guide will help you figure out what to do in almost every situation:

Understanding The Side Effects of Levothyroxine & What Causes Them

Should you experience any side effects when starting levothyroxine?

After all, why do we use hormone replacement therapy if not to replace low levels, to begin with?

If you are simply replacing the hormone deficiency in your body, shouldn’t you experience only POSITIVE side effects?

You would think so, but it isn’t as straightforward as you might think. 

Before we talk about levothyroxine we need to discuss some basics…


Levothyroxine is a T4-only medication and it is used in patients who have hypothyroidism. 

T4 (sometimes abbreviated LT4) contains the active ingredient Thyroxine which is the scientific name for T4 thyroid hormone. 

Hypothyroidism is a hormone imbalance in which there is insufficient thyroid hormone circulating in the bloodstream. 

Because thyroid hormone is required for life and optimal health it must be replaced if it is low which is the reason you are probably taking this medication. 

The entire goal of using this medication is to attempt to normalize thyroid hormone levels in your body


Levothyroxine is considered to be bio-identical, meaning it looks the exact same as the hormone that your thyroid gland produces naturally. 

the definition of bioidentical hormone replacement therapy from Wikipedia.

This may sound obvious and not worth mentioning but it’s important because many hormones that doctors prescribe are not actually identical to the hormones that your body produces naturally. 

Medications like birth-control pills contain altered hormone equivalents (1) that don’t always react the same way as natural hormones. 

Levothyroxine is bio-identical, meaning it looks the exact same as the hormone that your body produces (2). It is, however, synthetic in the sense that it is created in a lab

You can compare synthetic thyroid hormones to ‘natural’ thyroid hormones such as NDT (Natural Desiccated Thyroid) which is derived from crushing animal thyroid glands. 

With these basics in mind, we can talk about why some people experience side effects when taking T4 medication. 

It turns out that the reason many people react differently to certain medications is quite complex but we can break it down into an easy-to-understand way. 

There are 3 main reasons you may not tolerate medication including:

  • You are reacting to fillers, dyes, preservatives, or other inactive components of the medication –> You probably don’t even think about it when you take medications or supplements but these pills/capsules often contain a variety of strange ingredients all designed to keep the active ingredient safe and protected. Sometimes these inactive ingredients can cause problems for certain individuals.
  • You are taking the right medication but your dose is not sufficient –>  Even if you are taking the right medication, it won’t work properly if your dose is too small. A simple solution to this problem is to simply increase your dose. 
  • You are taking the right medication but your dose is too high –> On the flip side, even if you are taking the right medication you can still take too much which may lead to negative side effects. A simple solution to this problem is to simply reduce your dose. 


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Understanding Inactive Ingredients

Perhaps the most common reason that people react negatively to levothyroxine has to do with the inactive ingredients including fillers, dyes, preservatives, and so on. 

Certain people may react negatively to these ingredients and experience a hypersensitivity-like reaction which includes hives, headaches, irritability, and so on. 

These are not necessarily serious side effects but they may certainly interfere with your ability to take the medication! 

You can see a list of all of the inactive ingredients found within levothyroxine below: 

table showing different different color additives based on the strength of various doses of levothyroxine with the 50mcg dose boxed in.

This chart shows that each of the different doses of levothyroxine contains various fillers and dyes which serve to alter the color of the pill. 

The problem?

Some of these dyes (especially Red 40, Yellow 5, and Yellow 6) may cause hypersensitivity reactions and health problems in certain people (3). 

The simple solution to this problem is to take a formulation that contains the fewest amount of dyes and colorings. 

You can do this by switching to a different type of T4 medication like Tirosint or by switching to the 50mcg dose of levothyroxine/Synthroid. 

From the image, you may have noticed that the 50mcg dose of levothyroxine has no added colors or dyes which makes it the preferred choice for extremely sensitive individuals. 

And, when switching the mcg dose of your medication, you can still keep your total dose the same.

Consider this:

If you are on 150mcg per day of levothyroxine then all you would need to do is ask your doctor to switch to 3 tablets of the 50mcg dose. 

Using this strategy you would be keeping your total dose the same while removing the extra dyes and coloring agents. 

If you are wondering if you are reacting to these dyes you can see a list of symptoms that may indicate you have a hypersensitivity reaction to the inactive ingredients:

Hypersensitivity reactions to inactive ingredients of levothyroxine.

Symptoms of hypersensitivity include:

  • Hives, rash, flushing, swelling of the skin/face, intestinal pain, nausea, vomiting, unexplained fevers, sore joints, and even wheezing.

You can see a full list of inactive ingredients in levothyroxine here:

the complete list of inactive ingredients found inside levothyroxine with the word inactive highlighted.
  • Butylated Hydroxyanisole
  • Silicon Dioxide
  • Magnesium Stearate
  • Mannitol
  • Cellulose <—- many patients react to this component
  • Sucrose
  • Dyes and additives depending on the dose 

Many patients react poorly to these fillers and dyes (4) and simply changing your dose or type of medication MAY improve your symptoms. 

Is Your Dose of Levothyroxine Right for Your Body?

The next reason you may experience side effects has to do with your dosing. 

As I mentioned previously if your dose is sufficient for your body then you should not be experiencing negative symptoms. 

And it’s important to differentiate between the symptoms of the disease that you are treating (hypothyroidism) and the potential side effects of the medication. 

Hypothyroidism results in specific symptoms such as fatigue, weight gain, hair loss, pain, menstrual irregularities, constipation, cold intolerance, and so on. 

As you take levothyroxine, these symptoms should subside. 

If, however, you notice a slight improvement (but not a complete resolution) then that may be an indication that your dosing is off. 

If you notice completely new symptoms then that may be an indication that your symptoms are caused directly by the medication itself. 

But, back to your dose for a second.

It’s important to realize that levothyroxine may not work for up to 15% of the population. 

Certain patients, especially those with specific genetic defects in the DIO2 gene (5), may need a combination of T4 and T3. 

T3, sometimes referred to as LT3 or triiodothyronine, is the active thyroid hormone. 

How do you know if you fall into this category? 

Those who fit into this category are the ones who do not feel better despite taking levothyroxine

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They are the ones who continue to feel poorly despite having “normal thyroid lab tests”. 

If you fit into this category then you may need to seek out a combination thyroid medication (T3 in addition to your existing T3 dose). 

How do you know if levothyroxine is working in your body or if you are on the right dose? 

This can be assessed through a combination of your symptoms and by monitoring your lab tests

If levothyroxine is working for you, you should experience some of the following:

  • Your energy levels should be improving
  • Your weight should be dropping
  • Your hair, skin, and nails should be improving
  • Your mood should be improving
  • Your menstrual cycle should be more regular
  • Your joint pain or muscle soreness should be reduced
  • You should NOT be having constipation

These are what I would consider the “positive side effects of levothyroxine” meaning this is what SHOULD be happening when you take the medication. 

You SHOULD experience an improvement in symptoms not an INCREASE in symptoms.


Because the medication should be providing your body with the hormones that weren’t there previously.

If you are on levothyroxine and these symptoms are NOT improving or they are getting WORSE then 1 of 2 things might be happening:

1. You are not on a high enough dose. 

2. Levothyroxine (or Synthroid) is NOT the best medication for your body. 

You can read more about the specific and how to find your ideal dose in this post

Is Levothyroxine Causing me to Gain Weight?

Speaking of patients who may not tolerate T4 medications, let’s talk about levothyroxine and weight gain.

One question that I see asked fairly often is whether or not levothyroxine can contribute to weight gain. 

It seems like an odd question given what we know about thyroid physiology, but let’s explore it for a moment. 

Your thyroid helps to control your appetite, regulate your metabolism and help you burn energy. 

It makes sense, therefore, that taking extra thyroid hormone should result in increased metabolism and an increased ability to lose weight right?

But, that isn’t what happens to everyone. 

In fact, there are a number of people who seem to paradoxically gain weight when they use levothyroxine instead of losing weight. 

If you fit into this category you are not alone: 

text from other thyroid patients from an online forum explaining how they gained weight while taking levothyroxine.

How can this be? Why does levothyroxine cause weight gain in some people?

The exact reason is not clear but it may have to do with how your body processes the medication once it’s absorbed. 


In order for your body to utilize thyroid hormone, it must be converted from the inactive T4 into the active T3

This conversion process does not work equally for all patients. 

It’s plausible, then, that some individuals may not activate the medication after ingestion. 

In a sense, the medication is making it into their body but it’s not activating or making it into their cells. 

I have written about this phenomenon in this post and you can read more here if you want more information

What should you do if you fit into this category or what can you do to prevent this unwanted side effect?

Perhaps the best option is to switch medications or to use a combination of thyroid medications designed to provide you with both inactive and active thyroid hormones. 

You can also track your total T3 and free T3 levels as these levels tend to correlate well with your potential for weight loss (6). 

If you can find a doctor willing to prescribe Natural Desiccated thyroid or Liothyronine then you may be able to prevent weight gain from starting. 

Is Levothyroxine Making my Hair Fall out?

Another big potential (and concerning) side effect of levothyroxine is hair loss.

I know what you’re thinking:

Doesn’t hypothyroidism cause hair loss and shouldn’t levothyroxine, therefore, help you to regrow your lost hair?

The answer is yes, in most cases it should help you to grow your hair back. 

But this isn’t always the case. 

In fact, one of the potential side effects of levothyroxine medication is hair loss:

boxed in text from google search which shows that levothyroxine and other t4 medications can cause hair loss.

When it comes to hair loss there are actually several different variables that may be contributing. 

I’ve gone over all of the potential causes of hair loss in thyroid patients in this post and you can start there if you don’t fit the standard mold. 

So, how do you figure out what is causing your hair loss if you have hypothyroidism?

  • If you had hair loss PRIOR to taking levothyroxine and it was made WORSE by the medication then you may be experiencing a negative side effect
  • If you had hair loss prior to taking levothyroxine and you don’t notice any difference or your hair loss isn’t reducing, then you need to look into other causes.

It’s important to figure out if your hair is caused by persistent Hypothyroidism (meaning your dose is still too low) or if it is caused by the medication itself. 

When monitoring this make sure that you pay close attention to the amount of hair that you are losing both before and after you start your medication. 

So what do you do if you think that levothyroxine is causing hair loss?

The best thing you can do is to switch medications. 

You can switch from levothyroxine to Synthroid or from levothyroxine to NDT or from levothyroxine to Tirosint and so on. 

Simply switching the brand of your medication may be enough. 

If that doesn’t work you may need to focus on moving to a medication that contains T3 hormone (like NDT).

Some patients experience worsening hair loss when starting thyroid medication due to other micronutrient deficiencies (7) like zinc (8), Iron (9), Vitamin A, and Selenium (10). 

If you are deficient in these hair-growth-promoting nutrients then it may not matter if you are taking sufficient thyroid hormone. 

Furthermore, simply repleting these micronutrients is enough to stop hair loss in some individuals. 

Before switching thyroid medication make sure to check for these micronutrient levels. If you find yourself deficient make sure you find some high-quality supplements to replace them. 

You can find more information about these nutrients here:

It’s also worth pointing out that T3 medications like Liothyronine and Cytomel can also make hair loss worse but this effect is usually temporary. 

So make sure you watch out for this side effect when starting these T3-containing medications.

Negative Side Effects of Levothyroxine to Watch Out for:

As I mentioned above most of the negative side effects of levothyroxine come from the fillers, dyes, additives, and preservatives found in the medication. 

In addition, some people experience side effects because they aren’t taking enough of the hormone. 

On the flip side, it is also possible for you to get too much medication and suffer the side effects of too much thyroid hormone in your body. 

How do you know if your medication is too high?

You may experience the side effects of HYPERthyroidism:

boxed in text highlighting the symptoms of hyperthyroidism.
  • Worsening fatigue
  • Jittery sensation or nervous feeling
  • Insomnia or inability to fall asleep
  • Tremors or shaking/trembling of the hands
  • Anxiety or panic attacks
  • Worsening hair loss or hair thinning

If you experience any of these side effects you should see your physician as soon as possible to have your blood drawn and to potentially reduce your dose. 

Excessively high doses of thyroid hormone may put you at risk for a condition known as thyrotoxicosis

boxed in text showing the more serious and potentially life threatening symptoms of taking too much levothyroxine.

In addition to these side effects, I’m going to include ALL of the potential side effects as well.

Complete list of side effects of Levothyroxine:

Less common but serious side effects include: 

  • Chest pain or chest discomfort
  • Difficulty swallowing
  • Irregular heartbeat
  • Hives or urticaria
  • Menstrual irregularities
  • Sweating or hot flashes
  • Decreased urination
  • Worsening fatigue
  • Palpitations
  • Heat intolerance
  • Increased blood pressure
  • Muscles aches/pain
  • facial swelling
  • Shortness of Breath
  • Dizzy or lightheadedness
  • Fever
  • Increased pulse rate
  • Nausea or vomiting
  • Tremors of the extremities

If you experience these symptoms please seek medical attention as soon as possible. It’s important to realize that these are not very common (so the chance of them occurring is low) but if they do occur you may need some intervention which may include altering your dose or discontinuing your medication. 

More common and less serious side effects:

  • Abdominal pain or cramping
  • Diarrhea
  • Paranoia or distrust
  • Muscle weakness
  • Weight gain
  • Reduced appetite
  • Feeling “unwell” or “unhappy”
  • Headache
  • Feeling nervous
  • Worsening fatigue
  • Depression or anxiety
  • Sensation of warmth
  • Hair loss
  • Infertility
  • “Restlessness”
  • Weight loss

These side effects are much more common but far less severe. 

Some of these side effects may resolve over time without any intervention and others may require a small tweak to your dose. 

Do you need to Switch Medications?

Bottom line:

If levothyroxine or other T4-containing medications (Like Synthroid) are causing you multiple side effects then it’s worth a trial of a new medication to see how you respond.

If your symptoms are due to fillers, dyes, or additives then switching to a T4 medication like Tirosint may dramatically help (11).

If your symptoms are due to too much medication, then lowering your dose may help significantly.

If your symptoms are due to conversion issues in your body then adding medication containing T3 may improve your symptoms and side effects.

This information can serve as a rough guide to help you find whatever is optimal for your body. 

In my experience, many patients do better on a combination of T4 and T3, but a handful of patients do worse on these medications. 

Admittedly, I probably see a biased group of patients as most patients don’t come to see me unless other options have failed. 

Most Doctors are not comfortable dosing or adding T3 thyroid hormone so it may be difficult finding a physician willing to switch your medication entirely.

Most endocrinologists will not have a problem changing your medication to Tirosint or changing the tablets of levothyroxine to 50mcg tablets if necessary

Even though these are small changes they may have a large impact on how you feel. 

If all else fails, you can use these tips to try and help you find a knowledgeable doctor


Levothyroxine is a medication used to treat Hypothyroidism.

Even though it is bio-identical and is used to replace the lost thyroid hormone in the body many patients may react poorly to this medication.

Reactions or sensitivities may be caused by fillers, dyes, additives, or simply because it is not the RIGHT medication for your body.

Some patients may actually experience an increase in weight and an increase in hair loss while taking Levothyroxine. These symptoms may be directly due to the medication itself or because of how your body converts (or doesn’t convert) T4 to T3

If you fall into any of the above categories or are experiencing negative side effects of Levothyroxine you can consider switching to a different thyroid medication like Natural Desiccated Thyroid (Armour thyroid, WP thyroid, Nature-throid) or medication containing T3 (Cytomel or Liothyronine). 

Now it’s your turn:

Are you experiencing negative or positive side effects of Levothyroxine? 

Is this medication right for your body?

What medication has worked best for you?

Leave your comments below!

#1. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4960754/

#2. https://www.ncbi.nlm.nih.gov/pubmed/23072197

#3. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2957945/

#4. https://www.ncbi.nlm.nih.gov/pubmed/25017684

#5. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2950057/

#6. https://www.ncbi.nlm.nih.gov/pubmed/28138133

#7. http://www.ncbi.nlm.nih.gov/pubmed/20172476

#8. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3746228/

#9. https://www.ncbi.nlm.nih.gov/pubmed/20888066

#10. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5315033/

#11. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3726924/

levothyroxine side effects that you should know pinterest image.

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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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