Synthroid Side Effects Explained - 3 Main Reasons

Synthroid Side Effects Explained – 3 Main Reasons

Experiencing Side Effects? It Doesn't Mean you Need To Stop Your Thyroid Meds

This is a topic that can be confusing for thyroid patients.

And, by the way, the information we are about to talk about applies to ALL thyroid medications even though today we are primarily focusing on Synthroid

Synthroid is a thyroid medication used to treat low thyroid function

While it isn't the most popular thyroid medication (levothyroxine reserves that trophy), it's VERY similar to levothyroxine in terms of its ingredients - both inactive and active. 

And because it's so common, there are some things you need to know about it. 

Especially as it pertains to its side effect profile. 

But there's one big problem...

Most thyroid patients (and doctors, by the way) do not look at side effects correctly when it comes to thyroid medications. 

Most people, when they think about side effects, consider them to be something like this:

If you take a prescription medication and you experience a negative side effect then you simply stop taking the medication or deal with it if the medication is required. 

Most people view side effects in this way but this sort of thinking doesn't really work for thyroid medications. 

Unlike other medications, experiencing side effects while taking thyroid medication like Synthroid is actually very common and may not be a reason that you need to stop taking your medication. 

In fact, unlike other prescription medications, experiencing side effects while taking Synthroid can actually provide you with some additional and very beneficial information. 

The type of information which can go a long way to helping you feel better and get back to 100%. 

Today we are going to be discussing...

  • The 3 main ways that Synthroid can cause side effects
  • The difference between inactive and active ingredients and how these ingredients can cause side effects
  • What happens if you aren't taking enough Synthroid
  • What happens if you are taking too much Synthroid
  • How to tell the difference between these conditions
  • And much more

Let's dive in.

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3 Areas of Side Effects From Synthroid

You should be aware that Synthroid can cause side effects in three very distinct ways. 

And, just as a reminder, the following information applies to ALL thyroid medications and not just Synthroid. 

But back to Synthroid...

Because Synthroid is a bio-identical hormone, it's not really the same as other prescription medications you might be taking. 

Prescription medications for things like high blood pressure and high cholesterol are foreign to the body. 

Sure, they may alter your physiology, but they are completely different from bio-identical hormones. 

Synthroid contains the T4 thyroid hormone which looks EXACTLY like the same T4 thyroid hormone that your own body would produce if you didn't have low thyroid function. 

This means that, for the most part, your body knows what to do with the hormone once you ingest it and get it into your bloodstream (I use the word mostly here because Synthroid isn't used as well as other thyroid medications). 

Given this, it may make sense that the side effects you would experience when taking Synthroid should be completely different from the side effects you would experience when taking a completely foreign substance/medication. 

And if you are thinking that, you are 100% correct. 

There is some overlap between standard medication side effects, though, so we need to tease out the difference. 

There are 3 main ways that Synthroid can cause side effects which I will elaborate on below: 

#1. Side Effects from the Medication Itself or from Fillers/Dyes/Binders.

The first area should sound familiar to you.

Like other medications, taking Synthroid can cause side effects that are directly related to the medication itself. 

These side effects are separate and distinct from the DOSE (which we will discuss in detail below). 

These side effects tend to be related to the medication itself and are often related to a combination of the active and inactive ingredients found inside. 

If you aren't aware, here's a crash course on the difference between active ingredients and inactive ingredients. 

Active ingredients are those ingredients that actually have the desired effect that you want from the medication. 

In the case of Synthroid, the active ingredient is thyroxine or T4. 

But all prescription medications also contain inactive ingredients (1). 

These inactive ingredients assist in the manufacturing process, help stabilize the active ingredient, and may serve other purposes such as delaying absorption or assisting with rapid absorption. 

In the case of Synthroid, these inactive ingredients (2) include things like: 

  • lactose monohydrate (YES, this is lactose)
  • acacia
  • confectioner's sugar (with corn starch)
  • magnesium stearate
  • povidone
  • talc
  • Various dyes and or color additives based on dose such as yellow No. 6, Red No. 40, Blue No. 2, Yellow No. 10, Red No. 27 & 30, Blue No. 1, and many others
synthroid inactive ingredients

As you might have already imagined, it's quite possible for you to ALSO react to THESE ingredients. 

I mean, look at the list. 

Lactose makes an appearance! 

How many of you reading this don't tolerate dairy products very well? Probably many of you. 

The side effects you may experience from these inactive ingredients are completely different from the active ingredients! 

But you need to be aware of both. 

You'll know if you are reacting to these inactive ingredients because you may experience any of the following symptoms:

  • Hives
  • Rashes
  • Flushing
  • Swelling
  • Intestinal pain - Taking your medication should never HURT your stomach
  • Nausea
  • Vomiting
  • Sore Joints
  • Unexplained Fevers
  • Wheezing

These side effects are almost always from the inactive ingredients are represent a sensitivity to those ingredients (3). 

One last point I want to mention here before moving on is that Synthroid can cause directly cause hair loss. 

It's not clear where exactly this side effect comes from, but you should be aware that it can exacerbate hair loss independent of the dose that you are taking. 

#2. Side Effects From Not Taking Enough. 

These next two sections are DOSE related. 

This means that any of the symptoms you experience in these sections are either because your dose of Synthroid is NOT enough or because it is TOO much. 


Synthroid contains the exact same hormone that your own body is capable of producing. 

So as long as you are taking enough of it, it should be sufficient to reduce or eliminate your low thyroid symptoms (this assumes that your body can use completely and efficiently use Synthroid which isn't always true). 

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It is very possible that Synthroid is the right medication for you, but you are, perhaps accidentally, blaming your low thyroid symptoms on your medication. 

Either because your thyroid symptoms are worsening, or because your dose of Synthroid is not enough to keep up with the demand from your body. 

How do you know if your dose is sufficient?

Start by asking yourself these questions: 

  • Do you feel like your energy has improved since you started taking Synthroid?
  • Have you lost weight or has your weight stayed the same since starting Synthroid?
  • Have you noticed an improvement in the quality of your hair, skin, and nails since starting Synthroid? 
  • Have you noticed an improvement in your mood since starting Synthroid?
  • Has your menstrual cycle become more normal since starting Synthroid?
  • Have you noticed a decrease in joint pain since starting Synthroid?
  • Have you noticed more regular bowel movements or reduced constipation since starting Synthroid?

You should have answered yes to the questions above. 

Put in other words, you should see SOME improvement in your low thyroid symptoms about 6 to 8 weeks after you start taking Synthroid. 

Some improvement is a good sign and it means that your body is able to use the medication you are giving it. 

But if you aren't getting back to how you felt BEFORE you were diagnosed with your thyroid problem then you aren't quite where you need to be. 

If you see even minor improvement,(and assuming you aren't experiencing the side effects from section one) then it's highly likely that you are simply not taking enough or that your dose is too low. 

But what if you haven't noticed any improvement or if your symptoms actually got WORSE around the time you started Synthroid?

This is a different problem entirely and is usually related to how efficiently your body can or can't use the medication. 

In cases such as these, it's almost always preferable to try a completely new thyroid medication which you can read more about here. 

#3. Side Effects From Taking Too Much. 

The next section is basically the exact opposite when compared to section #2. 

Just like it's possible to not take enough, it's also very possible that you are simply taking TOO much. 

And, believe it or not, this happens quite frequently. 

Due to genetics, some people are far more efficient at others than using certain types of thyroid medications. 

Some people, people I like to call super converters, can take Synthroid and very efficiently activate it and use it in their body. 

For these people, it's actually easy to take too much. 

It's also possible, for various reasons, that your doctor accidentally just gave you too much. 

Whether you are just really good at using it or your doctor gave you too much, there are certain side effects that you may start to experience in these settings. 

These side effects include: 

  • Tiredness
  • Irritability
  • Sleep issues
  • Shakes/Tremors
  • Nervousness
  • Hair Loss
  • Muscle Weakness
  • Fertility issues
  • Rapid Heartrate
  • Heart palpitations

These side effects all have one thing in common:

They occur because your body is getting too much Synthroid and too much thyroid hormone. 

These are the exact same symptoms that patients experience when they are hyperthyroid, by the way. 

The solution for taking too much Synthroid is to simply reduce your dose. 

That's it. 

Switching to Another Medication

What are your next steps if you find that you are experiencing side effects while taking Synthroid?

The answer really depends on what type of symptoms you are experiencing, the severity of those symptoms, and where they are coming from. 

But what if you are experiencing symptoms from the inactive fillers, binders, or dyes in Synthroid?

What if you just can't seem to dial in your dose and you seem to experience fluctuations in symptoms no matter what you do? 

What if you've been playing around with Synthroid for years and years and you can't seem to feel better?

At some point, you may want to seriously consider switching thyroid medications entirely. 

And I have great news for you:

There are MANY different thyroid medications available outside of Synthroid and levothyroxine which, in my opinion, tend to be far more effective. 

These medications include:

Switching to a completely different thyroid medication isn't always easy but if it means the resolution in your symptoms and side effects then it will no doubt be worth it. 

Final Thoughts

Synthroid is a popular thyroid medication used to treat low thyroid but can carry with it many different side effects. 

These side effects are either associated with the dose of the hormone but they can also be secondary to the inactive fillers, binders, or dyes found inside. 

Identifying which side effects you are experiencing and what is causing them can help you determine whether or not Synthroid is working for you. 

If you find that Synthroid isn't working for you you have plenty of other additional options to choose from!

Now I want to hear from you:

Are you currently taking Synthroid and experiencing any side effects?

If so, what side effects are you experiencing?

Do you think your side effects are dosed related or inactive ingredient related?

Are you thinking about switching thyroid medications cause of your side effects?

Leave your questions or comments below! 

References (Click to Expand)

3 main reasons for side effects from synthroid

Dr. Westin Childs

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 45,000+ people have used them over the last 4.5 years. You can read more about my own personal health journey and why I am so passionate about what I do here.

10 thoughts on “Synthroid Side Effects Explained – 3 Main Reasons”

  1. Is it normal to have a free T3 to reverse T3 ratio of .38 (good) and a serum leptin level of 39 (not good) and not be able to lose weight? Shouldn’t my high T3 make the weight fall of? Most of my other hypo symptoms have subsided except for weight. Thanks in advance for any response!

  2. I’ve been taking synthroid for 2 yrs now & I experience every all of the Symptoms from taking too much except I also have went up 13 sizes in my weight in just one year I need to know what is going on with me, but I haven’t been on the same dose the whole Time it has increased over the years so I don’t understand why it’s doing this to my body I also take Cytomel with this

  3. In May 2012 I was diagnosed with stage IV Papillary Thyroid cancer. In June I had an 8 hour surgery, the Dr removed all of my thyroid and a portion of my parathyroid. After a biopsy there was “tall T cell” which I was told would need additional aggressive treatment. I then underwent internal radiation followed by external beam radiation.

    Since then I have been on synthroid. I have experienced fatigue and joint pain that has gotten worse every year to he point I have reduced my work by 75%.

    I hope the product I ordered, Glandular thyroid + dedicated thyroid helps me. I will be sure to share my results.

    • Hi Tony,

      Yep! You are on the right track with Thyroid Glandular+ 🙂 You may ultimately need more supplements but that is a good place to start.

  4. Hi, I had 1/2 of my Thyroid out in 1997 and in about 2005 had a Parathyroid Surgery, I was on Synthroid since 1997. About 8 Years ( 2013 ) ago my Dr. did the Thyroid Test and it came out that my Numbers are good and I don’t need the Synthroid Meds anymore. I didn’t believe my Dr. so I whent to a Thyroid Specialist and she confirmed thesame . So I am OFF Synthroid Meds since about 8 Years. I am fighting with my weight and I am very quickly irratable and short Tempered. Could it be that I need to be on the Medication again ??? But my Weight is my biggest Problem, I am 5’4” and weigh now 165lb and I am misserable. What is your advice on this??? Thank you very much for your time,

  5. I have been on Synthroid (50 mcg) for about one year now. The dose was less, but the doctor changed it to a higher dose because I wasn’t feeling great, and I think was always tired also.
    I am now so tired as well as dizzy and have no energy…..not good, as I want to work. My last test about 3 weeks ago indicated I was low on T3.
    The doctor never gave me the results — I had to ask him for it. He said it was “normal”…….and I said but “I saw the results that were emailed to me when I requested, and T3 was low”. He said “Oh that’s nothing”.
    Could you recommend a Thyroid Specialist in Beverly Hills, California ?
    Many thanks

  6. Hi. I just started synthroid 25mcg 3 days ago. I have metallic taste in my mouth. So far that’s the only side effect. Is this bad? I just was on np thyroid for 18 days. The first 4 days I was ok then got sore throat and a hoarse voice which I took allegra for and was better next day. Then I kept feeling good then bad. Finally felt like I was going hyper .

    • Hi Julie,

      The metallic taste in your mouth may be acid reflux. It’s possible your thyroid medication is causing that.


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