Surprising Side Effects Of Levothyroxine That You Should Know

Levothyroxine is, by far, the most commonly prescribed medication used to treat hypothyroidism and many thyroid patients have no idea the problems that it can cause.

This is probably part of the reason that thyroid patients score it a 5.1 out of 10 on average (1) and why so many thyroid patients prefer other thyroid medications like liothyronine or Armour thyroid.

While it still can be effective for many, if you are someone using it, you should be aware of these alarming side effects:

#1. Weight Gain

Most thyroid patients believe that taking thyroid medication will help them lose weight or, at the very least, improve their metabolism.

But, unfortunately, that’s just not the case.

Instead, what many thyroid patients find is that as they take levothyroxine their weight increases instead of decreases.

And this is not a minority of patients either (2).

It’s hard to assess the actual percentage of people who experience this side effect, but I would estimate it probably in the range of 30-40%.

This is based on my own experience of interacting with tens of thousands of thyroid patients over the years.

If you are someone who has experienced this side effect, or any of the others we are going to discuss, please leave your comment and experience below!

But getting back to the topic at hand, why does levothyroxine lead to weight gain?

The exact cause isn’t known, but there are a few suspected ways it can contribute:

The first is that most people who take it probably aren’t taking enough.

So in this setting, it’s probably not a side effect of the medication itself but just an inadequate level of treatment that allows low-thyroid symptoms like a sluggish metabolism to persist.

In this setting, thyroid patients may incorrectly attribute their weight gain to the medication when they likely would have gained that weight anyway had they not started it.

But there are certainly some people who experience weight gain that is directly related to the medication.

And in this setting, the mechanism of action likely comes from the suppression of the thyroid that occurs when taking it and the inability of the body to activate it.

You see, when you take levothyroxine, you are effectively shutting down your body’s ability to create its own T4 and T3.

And by taking levothyroxine, you are substituting whatever amount it would normally produce with 100% T4 thyroid hormone (the active hormone found in levothyroxine).

But if your body is unable to take that T4 and convert it into T3, through the process of thyroid conversion, then you may be filling your body and bloodstream with inactive thyroid hormone that eventually turns into the thyroid-blocking hormone known as reverse T3.

This malfunction of the thyroid conversion process is likely the root cause for many thyroid patients who experience weight gain when taking levothyroxine.

The good news? It’s completely solvable with the use of the right thyroid hormones.

This is why I always recommend using the full suite of thyroid hormones if you are taking any, which includes a combination of T4, T3, and T2.

If you are only taking levothyroxine, then you are only getting one of those thyroid hormones instead of all three.

#2. Increased Cancer Risk

This next one is probably the most alarming on the list and the one that gets thyroid patients riled up.

But before you get upset, here are the facts:

Several large-scale studies have shown that there appears to be some link between cancer risk and levothyroxine use.

I will give you my own thoughts on this in just a minute, but stay with me so we can lay everything on the table.

One study evaluated over 600,000 patients and compared them to 400,000 controls over the course of 10 years and found that those people using levothyroxine during this time had a 50% increased risk of developing cancer at any site (3) (brain, skin, pancreatic, and breast).

While this may seem concerning, and it is, here are a few things to consider before you start getting stressed:

The first is that this study does not prove causation, it just shows correlation.

So it may very well just be the case that people with thyroid problems have a higher risk of developing cancer regardless of whether or not they are taking any thyroid medication.

But one thing is for sure, this is a finding that needs to be further evaluated because if it is the case that long-term levothyroxine use causes cancer, it’s definitely something that all thyroid patients should be aware of.

If you are asking me, I do not believe that levothyroxine itself is the cause.

Instead, what I think is happening is this:

When thyroid patients take levothyroxine, they are not being adequately dosed with the correct amount of thyroid hormone for their body given how doctors approach thyroid medication dosing.

Because of this, they are in a perpetual state of thyroid dysregulation despite taking their thyroid medication regularly.

And it is this state of thyroid dysregulation (inadequate T3 relative to T4) that leads to the increased risk of cancer.

In this scenario, the link to cancer is related to the levothyroxine dose, but not the medication itself.

Because if the medication were to be used appropriately, and with other thyroid hormones, it’s my belief this risk of cancer would dissipate.

But this is just my own personal theory and more data is needed to better understand this connection.

The good news is that up to 30% of thyroid patients taking levothyroxine may be able to stop taking it (4).

So don’t get depressed thinking that you are someone that MUST take it forever as the data doesn’t support that.

#3. Hair Loss

Hair loss is another tricky side effect associated with levothyroxine use because it can be triggered by multiple mechanisms:

The first is that levothyroxine itself can cause hair loss as a side effect of its use, regardless of the dose you are taking.

The second is that levothyroxine may appear to contribute to hair loss if you aren’t taking enough (remember hypothyroidism leads to hair loss, and if you don’t take enough thyroid medication, then you will remain in this state).

And the third is that levothyroxine may appear to contribute to hair loss if you are taking too much.

Excessive use leads to a state of hyperthyroidism and hair loss is a known side effect of this state (5).

The hard part with hair loss in the setting of levothyroxine use is figuring out which of these mechanisms is behind it.

But the good news is that all of them are treatable.

If your hair loss is related to levothyroxine use directly, it tends to fade over time and can even be managed by switching to another type of thyroid medication like Tirosint.

If it’s related to an insufficient dose then you can simply increase your dose to compensate.

And if it’s related to too much levothyroxine, then you can reduce your dose.

Either way, don’t stress too much about this side effect because you can almost always manage thyroid-related hair loss.

#4. Dementia Risk

Some new research shows that there may be a connection between levothyroxine use and cognitive decline or dementia, especially in people who are older.

We don’t completely understand the exact relationship between these two conditions, but here’s what we know:

One large-scale study of over 15,000 people found that people with hypothyroidism have about an 81% increased risk of developing dementia compared to the average person.

This is concerning by itself, but what’s even more concerning is that this number increased more than 3-fold in hypothyroid patients who are also taking thyroid medication (6).

This seems to suggest that there’s definitely something about the hypothyroid state that contributes to dementia but that this problem is made worse in those who are taking thyroid medication.

Not much else is known about this connection except what I’ve shared, but my own personal suspicion is that this is another situation that is similar to that of levothyroxine use and thyroid cancer.

In this way, it’s probably not the medication itself that is the problem but instead, the way that it is used by doctors and patients.

But what makes this connection even more complicated is that some studies show that treating thyroid dysfunction may actually help prevent dementia as well.

As a result, it’s hard to discern exactly what is going on here, but I still think it’s something every thyroid patient should be aware of.

#5. Bone Density Concerns

This is another concerning side effect, especially for post-menopausal women who already are at an increased risk of bone loss due to the decline in estrogen.

And it’s long been known that there is a connection between altered thyroid states and bone health, but now it’s looking like the use of levothyroxine may make this situation worse.

Here’s what I mean:

One study of 81 thyroid patients and 364 controls (7) showed that even with a completely normal TSH, the patients taking levothyroxine experienced a greater loss of both bone mass and bone density compared to healthy individuals.

The big concern here is that this effect still existed even after controlling for TSH level, age, gender, medication use, and medical history, suggesting there may be some direct connection between bone loss and levothyroxine.

Again, it’s hard to pinpoint the exact mechanism of action and whether or not this is causal or just an artifact in the data, but it’s certainly concerning for women who take it.

There’s also additional research that seems to support this connection as illustrated in this meta-analysis (8).

Your Next Steps

Does all of this mean that you need to panic because you’re taking levothyroxine?

Not necessarily and here’s why:

First, we don’t know for sure if these side effects are directly related to the use of levothyroxine or just the dysfunctional thyroid state.

It does appear for some that it is connected, but more research is still needed.

Second, and perhaps more importantly, as I mentioned in the beginning, research shows that up to 30% of all thyroid patients taking thyroid medication may be able to get off it!

That is a huge percentage of you reading this right now.

And to make things even better, there are things you can do right and supplements you can take to help facilitate this process.

If you want to see a list of supplements that may be able to help you get off your meds, check out this article next.