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.
Scientific References
#1. https://www.drugs.com/comments/levothyroxine/
#2. https://pmc.ncbi.nlm.nih.gov/articles/PMC8259075/
#3. https://pmc.ncbi.nlm.nih.gov/articles/PMC8177794/
#4. https://pmc.ncbi.nlm.nih.gov/articles/PMC8110016/
#5. https://pmc.ncbi.nlm.nih.gov/articles/PMC4738522/
#6. https://www.neurology.org/doi/10.1212/WNL.0000000000200740?utm_source=chatgpt.com
#7. https://www.news-medical.net/news/20241125/Study-finds-bone-density-loss-associated-with-levothyroxine.aspx
#8. https://pubmed.ncbi.nlm.nih.gov/39810175/
I’m a 76 yr old man who is doing testosterone cream every day. Taking levothyroxine 25 mcg every day. If I do not do the testosterone cream I have to ly down in the afternoon due to being tired and weak. I’m 5′ 10 AND WEIGH 140 Lbs. I have a throat condition called LPS which the sphincter valves at the top of my stomach and below my voice box don’t go closed properly due to age. I eat a Mediterranean diet, no smoking or drinking, not much pop or sweet stuff. Is it because the thyroid isn’t producing enough hormone?
Hi Bernie,
Are you asking if your fatigue is related to your thyroid? If so, that is certainly part of the issue but probably not all of it.
Does NDT come with these same long term risks? Thanks!
Hi Melanee,
Studies don’t exist for NDT since it’s not as popular, but I would suggest that if you are being underdosed while taking it, all of the same principles apply.
Page 59. Post menepausal for 25 years, take high estrogen, progesterone and testosterone. Been taking 1.25 of levithyroxide 5mg for years plus 1.25 0f Synthroid for decades. Yet, my TSH, reverse R3, free T3 and T4 are off charts high. Doc keeps saying “it’s how it should be”. Suggestions?
*age
Hi Rhonda,
If you aren’t symptomatic, then your labs aren’t that important. You never mentioned anything about how you are feeling so there’s not much else I can add.
I was taking levothroxine 157 mg. I was having high blood pressure 200plus, fatigue, falling, tremors, weight loss, just had a MRI last week and came back reduced mass on the report. Emergency room doc. put me on 100mg. Blood test 1 week ago and my thyroid level was in range. Prior to that it was negative. Needless to say I’m going to new doc in 2 days. I’m 82. Will the bone mass come back? I’m still very weak. Thank you.
Hi Fay,
Probably not, though there is a possibility. Bone mass at your age is hard and slow to recover.
I would like to have an independent thyroid test other than my doctor. Can you suggest where I could go online for that possibly? In the past decades three times I have been prescribed synthroid and levothyroxine and have ceased taking them. My levels in their tests were slightly elevated which was why I was given the prescription. After a number of months on each, I sustained side effects such as clumsiness, brain fog, muscle aches and pains, and general bitchiness (female). The side effects for each med was different but intolerable for a happy manageable life which is why I ceased. I was given an alternative that had some kind of bovine ingredient and was told that would be better. It was not. So, I feel like I need to have an independent test since I won’t be going back until November for my annual exam. If the independent tests come back high, then I will at least have an intelligent place to start with your supplementations.
Hi Pat,
You can definitely get another set of labs if you’d like online from Everlywell: https://www.everlywell.com/products/thyroid-test/
But I don’t think that’s the problem here. The issue is more likely the intepretation of the labs, not the results, which is where most doctors and patients go wrong. Thyroid lab tests can be inaccurate, if not taken the right way, but this is usually only important when monitoring someone who is already taking thyroid medication.
As far as lab tests go and the need for supplements, that’s also unnecessary. Supplements target symptoms, not lab tests, so if you are symptomatic then you have all of the information you need.
As I mentioned, you can certainly get those labs drawn and tested, I just don’t think they will provide you with the clarity you are hoping they will.
My labs have always been wnl except for my TPO’s they range from 4500-6500!
I am on Levothyroxine and would love to get off it all together as I don’t feel it has had any effect on my- what’s best advise to taper off all together –
Hi Therese,
I would not try to taper your Levothyroxine until your antibodies are under control. When antibodies stay high, the immune system keeps attacking the thyroid, which increases the chances of permanent thyroid gland damage. If you taper while antibodies are elevated, you almost guarantee a poor outcome.
The best next step is to focus on lowering antibodies first. My Hashimoto’s Bundle is designed for this exact situation and is the best option to help reduce your need for thyroid medication over time while supporting immune balance:
https://www.restartmed.com/product/hashimotos-bundle/
Track TPO and TgAb every 8 to 12 weeks, along with TSH, Free T4, and Free T3. Combine the bundle with an anti inflammatory diet, adequate protein, stress management, and good sleep. Once antibodies are trending down and stable, you and your doctor can consider a slow, supervised taper.
This is a very upsetting article for those of us who do not have a thyroid. I can’t take NDT, the amount of T3 in that does not agree with my body at all. I was overmedicated on it for years and my body decided it does not like it after having my thyroid out. Seems I do better with Levo and a smaller amount of T3 but peripheral conversion can still be a struggle.
Hi Nicole,
Thank you for sharing this. I believe even without a thyroid you can optimize your thyroid medication so that you limit potential side effects from levothyroxine. It usually takes the right dose plus targeted supplements and lifestyle changes, but it is absolutely possible.
For patients in your situation, the most helpful additions tend to be:
• T3 Conversion Booster to support T4 to T3 conversion:
https://www.restartmed.com/product/t3-conversion-booster/
• Thyroid Glandular+ for natural glandular support:
https://www.restartmed.com/product/thyroid-glandular/
• Essential T2 to help metabolism and complement levothyroxine:
https://www.restartmed.com/product/essential-t2/
Alongside these, focus on nutrient repletion, protein intake, stress management, and sleep. Track how you feel, and review Free T3, Free T4, Reverse T3, and TSH together so you can fine tune your levothyroxine dose as your physiology improves.
I take over the counter Nutri-Meds which have been very successful at managing my Hashimoto’s. Nutri-Meds is a full complement of thyroid hormones sourced from crushed thyroid of grass-fed cows. I have severe bone loss for a 59-year-old – and I read it’s because my TSH was suppressed for too long. No idea if it’s true, but my bone loss is not related to Levothyroxine. I also worry about heart damage that may have been caused by my medication. I need to have my heart evaluated.
Hi Kari,
It’s certainly possible that long-term TSH suppression contributed, especially if your T3 or T4 levels were elevated for an extended period, to your bone loss.
That said, many factors also play a role in bone density, including nutrient status (vitamin D, calcium, magnesium, and vitamin K2), hormone balance, and overall inflammation. The same is true for heart health, which is susceptible to other factors like cholesterol, diet, and lifestyle.
The goal now is to make all of the necessary changes to your lifestyle to optimize for your heart health and bone health while also providing natural support for your thyroid. My recommendation is to use Thyroid Glandular+ which contains desiccated OTC thyroid gland powder and all of the cofactors, enzymes, and hormones found in the thyroid gland. 300-600 mg per day is great for most thyroid patients: https://www.restartmed.com/product/thyroid-glandular/
Dr Childs,
On the subject of side effects, can thyroid meds cause periodic moments of feeling light-headed if a person is over-medicated? I switched to tirosint and cytomel after reading your recommendations for 2025 best thyroid meds. Prior to this change, I took NP thyroid a couple of years ago and then my doctor transitioned me to Levothyroxine. I just switched to tirosint and cytomel a couple of months ago, but still have moments of feeling light-headed which I also felt every now and then on the previous meds.
Thank you for your advice.
Hi Rebecca,
Yes, feeling light-headed can happen if your thyroid dose is a bit too high, but it can also occur for other reasons such as fluctuations in blood pressure, hydration, or even cortisol levels. Since you’ve felt this way on multiple medications, it might not just be from being over-medicated.
I’d suggest checking your Free T3, Free T4, TSH, and Reverse T3 to make sure your dose of Tirosint and Cytomel is properly balanced. Sometimes a small dose adjustment or taking the medications at different times can make a big difference.
If you’re having trouble optimizing your medication, you can also try making more refined adjustments with supplements like T2: https://www.restartmed.com/product/essential-t2/
I have had thyroid cancer so now I am on thyroid meds. I take both T4 and T3 and I think I do not convert well as I do really poorly when just on T4. My issue is I still have hypothyroid symptoms even though my TSH is very low though my T3 levels are low normal. I cannot take any more T3 as it brings my TSH down even more. It is just above the level to bring on bone loss. I am really sick of not having a well regulated thyroid and the hypothyroid symptoms even though my TSH numbers are very low.
Hi Sue,
Thank you for sharing that. This situation is actually quite common after thyroid cancer. When TSH is suppressed for cancer follow-up, it can sometimes leave you with persistent hypothyroid symptoms even though your labs appear “within range.”
In your case, the problem is likely related to low T3 availability at the cellular level. Even though your Free T3 is technically in the normal range, it may not be optimal for how your body functions. Since you can’t safely increase T3 without lowering TSH further, the best approach is to improve T4 to T3 conversion naturally and enhance how your cells use thyroid hormone.
You can do this with targeted supplements and lifestyle changes that support conversion and thyroid signaling. I recommend starting with:
• T3 Conversion Booster: https://www.restartmed.com/product/t3-conversion-booster/
• Essential T2: https://www.restartmed.com/product/essential-t2/
• Thyroid Glandular+: https://www.restartmed.com/product/thyroid-glandular/
These can help improve how your body utilizes thyroid hormone without needing to raise your medication dose.
Dr. Westin, I was diagnosed 35 years ago. Started livothyroxine. Immediately found i could only take synthroid .275 mcg. Gained in 30 200 lbs. Was told I was stable till I read one of your articles about knee pain in 1998 I was able to get my provider to put me on armour. She was afraid of dosing it so when she needed to increase the dose she added synthroid. I lost 125lbs in 1 1/2 years. Till medicare kicked in and my provider changed specialties. Next provider put me on livothyroxine even tho she was told I couldn’t take it.so changed providers he sent me to endocrynologist. She put me on tirosint and leothyronine. Put back 75 lbs, hair falling out and exhausted. She says she will never put me on armour and will not change it. I was able to see my labs back to 2017 the only time my tsh was normal was while I was on armour. Went to surgeon yesterday. I need both knees replaced and a spin infused. I have paid cash for a wellness dr hoping I can get back on armour. I will have to pay for it big told medicare won’t cover it. I ran out of tirosint for 7 days I lost a pound a day.
Hi Patricia,
It sounds like your body does much better on Armour. Either that, or you were severely underdosed when you made the switch to levo + lio (which is also common). In either event, you can do a lot of good with the right support supplements to bridge the gap between what you are taking now and where you previously felt.
For your situation, I’d start with:
• Thyroid Glandular+ – Supports natural thyroid hormone production.
Plain link: https://www.restartmed.com/product/thyroid-glandular-plus/
• T3 Conversion Booster – Helps your body efficiently convert T4 to the active T3.
Plain link: https://www.restartmed.com/product/t3-conversion-booster/
• Essential T2 – Supports metabolism and cellular thyroid activity, even when medication isn’t optimized.
Plain link: https://www.restartmed.com/product/essential-t2/
These won’t replace Armour, but they will make you far less dependent on high doses, help stabilize energy and weight, and support your body while you transition back to the treatment that actually worked for you.
What can we do about taking too much levothyroxine? Without it, I can’t function. I take a high dose of NPTHyroid because I can’t function without it. This scares me. What else can i do?
Hi Pat,
This is actually very common. When you feel like you can’t function without high doses of thyroid medication, it usually means your body isn’t converting or using thyroid hormone efficiently, not that you truly need that much medication.
The right supplements can improve your thyroid function, support better T4-to-T3 conversion, and often reduce the need for higher doses over time.
For your situation, I recommend:
• Thyroid Glandular+ – Supports normal thyroid gland activity.
Plain link: https://www.restartmed.com/product/thyroid-glandular-plus/
• T3 Conversion Booster – Helps your body convert T4 into the active T3 you actually feel.
Plain link: https://www.restartmed.com/product/t3-conversion-booster/
• Essential T2 – Enhances cellular thyroid activity and metabolism.
Plain link: https://www.restartmed.com/product/essential-t2/
Using these will help reduce your dependence on the high dose of levothyroxine which will help minimize unwanted side effects.
I’m 50. Complete hysterectomy 2009. Total thyroidectomy 2011. Subsequent radiation therapy. I requested Armor after having nothing better to do than research while I was sequestered. Recovering, felt OK but not great but not gaining weight and feeling pretty ok. 6 months bloodwork comes back and we radiate again. They tell me Armor isn’t working and put me on generic levithyroid. 6 months bloodwork comes back. 3rd round of radiation. They switch me to name brand levithyroid. Post 18 months and I’ve gained 60 lbs. 6 months later bloodwork says range wont stabilize even given that every 2 months they are changing dosage to attempt to get me straight. They change me to Synthroid all the while changing dosage as the range won’t stabilize. Sometimes I’m low sometimes I’m high. Just all over the board. Put on another 20 lbs. Mind you I’m not eating junk food or fast food and I’ve changed my diet to a leaner, less carb more Mediterranean diet. Sick all the time. Nausea and exhaustion. In 2019 I spent 15k on a gym membership and personal training. I have a dietician, trainer, I wear a hr tracker and I’m tracked 6 out of 7 days. My heartrate is really high at the slightest exercise. They watch me closely and have me interval training. Unable to lose any weight. Now I will say that I gained muscle but no change in total weight or clothes sizes. Last year they changed meds to Triosent as the tablets were getting stuck in my esophagus. Im exhausted all the time, sleep isn’t good, I’m as active as possible (specifically squats, push ups and a few yoga stretches) my passion is riding motocross. My food intake is half what it used to be. Im 5’7″ and I’m sitting at 217 – 219 pounds regularly.
10 % of the population cannot take a synthetic drug. I was on levoxin 9 years ago gained 15 pounds,largic I got on the computer right away looked up Armour thyroid, ratio of Armour to Levoxin callled my doctor made appointment and off levoxin in 2 months. Armour thyroid has been the best, been on since 2016, great results.