Is Low Carb Bad For Hypothyroidism?


[Last updated 9th January, 2019]

Hypothyroidism is becoming increasingly more common in Western countries.

One of the main symptoms of this hormone disorder is a slower metabolism and gradual weight gain.

Low carb and ketogenic diets have emerged as popular approaches to weight loss, at least in otherwise healthy individuals. But there is some controversy over the safety of these eating patterns for hypothyroidism.

This article reviews the scientific evidence available.

What is a Low Carbohydrate Diet and Ketogenic Diet?

What is a Low Carbohydrate Diet and Ketogenic Diet?A low carbohydrate (low carb) diet is any eating pattern that limits carbohydrate consumption.

The standard Western diet is about 50-60% energy carbs, or roughly 300 grams per day. Low-carb diets are typically 30% energy or lower, although there is no set criteria.

However, there is a clear distinction between a low carb diet and a ketogenic diet.

A ketogenic diet (keto diet) is a very-low carb diet that restricts carbs to less than 20-50 grams per day, or less than 10% of total energy intake.

This makes the body switch to ketones for energy – produced from fats – rather than glucose from carbs. Hence the name ketogenic diet.

Summary: Low carb diets restrict carbohydrates to less than 30% of total energy intake, while ketogenic diets restrict to less than 10%. A ketogenic diet causes the body to shift to using ketones as energy, rather than glucose.

Carbohydrates and Thyroid Health

Carbohydrates and Thyroid Health

Thyroid hormones are essential to maintain and regulate carbohydrate/energy metabolism (1).

Conversely, the energy (glucose) we get from carbs is required to fuel the production of thyroid hormones.

This is because the parts of the brain ultimately responsible for thyroid hormone regulation – the hypothalamus and the pituitary gland – require glucose to function.

In fact, the main regulation hormone, called Thyroid Stimulating Hormone (TSH), is made up partly of glucose molecules (2).

thyroid gland and thyroid hormones

Click to enlarge.

In addition to these important roles, carb intake appears to influence the amount of T3 that gets converted from T4 thyroid hormone. This is important for hypothyroidism as T3 is the active thyroid hormone that you need to increase.

It appears that when carb intake is drastically reduced, conversion of T3 from T4 declines (3).

This could be explained by the possible interaction between insulin and the enzymes that convert T4 into the T3 (1, 4).

Summary: Glucose and thyroid hormones rely on each other synergistically. The amount of carbs eaten can affect amount of T3 converted from T4.

Will Cutting Carbs Inhibit Hormone Production?

Will Cutting Carbs Inhibit Hormone Production

It’s known that a state of prolonged calorie restriction or fasting reduces T3 hormone production.

This occurs to slow down metabolism, increasing chances of survival when our ancestors were in times of food shortage (6).

Of course, lower T3 (and therefore a slower metabolism) is typically undesirable when food is abundant… especially for those with an underactive thyroid.

A ketogenic diet mimics starvation or fasting, at least from a metabolic point of view (7).

This means it may limit T3 production and potentially influence metabolism.

A number of small scale studies have shown that after a period of starvation, refeeding with carbohydrates – but not with protein or fats – normalised thyroid hormone levels (8, 9, 10).

In one of the studies, researchers evaluated the effects of restricting carbs at various levels (85, 44 and 2% of total energy intake) on thyroid hormones in 6 healthy male participants.

Results found that the high carbohydrate diet had no impact, whereas the very low carbohydrate diet did. It caused decreased T3 levels, increased rT3 and free T4 levels (8).

t3 concentration after carb restriction

Average T3 plasma concentrations after 11 days of high carbohydrate diet (white), control diet (grey) and low carbohydrate diet (black) in 6 healthy males. The * indicates a significantly lower T3 level for the low carbohydrate diet, which is not desirable in hypothyroidism.

These results are similar to those of another study where participants received an 800 Kcal low calorie diet comprising of either 0%, 25% or 100% carbohydrate, for a period of 2 weeks.

The results showed T3 levels were reduced from both fasting and the 0% carbohydrate diet, but not from the 100% carbohydrate diet (11).

What About Lower Carb, Not Ketogenic?

Simply reducing carb intake lower than normal – without ketosis – is unlikely to influence thyroid hormone levels (5).

A study of 17 obese patients on a 440 kcal diet for 3 weeks found those on the 1% carb diet had greater decreases in T3 levels compared to the 55% carb group (12).

So perhaps the further carbs are restricted, the more T3 hormone is reduced.

However, we must acknowledge the limitations of all these clinical trials I’ve mentioned:

  • Tiny sample sizes
  • Relatively short duration
  • Tend to use obese individuals, and may not be applicable to non-obese individuals
  • Studies were also not specific to individuals with existing hypothyroidism.

How directly we can apply the findings to a middle-aged man or woman with hypothyroidism is difficult to say.

But considering a long-term ketogenic diet can cause other hormonal imbalances (particularly in women) it seems unnecessarily risky in these circumstances.

Summary: Severe calorie restriction appears to limit the conversion of T3 from T4, which is undesirable for hypothyroidism. Studies suggest a very low carb ketogenic diet may have a similar effect because of the way it mimics starvation or fasting (metabolically). However, moderate low carb diets are likely safe.

Other Considerations

Other ConsiderationsIf you exercise frequently for prolonged periods, your carbohydrate and calorie requirements will be greater than average.

It’s important to consider this if planning to reduce carb intake.

Additionally, low carb diets have been known to increase the “bad” LDL cholesterol in select individuals, likely due to an increased saturated fat intake.

Consider this if you are at risk for heart disease, and be sure to have routine blood tests.

As always, it’s recommended you speak with your doctor or dietitian before making any major diet changes.


A low carb diet can help with fat loss and maintenance, particularly for those with an underactive thyroid.

Current research on the topic is limited, but it seems safe.

However, a very low carb or ketogenic diet is not recommended.

Studies suggest it inhibits the conversion of T4 into T3 (active thyroid hormone), which is undesirable for those with hypothyroidism.

Additional tips for weight loss with hypothyroidism can be found here, and a sample 2-week meal plan here.


About Eleise Britt (MSc Nutrition)

Eleise is a university qualified Nutritionist from Melbourne, Australia.

After studying a Bachelor of Health Science (Paramedics), she later went on to follow her passion for nutrition science and completed a Master of Human Nutrition at Deakin University.

Learn more about her on the About page

36 responses to “Is Low Carb Bad For Hypothyroidism?”

  1. Hi I suffer from non working hypothalamus..

    It really seems like I cant loose weight, but only gain weight. My current weight is 105 kilo and before it was 70 kg.

    My doctor wants to put me in directly T3 medicine but that also inhibits my own body production of testestorone. Can you recommend any diet or living style that could lower my weight ?


    • Paleo for Autoimmune works for me basically you eliminate gluten, sugar, process food, corn, soy and nightshades vegetables is a pretty balance diet I recommend you to research more about it and if is possible get this book “Hashimoto’s Protocol” by Dr Izabella Wentz. Good look and one love.

  2. I am 47 years old and was diagnosed with an underactive thyroid in my late 30’s. They did a blood test for tsh levels and started me on meds i was never leveled for several reasons, but my real concern is the doctors never looked at my thyroid or sought a cause just assumed it was autoimmune disease. It’s seems that Western medicine doesn’t care of causes they what to treat with meds to help symptoms, what do you suggest?

  3. I’m. 53. Years. Old. And. Been diagnosed. With. A inlarged. Thiroide. I’m so. Confused. They. Keep. Switching. My. Meds. From. 50 micgram. To. 100. Micgrogram. And. Now. To. 150. Microgram. Am. I. Eating the. Wrong. Foods. Or. What. Kind. Of. Food. Or. Died. Is. Gonna. Help. Me. Get. This. Right

  4. I’m on 90mg of Armour thyroid medecine. I’ve been hypothyroid for 4 years now and this runs in my family. If you want to lose weight and feel better I suggest you research every free moment you have and try it all. Everyone is different and reacts different. I’ve yo-yoed with my weight due to my Dr. changing meds or my thyroid getting worse. I’ve lost and gained the same 20 pounds probably 12 times by now but I never give up and neither can you! I’ve tried atkins, fasting, intermittent fasting, up at 4am to work out for 2 hours (hour weight training) with a good diet. The working out with a good diet worked best but with a slight change in my meds 19 pounds came back in 6 weeks. A year of hard work for nothing! Don’t trust your doctor’s. Do your research. Because low carb effects your t3 production I’m starting atkins this time with a t3 supplement called cytomel which is straight up T3. Atkins works well but if you have a bad thyroid you’ll gain weight instead. Find a place to but T3 and go from there. Also look at body building sites. They have a lot of knowledge in the dept.

  5. I’ve been doing very low carb. Meat cheese fish green veg salad. I have lost no weight in 8 weeks. I eat only sweet potato as carbs. My thyroid is underactive. I’m 59 year old lady feeling very down Can anyone help?

    • Hi Shaz,

      Make sure to eat the proteins and fats earlier in the day and carbs at night. Carbs help stimulate serotonin that triggers melatonin for sleep. This blog also showed that carbs are essential for thryoid production. THS followed by T3 are manufactured during sleep and peaks before awakening in conjunction with cortisol rhythm ( This is to address thryoid and sleep function, along with mood. As a sidebar, protein and carbs are esential neurotransmitter and hormone regulators while fat isn’t.

      Fat loss is a function of calorie restriction relative to energy expenditure. So, if your body burns 1300 calories a day and you have 30% fat that you want to trim, you have to restrict calories by for e.g. 5% while ensuring you get adequate protein and carbs. If you eat 1300 cals (weight maintenance) or more (energy surplus), you won’t lose fat even if your’e on a keto diet. This is why it’s possible to loose fat even on a non-keto diet. It takes approx 3500 calorie deficit to lose 1 pound of fat. Look into intermittent fasting. E.g. do not eat anything for 16 hrs after dinner. Begin your first meal with protein, fat and greens. Then consume your dinner/ last meal within 7-8 hr after your first meal. Dr. Eric Berg has some great videos that could help along the way:

    • Shaz, how many grams protein daily are you getting? May not be nearly enough. Please visit Optimal Ketogenic Living FB page…..most effective lower carb way of eating I have found…..all the best….

    • I hear you! Last October i went to my doctor and again they said the tests were normal and I felt so horrible, rashes, stomach problems, not sleeping, joint pain and much more, my legs were swollen and I was told NORMAL??? . that was it! i left the office crying then I got MAD AS HELL! So, i went online to amazon , ordered ‘Stop the thyroid madness’ book 2. This led to me ordering thyroid meds online , they are called ‘THIROID’ (not a misspelling) ) They have a Facebook page. I ordered a 1000 count. All natural and with these I take Hypitrol which is a supplement I order from Amazon, formulated by the lady who wrote the STTM book. After 1 month, no joint pain, inflammation reduced, no stomach issues. It was the Levothyroid, Synthroid which I took previously was even worse. My hair loss is lessened, I am so much better.

      • My doctor told me the same thing. I’m interested in what you are saying. I’m going to order the book and read about it. Thanks. Are you still seeing a doctor to monitor your levels?


            Here is a link to order a complete thyroid panel online. In my neck of the woods the testing is done by Quest Diagnostics. I haven’t ordered these yet. Giving my doc one more chance to help me out before he gets fired. I’m exhibiting all of the classic signs of an under active thyroid but my TSH is “normal” so my doc says I just have anxiety and depression.

            I am 37 and an athlete. I love lifting heavy shit but that has gone to crap in the last couple years and my doc keeps saying its my diet or i need to exercise more. I’m not sure how to exercise more than doing CrossFit or power lifting 5 days a week. So recently i started learning everything I can.

            Here is another link i recently found It is a comprehensive list put together by patients in the US of all thyroid friendly doctors. If my doc won’t help me out I’m going to print off all my labs and records from his site (thankfully they post those online) and pick one off this list to try out.

  6. was told high thyroid levels been taking medication for 5 yrs last 4 years numbers would in normal range, however was unable to lose weight actually gained weight and nothing I could do worked. I went on low carb high protein diet and excerise every day and was able to lose 10 pounds (had to work very hard at this) and just had tyroid test and it is higher (was .39 now 5.40) . what the heck?

    • Were you told you were hyperthyroid? If you were overweight, that seems doubtful. There are many different thyroid tests. Was this the usual TSH test that all doctors do? That is actually a pituitary hormone called “thyroid stimulating hormone”. If it is raised, then your body is trying to make more thyroid because it is low. It does not mean you have “high thyroid levels”. If you went on a very low carb diet, your body would have trouble making the active T3 hormone or you would develop high reverse T3 which cancels out the T3. Very low carb is bad for hypothyroids.

  7. Thank you all for your comments

    I’ve slightly increased carb intake to include occasional rice and white potatoes

    I lost 4 lb and my mood improved ?

  8. “Summary: Severe calorie restriction appears to limit the conversion of T3 from T4, which is undesirable for hypothyroidism.” Low carb and keto diets should not be equated with calorie restrictive diets.

    I just read an article suggesting that lowered T3 in response to keto diet may in fact indicate higher thyroid hormone sensitivity, as it did not coincide with hypothyroid symptoms. We need more studies! Food for thought!

  9. I had RAI as a child, no thyroid. meds ever since. LCHF changed my life and gave me a normal bmi for the first time in my adult life after a handful of kids. do your research and find out for yourself. don’t listen to anyone on the internet who doesn’t know you and your body. do what works for you. keto on.

    • I too have no functioning thyroid and was hoping to find answers. Like is Keto right for me? I think you answered my question Rebekah I think I will keto on. I think it will be my best bet in losing and controlling my yo yo dieting and weight loss/gain. Thanks,
      Nancy M.

  10. If my thyroid is 2.5 and I am on 50mkg of syntheoid world keto be ok for me? I am on week three and don’t own a scale but clothes feel the same and I am frustrated. Also it’s hard to eat all the calories bc I am full from the fat.

  11. I had my thyroid removed last year and have gained 20lbs. While I like to eat and don’t exercise I can’t lose anything when I try. I’m depressed, achy, sweat and always tired.
    I take 125 mcg of synthroid. What should I do to lose weight and correct myself.

    • Hi Jean, the way to lose weight effectively is to be in a caloric deficit, i.e, consume less energy than you expend. Dieting is the obvious and easiest way to do this. Exercise adds a bit to the calorie deficit but mostly promotes overall health more than anything.
      In order to be in a calorie deficit, choose foods that are low in calories but have a high level of satiation (they make you feel full), such as lean protein sources, plain white potatoes (no butter or cheese added!), all kinds of vegetables, fruit, plain yoghurt etc. Choosing these foods and eating until you are 80% full each time should put you in a caloric deficit.

    • New publications can be found (and great authors, too) that claim 1) TSH is highly flawed to monitor treatment, 2) T4 monotherapy (Synthroid) does not provide euthroid status in all tissues in rodents yet no one has looked in humans to see if this is the case for us, 3) rodents have exponentially higher MK-4 (an endogenously-made vitamin K2) in their thyroids than in any other tissues/organs.
      Does this apply to humans?
      No one knows.
      Consider adding T3 to your treatment
      Look to get better K status by eating more vitamin K2 from real cheeses, liver, natto and…if you are blocking K status by some drugs (statins, bisphosphonates, warfarin) or eat lots of processed/fried foods then change.
      Consider a K2 supplement

  12. Interesting indeed, but quid goitres ? Nodules ? Do they react to keto and/or weight loss, just like T3 does ?

  13. I completed the Whole 30 which limited a lot of different types of food. The result was a loss of 15lbs, great skin, more energy and better eating habits. It also threw my levels way off and I had to increase my medication from 75 to 125mcg within 3 months. I am back up in weight and all the good of the 30 days is gone, but my levels are normal. My question… Which is more important, the levels being right or healthier weight and habits?

  14. Interesting read. However, I have found inspite of working out 5x a week 3/4 classes of spin-hiit and 3/4 classes of bodypump (yes I double up some days) that I do not lose weight unless I am on an atkins, ketogenic, extremely low-carb diet. I have Hashimoto’s and was diagnosed at age 12 (I am 39) and my body disintegrated my thyroid – so no thyroid. I started out taking just synthroid, then in mid 20’s I put on 80 LBS in 3 months and my doctor added Armour and I lost the weight with diet/exercise (extreme intensity) and kept it off until I was 36 (had my 1st child at 32). At 36 I experienced exactly the same as when I was in my mid 20’s gained and excessive amount of weight in 3 months and could not lose it. My doctor increased my synthroid and took me off of Armour and put me on 10 MCG of Liothyronine and as stated above work out 5x a week pretty intensely, eating a pretty well rounded diet and sticking to under 1800 calories. This has been since Sept and it is now April and I have lost/gained the same 5lbs. So today I go back to the atkins/extremely low carb/ ketogenic model because for me that is the only thing that has worked in weight reduction.

  15. This was helpful. I started keto and my body temp is completely off. I learned the month I spent on the diet affected the hormones and caused some thyroid issues. Also discovered I have thyroid nodules.

    I’m getting treatment. Still, I had no clue. Wish I’d read this first. 🙁

  16. I have hypothyroidism and take levothyroxine. I only lose Weight when I do Keto. I exercise and eat a balanced meal but this usually maintains my weight. How can I ultimately utilize keto for my healthy lifestyle?

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