Is Low Carb Bad For Hypothyroidism?

Is a low carb diet bad for hypothyroidism?

[Last updated 9th November, 2017]

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.

Low carb and ketogenic diets are now popular approaches to health and weight loss. But are they safe for hypothyroidism? This is a review of the evidence. See more here:


  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 ?


  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 dugg some of you post as I thought they
    were invaluable handy.

  4. 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

  5. Hypo chick says:

    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.

  6. 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….

    • irene schreiber says:

      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?

  7. 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?

  8. 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 😊

  9. Jennifer Kondor says:

    “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!

  10. 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.

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