Is Intermittent Fasting Bad for People with Thyroid Issues?

hands holding empty plateIs intermittent fasting a good idea for people with thyroid issues?

It’s a common question. After all, we know that the thyroid gland is a sensitive barometer of overall caloric sufficiency in the body. If a fast sends a message of caloric insufficiency, and the body thinks “times are tough,” the thyroid may presumably down-regulate its function to slow down the metabolic rate and preserve energy and nutrients. Caution is justifiable.

What Happens to Thyroid Function When You Fast?

It’s all going to be specific to the individual fasting, of course, but here’s what studies of humans have shown:

In 1978, researchers placed 45 obese human subjects on a 4-day fast, studying their thyroid hormones levels before and during the fast, and after refeeding. Everyone’s TSH (thyroid stimulating hormone) and T4 (inactive thyroid hormone) went up during the fast. Everyone’s T3 (active thyroid hormone) dropped. Men saw bigger changes than women. When they supplemented T4 during the fast, T3 still dropped. When they supplemented T3, T3 was maintained.

  • After the fast, what the people ate affected whether their thyroid hormones recovered.
  • Those who ate pure carbohydrate saw their T3 return to normal.
  • Those who ate mixed meals of fat, carbs, and protein saw their T3 return to normal.
  • Those who ate only protein still had lower T3.
  • Those who ate only fat still had lower T3.

Summary: A four-day fast depresses thyroid function in obese people without preexisting thyroid issues. Refeeding with carbs and/or a mixed meal restores it; protein or fat alone does not.

In 1980, researchers placed obese human subjects with normal thyroid function on a 10-day fast. Prior to fasting, they ate 1500 calories per day for five days. During the fast, their T3 levels dropped from 167 ng/dl to 86 ng/dl despite repeated infusions of thyrotropin releasing hormone (TRH), to which they were unresponsive. Normally, TRH will cause TSH to rise, thereby stimulating thyroid hormone production, but this didn’t happen during the fast.

After fasting, five days of 1500-calorie refeeding still weren’t enough for T3 and TSH responsiveness to TRH infusions to fully recover.

Summary: A 10-day fast depresses T3 levels and thyroid responsiveness to thyrotropin releasing hormone, even after five days of refeeding.

What about shorter, less extreme fasting? Most people reading this aren’t doing 4- or 10-day fasts. They’re doing truncated eating windows. They’re skipping meals for a day, here and there. Do we have any studies on thyroid function during those types of fasts?

In Ramadan fasting, you eat before sunrise and fast until sunset. During the fast, you abstain from both food and drink (even water). Ramadan fasting is less extreme than four-day or 10-day all-out fasts and thus a bit more relevant to the average IFer.

In healthy Muslim males, Ramadan fasting has no effect on thyroid function or hormone levels. Other than minor fluctuations, TSH, T3, T4, and all other measures of thyroid function remain virtually unchanged in males during Ramadan.

Women may experience drops in T4 and T3 during the last few days of Ramadan.

Most interestingly, Ramadan fasters who are hypothyroid patients taking thyroid hormones generally do not have to increase their dosages during Ramadan. In fact, they see improved quality of life following the month of fasting.

How About Alternate-Day Fasting?

In one study, groups of people with subclinical hypothyroid and normal thyroid status were either placed on alternate-day fasting (25% of calories on fasting days, 125% of calories on non-fasting days) or calorie restriction (75% of calories in perpetuity) for six months. Both groups lost about the same amount of weight and body fat. In hypothyroid patients, fasting produced greater improvements in fasting insulin and insulin resistance.

In other words, it worked about as well as (and in some respects, better than) calorie restriction.

That’s about it for published research on the effects of fasting on thyroid function in both euthyroid and subtly hypothyroid people. While there weren’t any studies I could find looking at the effects of intermittent fasting on clinically hypothyroid people, I think we can make some inferences on fasting and hypothyroidism.

Tips for Fasting When You Have Thyroid Issues

1. Make sure you’re replete in the thyroid-relevant nutrients.


  • Selenium: for converting T4 to active T3
  • Iodine: for making thyroid hormone
  • Zinc: for thyroid function

2. Don’t combine fasting with extreme calorie restriction.

What really seems to destroy thyroid function is chronic calorie restriction—going “without” for weeks, months, and years on end. Your thyroid is always watching you, watching the signals you send. Feasting and fasting sends a signal of abundance. Reducing calories by 20% for months sends the opposite signal, and reduced thyroid function may be the result.

If you fast, make sure you eat enough food when you actually eat.

3. If you’re hypothyroid but have it under control with thyroid hormone, fasting will probably work for you.

Taking thyroid hormone the right way effectively renders you euthyroid (normal thyroid function). You can proceed with fasting without much worry; just monitor your thyroid hormones and track your subjective impression of how it’s going.

4. If you’re subclinically hypothyroid, fasting is probably okay.

Research linked above shows that people with subclinical hypothyroid can benefit from alternate day fasting (where the “fast” day is actually a super low-calorie day and the “fed” day involves eating extra food).

5. If you’re hypothyroid and trying to fix it naturally, long fasting is probably not a great idea.

One of T3’s roles is to protect the muscle from atrophying during fasts. If you have low T3 and want to handle it naturally, launching into a fasting routine will probably be counterproductive and may even speed up muscle loss. This is exactly what you don’t want to happen during a fast.

Get the hypothyroidism handled before you try a serious fast (~24 hours+).

6. If you’re hypothyroid and trying to fix it naturally, shorter eating windows may help.

I don’t recommend 24 hour+ fasts for hypothyroid patients going it alone, but I do think there’s an argument for a shorter eating window, especially if that eating window falls during the daylight hours.

Like everything else in the body, thyroid function follows a circadian rhythm. Food is a powerful circadian entrainer. If you’re eating at all hours of the day and night, or packing most of your calories late in the day and on into the evening hours, this may throw off your circadian rhythm and your thyroid function along with it.

Conversely, if your eating window is from 9 a.m. to 5 p.m., you’re eating with the sun, which syncs your biological clock with the objective daytime.

7. Be careful on the extended fasts.

I don’t recommend them for hypothyroid patients, but I know you’re a curious, experimental bunch who will try it regardless. If that’s you, be cautious. Listen to your body. Track your numbers. Don’t be afraid or feel ashamed if you have to cut the fast short with a meal.

8. If it feels stressful, it probably is.

Stress inhibits the conversion of T4 to T3, thereby reducing the level of active thyroid hormone in your body. Don’t ignore stress, especially if it’s the fasting that seems stressful.

9. If fasting affects your sleep, it’s probably affecting your thyroid.

Adequate sleep is a bare minimum requirement for good thyroid function. Some (but not all) people report worse sleep when fasting. Others (but not all) report better sleep when fasting. This could go either way—just make sure you know which way it’s affecting you and your sleep.

That’s about it. The subject isn’t as extensively researched as I’d like, and there’s a lot of individual variation, but we have a decent amount of info to go on and can make some decent inferences. If you have any questions about fasting and hypothyroidism, ask down below. I can probably pick Elle Russ’ brain (the real thyroid expert around here) for some input, too.

Take care, everyone!


Azizi F. Effect of dietary composition on fasting-induced changes in serum thyroid hormones and thyrotropin. Metab Clin Exp. 1978;27(8):935-42.

Burman KD, Smallridge RC, Osburne R, et al. Nature of suppressed TSH secretion during undernutrition: effect of fasting and refeeding on TSH responses to prolonged TRH infusions. Metab Clin Exp. 1980;29(1):46-52.


Akasheh RT, Kroeger CM, Trepanowski JF, et al. Weight loss efficacy of alternate day fasting versus daily calorie restriction in subjects with subclinical hypothyroidism: a secondary analysis. Appl Physiol Nutr Metab. 2020;45(3):340-343.

Ucci S, Renzini A, Russi V, et al. Thyroid Hormone Protects from Fasting-Induced Skeletal Muscle Atrophy by Promoting Metabolic Adaptation. Int J Mol Sci. 2019;20(22)

About the Author

Mark Sisson is the founder of Mark’s Daily Apple, godfather to the Primal food and lifestyle movement, and the New York Times bestselling author of The Keto Reset Diet. His latest book is Keto for Life, where he discusses how he combines the keto diet with a Primal lifestyle for optimal health and longevity. Mark is the author of numerous other books as well, including The Primal Blueprint, which was credited with turbocharging the growth of the primal/paleo movement back in 2009. After spending three decades researching and educating folks on why food is the key component to achieving and maintaining optimal wellness, Mark launched Primal Kitchen, a real-food company that creates Primal/paleo, keto, and Whole30-friendly kitchen staples.

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15 thoughts on “Is Intermittent Fasting Bad for People with Thyroid Issues?”

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  1. Hi Mark, thanks so much for all the work you and your staff do to educate us. I have a different issue that I don’t hear much about anywhere, the focus always seems to be on hyPOthyroidism. I’ve had hyPERthyroidism and even though it’s not as severe as it once was I’m still considered borderline and I hope you can give me some information regarding this and IF. Thank you.

    1. I second this. Hyperthyroidism doesn’t really get talked about whenever someone talks about thyroid issues.

      I’ve been showing signs of hyperthyroidism for years, but I’m in the military and they follow conventional wisdom when it comes to testing. I’ve even had a doctor tell me it sounds like I have it, but the labs came back normal so I’m fine. End of discussion.

      I’d like to know what I can do to help resolve hyperthyroidism naturally, if I do actually have it.

  2. I started fasting consistently (8-9 hour eating window) a few months ago. I was feeling fine, but asked for a TSH test because I noticed my outer eyebrows were thinning again. I usually stay around 1-2 mIU/L, but my test last week now shows me at almost a 5! Not good! Maybe this fasting thing isn’t for me.

  3. Just a note that for someone with labile TSH levels or hyperthyroidism, taking extra zinc or iodine can push a person into hyperthyroidism (racing heartbeat, anxiety, night sweats, etc.). The advice presented seems to be geared towards those with hypothyroidism.

  4. As someone in this boat, I appreciate the article. I find for myself that everything is better if I wait until 12:00p.m. to eat. I’ll heed the warning about too little calories, though. Thanks.

  5. Thanks super helpful! I’ve always struggled with fasting even after going keto. It didn’t make sense, but now it does!

  6. What is your/Mark’s personal hormone replacement regime? I cannot find the info on the web site?!
    Thank you!

  7. I had thyroid ablation (destruction of the thyroid with radioactive iodine) 25 years ago. I have practiced intermittent fasting for several years without ill effect. I am a 54yo woman entering menopause and haven’t had to adjust my dose for a few years. I take both T3 and T4 supplements. My doctor recommends both for women past 35, since conversion efficiency begins to drop around that age. I feel great when I practice IF, and I have excellent blood sugar levels.

    1. Hi there.. inspiring hearing It from you… I had similar issue 7 years ago (thyroid removal + radioactive iodine), but here in the UK they only prescribe levothyroxine (T4).. they don’t even test my T3.. ever… which I have insistently asked for. I’m 38yo now and never felt worse.. hair loss, opaque, breaking nails, weight gain.. but GP says my T4 is fine, and that’s the end of the talk! May I ask how do you take T3? Is it allowed wherever you live? May I ask where is it? Cheers and Thanks for sharing!

  8. Hi Mark,
    I have a question concerning your article on fasting and thyroid disease. What about intermittent fasting for women with hyperthyroid. How does this effect T3-T4? I have been fasting 18 hours daily with low carb intake.

  9. Hi Mark, My thyroid was removed a year and a half ago. I take synthroid daily and l have not missed a beat. I normally eat between noon and 9 pm. So far so good. Anything I should be aware of for intermittent fasting?


    1. Hey Peter, May I ask how do you feel on your daily basis under synthroid, regarding energy level, weight loss/gain, etc?

  10. HI there,

    I am awaiting to start medication for Hypo, I’m wondering if its a bad idea to start IF (16.8) will this lower my T3 level and make weight loss even harder. Should I wait until I start my medication and I’m receiving the addition T3 from the medication ?

    Thank you!