Is fasting harmful when you have thyroid disease? If you have considered fasting, but aren’t sure about how it might affect your thyroid, let’s break down this topic for you so you know what action steps, if any, you should be taking.

Here’s the quick synopsis: There are some versions of fasting that can slow your thyroid, causing it to make less hormone, and impede your body’s ability to convert it.

But, there’s a lot more to that story. I’ll help you get familiar with the terms, the processes, and exactly how everything works (and the science behind it). That way, you have a clear picture of why fasting might not be optimal for your thyroid.


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Terms Concerning Fasting

So, let’s start by defining some common terms that we’re going to be using throughout today’s discussion:

  • Fasting – This is the process of having no foods during a set period (or set periods) of time.
  • Intermittent Fasting – Choosing windows of time in which you will not eat.
  • Time-Restricted Feeding – Choosing windows in which you will eat.
  • Protein-sparing modified fasts – A type of fast that is important to keep in mind now, as we will explain a bit more about it later on in this article.

Metabolism & Thyroid Function

Did you know that one of the biggest things that controls metabolism is thyroid function? One study, involving time-restricted feeding, resulted in a significant drop in T3 levels. But, no change in TSH (Read More: Should You Ignore Your TSH Levels?)1.

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This is important because these participants were not losing weight. Their total food intake did not go down, but in the process of narrowing their food window, they slowed their metabolisms by decreasing their food output.

What we now know is that there are different contributions that different pathways have in the overall regulation of thyroid hormones. These can be radically affected by fasting and by illness.

The thing that occurs most is a drop in T3 levels (Read More: How High Should Your T3 Levels Be?), or a drop/elevation in T4 levels. When this goes on long enough, it is not uncommon to see TSH levels get higher2.

There are a lot of ways in which the ‘peripheral’ use of thyroid hormones, outside your thyroid, can change. One of which is your body actively breaking down less T4 into T3 and making more of reverse T3.

We will also see your cells becoming more resistant. This means that they block the entry to thyroid hormones.

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The same has been observed in diets lower in protein. And, this is also true even when they are not “starvation” or low-calorie diets.

What happens, in this case, is that the body will lower T3 levels to spare muscle loss3. It’s like your body is protecting itself, due to the lack of protein, by lowering T3 levels.

All of this is to say that your thyroid, your metabolism, and your diet have a special relationship. And it’s one that we can’t ignore.

What About Fasting During Ramadan?

One of the most popular examples of fasting occurs during the religious tradition of Ramadan. In fact, time-restricted eating is a huge part of this process.

This is where eating only occurs during hours of darkness (before sunrise and after sunset).

Studies have shown that this can lead to a significant change in overall thyroid function. Those with thyroid disease found that their TSH scores had roughly doubled4. This was in addition to free hormones also being lower, too.

However, it was noted that there was not a dramatic change in symptoms or quality of life. That said, elevations to TSH may last as long as five months after the process of fasting.

In this case, if Ramadan were to theoretically end at the conclusion of January, we would likely still see effects all the way until June for thyroid function to come back again.

That is a big deal.

Breaking Down The Data

In the study referenced above, those participating in Ramadan were fasting for around 15 hours. This would be similar to an intermittent fast of 16/8.

Here are some of the results that they found:

  • The mean weight change over the 26.5 day fast was 0.4 pounds.
  • Caloric deficit would then be 52 calories per day less than the period prior to fast.

This suggests that the effects may have been more related to the timing of feeding, rather than the total food reduction. Thyroidal changes do not show up with food reductions under 500-700 calories per day.

Key Insight: There have been suggestions that thyroid medications may need to be changed preemptively during the period of Ramadan, to help protect those during times of time-restricted eating.

Protein-Sparing Modified Fasts

So, where does this lead us? Basically, we need to take great care and caution whenever we undergo a fast. Whether for religious or health reasons, we want to make sure we are doing things the right way.

Let’s talk about protein-sparing modified fasts. This is a version of fasting and happens to be one that I used in the Metabolism Reset Diet. I want to tell you a bit more about it.

It is a different kind of fast because it is thoughtful enough to give a maintenance amount of protein while having food timing remain frequent throughout the course of the day.

Also, it uses high amounts of fibers, like resistant starch, to keep blood sugar stable and steady throughout the daily process. We have seen that protein-sparing modified fasts can get the benefits of weight loss, without seeing the decrease in thyroid function.

Key Insight: That is when protein-sparing modified fasts are curtailed to windows of under six weeks, though, meaning that more than six weeks could reveal metabolic compromises that you want to avoid.
In the process of writing the Metabolism Reset Diet, I opted for a four-week process. This was to keep things conservative and safe. That’s because we know that this window is safe when you are:
  • Getting protein
  • Enjoying enough fibers

If the goal is simply general health and weight loss, fasting can be a simple way to do that. But, by doing a protein-sparing modified fast, you are less apt to move into a space where your thyroid is being slowed down as a byproduct of your fast.

The perks about a protein-sparing modified fast is that:

  • Your thyroid will remain healthy.
  • There will be less suppression of your metabolism.

In short, there is a good reason why I placed great emphasis on protein-sparing modified fasts during the Metabolism Reset Diet. They are a safer alternative to what we traditionally think of as fasts, and ensures that fasting does not hurt your thyroid.

Action Steps: What Should You Do About Fasting?

In general, we have only seen some studies about intermittent fasting and time-restricted feeding. So, there may be more benefits that emerge over time with a larger sample of data.

That said, so far, these kinds of fasts have been on the detrimental side for those who are struggling with thyroid disease. To be blunt, it often does more harm than good.

On the other hand, it is important to know that a protein-sparing modified fast is safe for those with thyroid disease (when done correctly, of course, like in the Metabolism Reset Diet).

Key Insight: If you are thinking about fasting, but want to play it safe with your thyroid, you would likely be best suited to try a protein-sparing modified fast first.

Are You Concerned About Your Thyroid?

For you, fasting may just be one more consideration for you when it comes to the health of your thyroid. If you are curious about what you can do, in addition, to help your thyroid, please consider taking the Thyroid Quiz today. It can help reveal more about your health.


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P.S. Whenever you are ready, here is how I can help you now:

1. Schedule a Thyroid Second Opinion with me, Dr. C, Click Here for Details
2. Download and use my Favorite Recipes Cookbook Here
3. Check out my podcast Medical Myths, Legends, and Fairytales Here

Dr. Alan Glen Christianson (Dr. C) is a Naturopathic Endocrinologist and the author of The NY Times bestselling Adrenal Reset Diet, The Metabolism Reset Diet and The Thyroid Reset Diet.

Dr. C’s gift for figuring out what really works has helped hundreds of thousands of people reverse thyroid disease, lose weight, diabetes, and regain energy. Learn more about the surprising story that started his quest.