Does Fasting Hurt Your Thyroid?

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Dr. Khoshaba

February 6, 2023

Is intermittent fasting getting the buzz? Feel like you need to follow the latest trend when it comes to diet? Have you been hearing about all the health benefits of fasting and feel like you don’t want to miss out?

Well... ever heard the phrase, “One man’s medicine is another man’s poison?” When it comes to anything regarding one’s health, there is no universal answer to be deemed healthy for everyone. 

Does fasting hurt your thyroid? There are many different types of fasting, too, so we can’t generalize and say all fasting hurts thyroid function.

What is Fasting?

Fasting is the act of abstaining from food or calories for a certain time. Now the timeframe is significant here. Some fasts can be 12 hours (think 8 pm until 8 am), and others could be anywhere from 24 hours to 3-4 days. Generally speaking, when one is fasting, they’re putting themselves in a caloric deficit.

Your thyroid is the master controller of your metabolism, so when you put yourself through a significant caloric deficit, particularly for long periods of time, you are essentially slowing down your thyroid function and, therefore, your metabolism. So how exactly does this happen? We’ve spoken about T4 and T3 thyroid hormones, but let’s dive deep into Reverse T3 (also known as RT3).

Benefits of Fasting

A glucometer,for does fasting hurt your thyroid?
A glucometer

There are many researched and proven benefits of fasting. It is important to mention that depending on your gender and age, the benefits of fasting and recommended fast lengths may vary. Some well-known general benefits of fasting include: anti-aging, improved cognitive function, increased weight loss, decreased risk of metabolic disease, decreased inflammation, and blood sugar control

What is Reverse T3? 

Reverse T3 is your metabolically inactive thyroid hormone that competes with your active T3 thyroid hormone. When your RT3 is high, your active and free T3 is low. A low T3 can cause symptoms of sluggish thyroid function, like unintentional weight gain, cold intolerance, fatigue, and hair loss.

Reverse T3 increases with dieting and competes with your active thyroid hormone (T3) to bind to your receptors. Several factors promote reverse T3, which inevitably decreases active T3 thyroid hormone serum levels. Certain things like stress, liver disease, illness, inflammation, nutrient deficiencies, lack of sleep, and extreme dieting can promote or increase Reverse T3.  

Reverse T3 and fasting

Extreme dieting, especially when in a significant calorie deficit, has been shown to be an added stressor on the body. Many studies have examined the relationship between extremely low-calorie diets on reverse T3 and overall thyroid function.

It is hard to deny the impact extreme caloric deprivation has on your metabolism and thyroid health. So is it the fasting that is “hurting” your thyroid, or is the calorie deficit that is to blame? This is a 2 fold answer and will be based on the individual. As mentioned, extreme calorie deficits via extreme dieting or long fasting could be an added stressor on the body.

Unfortunately, the body cannot distinguish between “good” stress and “bad” stress. You performing intense physical exercise, having a very stressful job, or running from a bear- those are all one and the same when it comes to your body and how it perceives stress. How we each respond to stress differs, and the amount of other stress we have in our lives will also make an impact.

Imagine having a 1-liter bucket set aside for all the stress you experience in daily life. One small stressor like a 12-hour fast a few times a week may not raise your cortisol and can cause an intense stress reaction, but if you add environmental stressors, extreme exercise, a stressful job, and stressful home life, that bucket can then more likely overflow and wreak havoc on your neuroendocrine system.  

What to do with this information

As you can tell, there are many benefits to fasting, but also some drawbacks. The only way to assess whether intermittent fasting is impacting your thyroid health is by experimenting with it and following up with bloodwork.

With most things, you want to practice moderation and address other areas in your that may be stressful. If you have a thyroid condition, perhaps fasting for long periods and eating a severe caloric deficit may not be the appropriate dietary changes to make. However, trying out a 12-16 hour fast once or twice a week may not be a bad idea. You do want to make sure that on the days you are eating during a restrictive time window, you are still consuming an adequate amount of calories. 

What if I am trying to lose weight and have a thyroid condition? Is dieting bad?:

A weighing scale and measuring tape
A weighing scale and measuring tape

No, you can still actively try to lose weight and follow a healthy diet without hurting your thyroid function. This goes back to moderation. You don’t want to embark on a quick-fix diet program and lose more than 1-2 pounds per week. The research shows that an extreme, quick-fix diet approach will not only often end in weight gain but will also negatively impact your thyroid function, slowing down your metabolism and making it more difficult to lose weight. 

If you want to try intermittent fasting for all of the health benefits, consult your Physician first. Knowing your medical history, they may be able to provide you with the best type of fasting to incorporate into your routine or assess whether you are undergoing too many other life stressors to initiate a fasting program in your diet. If you start to incorporate fasting, re-check your thyroid labs within 4-6 weeks to ensure your thyroid levels have been unaffected. 

Hopefully, this can provide you with some insight as to why fasting can impact the thyroid, but also give you the foundation to recognize the stressors in your life and how they can impact your thyroid and your overall health! 

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Dr. Linda Khoshaba is the Leading Integrative Health and Hormone Doctor in Scottsdale, Arizona. She has extensive experience working in the field as a Hormone Specialist and Natural Endocrinologist.

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