SIMPLE’s guide on intermittent fasting and hypothyroidism

Written by Trey Reck

Medically reviewed by Louise Bula, RD

Published on October 27, 2023 | Updated on May 3, 2024

When your thyroid doesn’t work like it’s supposed to, losing weight can be even tougher to achieve than usual.[1] Perhaps you heard about intermittent fasting and are wondering if, given your hypothyroidism, fasting could be an option for finally changing that number on the scale.

So, are intermittent fasting and hypothyroidism a good fit? Or do fasting and thyroids not gel well? Let’s check it out.

Hypothyroidism — what is it?

The thyroid gland makes two hormones: thyroxine (T4) and triiodothyronine (T3). These hormones are necessary for all the cells in your body to work normally. T3 — either made from T4 or released as T3 directly from the thyroid — influences your metabolism and regulates how fast or slow your cells work.[2] 

In some people, the thyroid is overactive and generates too much thyroxine. This means their cells work faster than they should. This is hyperthyroidism.[3] 

For others, the thyroid is underactive and produces too little thyroxine, so the cells and organs of their body slow down. This is hypothyroidism.[4]

What are the symptoms of hypothyroidism?

In hypothyroidism, as the thyroid gland isn’t active enough, it doesn’t produce enough thyroid hormones. Because thyroid hormones regulate energy metabolism, this leads to a decrease in energy expenditure. That in turn can lead to weight gain [5], as well as difficulty maintaining weight or losing weight. It can also lead to [6]: 

• tiredness
• a feeling of coldness
• inability to concentrate
• depression and anxiety
• low energy levels
• brain fog
• dry skin
• hair loss
• excessive sweating

Intermittent fasting and hypothyroidism

Intermittent fasting is known for helping those who want to lose weight and its health benefits more generally. So, could intermittent fasting be a way to hack the halting weight loss of hypothyroidism?

The answer: a resounding … maybe. 

If you have hypothyroidism and are struggling to lose weight, the impact of fasting has to be more than just restricting calories. Hypothyroidism slows metabolism, which means that you already burn fewer calories. The traditional advice of “just eat less!” isn’t going to cut it here. 

A couple contenders for how intermittent fasting might bring something new to the table to support the thyroid are its impact on fat burning, insulin resistance, and inflammation.

Intermittent fasting, hypothyroidism, and fat burning

One possible edge that intermittent fasting has over traditional calorie-restricted diets is the fasting itself.

When we eat regularly throughout the day, our energy comes from the calories we eat. By fasting daily, the body can get into ketosis, a metabolic state where — due to a lack of calories coming in for 12 or more hours — our fat stores are accessed and used for energy. 

This could be partially why some studies found that intermittent fasting does not slow metabolism while low-calorie diets do. We can eat less without our bodies reacting to a lack of energy coming in and lowering metabolic rate as a result. 

Intermittent fasting, hypothyroidism, and insulin resistance

Intermittent fasting has been shown to help reduce insulin resistance and increase insulin sensitivity.[7] As people with hypothyroidism are often insulin resistant [8], this effect of intermittent fasting could be a valuable one. 

Intermittent fasting, hypothyroidism, and inflammation

A leading cause of hypothyroidism is a thyroid autoimmune condition called Hashimoto’s disease.[6] This is where the immune system starts attacking healthy thyroid cells, damaging or destroying them, which affects thyroid hormone production and causes chronic inflammation in the body.

Reducing inflammation can help treat Hashimoto’s. Intermittent fasting has some skills in this area [9], having been shown in many studies to reduce oxidative stress (when you have more cell-damaging free radicals than cell-protecting antioxidants) and lower inflammation.[10]

The process here potentially goes like this: 

Intermittent fasting reduces inflammation. Reducing inflammation improves Hashimoto’s. Improving Hashimoto’s reduces hypothyroidism. Reducing hypothyroidism increases weight loss. 

Important note: Intermittent fasting will not cure Hashimoto’s, hypothyroidism, or any other chronic health conditions. It might just help alleviate the symptoms and potentially kickstart weight loss. 

If you’re eager to try it — and your doctor is on board — take our SIMPLE quiz and get yourself set up with an intermittent fasting schedule that suits you, your lifestyle, and your goals.  

Hypothyroidism and intermittent fasting — the risks 

Intermittent fasting has its risks, some of which could make hypothyroidism and thyroid function worse. 

If we fast for too long and/or don’t eat well-balanced meals, it’s easy to end up with a nutrient deficiency. Nutrient deficiencies are never good news, but especially so when your body is already fighting something like an under-functioning thyroid. 

Let’s be clear here: intermittent fasting is not just skipping meals. It’s eating a well-balanced, nutrient-rich diet during your eating window, then having a number of consecutive hours where you don’t eat or drink anything that contains calories. It’s both the multi-hour fast AND the nourishing diet that make intermittent fasting work. 

Intermittent fasting and thyroid function

Does the act of fasting directly affect our thyroid function? Let’s look at the research.

According to some studies, fasting can decrease thyroid-stimulating hormone (TSH), triiodothyronine (T3), and thyroxine (T4), though, in this particular study, T3 rapidly returned to normal levels once fasting ended.[11]

In one study, alternate day fasting (not an intermittent fasting schedule we recommend at SIMPLE because our upper limit for safe fasting is 18 hours.[12] Anything over that makes it tough to get all the nutrients you need) lowered levels of T3, though overall thyroid function was not impaired. Another alternate day fasting study demonstrated no impact on TSH and free T4.[13]

This review showed that T3 levels dropped within the first 24 hours of fasting, but TSH levels remained unchanged.[14] However, a study of Ramadan fasting showed that TSH levels exceeded normal levels after the Ramadan fast.[15] 

So, we have a big mixed bag of inconclusive evidence. 

Some studies say intermittent fasting affects thyroid function and lowers T3 and T4, which can worsen hypothyroidism. However, other research suggests that intermittent fasting can help alleviate hypothyroidism symptoms by decreasing body weight and improving insulin resistance without affecting thyroid hormones.

What do you do when the research is inconclusive? Talk to your doctor before trying anything. If you want to try intermittent fasting to see if it could help you lose weight and you have hypothyroidism, involve your doctor in the process so you’re not going at it alone. 

Once you’ve done so and got the all clear, give our SIMPLE quiz a try. It’s really easy to get started with intermittent fasting with our help, and the SIMPLE app has so many useful features, like:

• Avo, your personal wellness assistant who’s always there to answer your questions and keep you motivated;
• Nutrition Scores to analyze your meals and deepen your understanding of what to eat to support your goals; and
• food, movement, hydration, and progress trackers to keep you accountable and on track. 

Get your free account set up and see what you can achieve with some extra support.  

Does intermittent fasting affect thyroid medication?

If you have hypothyroidism or Hashimoto’s, you likely take medication to manage your symptoms and bring your thyroid hormone levels into a healthy range.

Because it’s not clear from the research how intermittent fasting affects thyroid levels, it’s also unclear how medication dosage could be affected. If your levels of TSH increase or decrease, your medication dose may need to be adjusted, so again, make sure your doctor is fully looped in if you are considering trying intermittent fasting. 

Check out our article for a more detailed look at whether you can take medication while fasting.

Tips on intermittent fasting with hypothyroidism 

When you’re fasting for a health reason, like supporting an underactive thyroid, it’s important to take care of and tune into your body. Here are some tips to help you do that.  

Work with your doctor

Get the go-ahead from your doctor before you start intermittent fasting and work with them throughout. You’ll need to make sure your thyroid hormones are checked regularly and that your medication dosage is adjusted as needed. 

This is even more true if you’re someone for whom intermittent fasting could pose some extra risks. If you: 

• have a medical condition
• take prescription medications
• are extremely active
• are under 18 years old, or 80 years old or more
• have a body mass index (BMI) < 18.5
• are pregnant, breastfeeding, or trying to conceive
• have (or are at risk of having) an eating disorder or have a history of one

again, get your doctor to sign off on your plans. 

Start with short fasts 

Starting out with a short fast, like 12 hours, is gentler on your body and gives you time to adjust to this new eating pattern. An easy way to do this is to stop eating an hour before you usually would before hitting the sack at night and/or delaying your breakfast for an hour after getting up in the morning. 

For more tips on safe fasting and choosing the right schedule for you, check out our guides on intermittent fasting for beginners and avoiding the most common intermittent fasting mistakes. 

Eat nourishing foods regularly during your eating window

As you’re adjusting to your fasting window, pay attention to what you’re eating. Let us show you how to meal plan to help you make sure you’re getting enough of what you need in your daily diet. That means getting enough lean protein, veggies, fruit, whole grain carbohydrates, healthy fats, and calcium-rich foods. To learn more, check out what to eat during intermittent fasting.

Listen to your body

Any time you try something new, listen to your body. If anything about fasting makes you feel worse — mentally, physically, or emotionally — stop. 

There are some common intermittent fasting side effects that are worth knowing about ahead of time. Check out our guide to give you a heads-up on what you might experience and what to do about it if you do. 

If you feel great on your intermittent fasting schedule, try easing into the practice by gradually lengthening your fasting time. You may notice that you’re eating less quite naturally and — if you’re hitting your stride with balanced meals packed full of high-quality nutrition — without feeling hungry.

SIMPLE’s expert opinion and final thoughts

Losing weight with hypothyroidism can be a frustrating experience, and intermittent fasting could be a worthwhile option to explore. 

That said, due to the uncertainty around how fasting affects thyroid function and hypothyroidism, it’s best to get some medical advice before you try it. 

Once you’ve got the approval of your doctor, take our SIMPLE quiz, and we’ll get you up and running with everything you need: insights to feed your fasting skill set, a full set of trackers to keep you focused, and daily feedback to help you make fantastic food choices (and a lot more). 

Frequently asked questions about intermittent fasting and hypothyroidism

Does fasting cause low TSH? 

According to some studies, fasting can cause low levels of thyroid-stimulating hormone (TSH). According to others, it does not. To say for sure, we need more research. The practical takeaway: if you try fasting, get your hormone levels checked regularly. 

Can you lose weight fast with hypothyroidism? 

Losing weight is more challenging with hypothyroidism, so it tends to be slow rather than fast, but it’s not impossible. Make sure you have a good support network around you to help keep you motivated and on track. 

Can you reverse hypothyroidism with diet? 

No diet has been shown to help the thyroid work better in people with underactive thyroids, so there’s no evidence that you can reverse hypothyroidism with your diet. Treating hypothyroidism is usually done via medication and lifestyle changes. If your hypothyroidism is caused by Hashimoto’s, diet can be part of the solution. 

Does coffee affect thyroid levels? 

Coffee, containing caffeine, can temporarily influence thyroid hormone levels by stimulating TSH.[16] It can also affect medication absorption if taken with thyroid drugs. People with thyroid disorders may be more sensitive to these effects and should manage caffeine intake carefully. We advise that you speak with your doctor for more guidance on this.

What are the worst foods for hypothyroidism? 

If you have hypothyroidism, your diet can be just as balanced as someone who doesn’t have this condition. However, you may find that limiting your intake of certain foods brings some general relief. This includes: 

• alcohol
• coffee (drunk within an hour of taking thyroid medication)
• high-fiber foods (if your fiber intake exceeds the recommended daily allowance)
• ultra-processed foods

These foods can interfere with thyroid function, thyroid hormone production, absorption of medication, and general health. Pay attention to how foods like these affect your body and thyroid health, and talk to your doctor or registered dietitian if you need help understanding which foods are — and aren’t — beneficial for you. 

Is intermittent fasting safe?

In general, yes, intermittent fasting is safe. However, there are some considerations to take into account if you have hypothyroidism, so be sure to talk to your doctor before you start.

Written by Trey Reck

Medically reviewed by Louise Bula, RD

Published on | Updated on May 3, 2024