Intermittent Fasting and Hypothyroid

The popularity of intermittent fasting has grown tremendously over the past few years. Benefits of intermittent fasting may include weight loss, reduced inflammation, and improved blood sugar balance. 

Yet, how do intermittent fasting and thyroid health fit together? Is it safe to practice intermittent fasting if you have hypothyroidism?

Intermittent fasting is the term used to describe various fasting patterns, in which individuals purposefully fast for a specific amount of time (except for water). Please note, this article is not referring to religious fasting, which some cultures practice during certain times of the year. There are many different types of intermittent fasting, but some of the most common include:

In this practice, a person would completely fast for alternating days throughout the week.

This is similar to the above method with the distinction that you can eat a small amount of food (usually less than 25% of calorie needs) during the fasting days.

We all practice some form of time-restricted fasting when we sleep. However, the 16:8 fasting method limits food intake to 8 hours per day. People often compress their eating window by eliminating breakfast or eating an earlier dinner to allow for a 16 hour overnight fast. 

A periodic faster would eat normally most days of the week, and fast for 1 to 2 days. Another form of this fasting schedule includes the 5:2 method. For this practice, individuals eat 25 percent of their calorie needs on two scheduled fasting days and eat normally for the other 5 days. 

Many health professionals and celebrities tout the benefits of intermittent fasting. Some research studies also show potential benefits. But, are there any drawbacks to intermittent fasting? 

The main benefits of intermittent fasting found in human research so far include weight loss (with fat loss), improvements in fasting blood sugar, and reduced insulin resistance (1, 2). Intermittent fasting may also reduce levels of inflammation within the body (3, 4, 5).

The main drawbacks of intermittent fasting for certain individuals include the potential for disordered eating behaviors, hunger, sustainability, and nutrient deficiencies (6).

Let’s discuss the potential impact of intermittent fasting on hypothyroid conditions. 

Limiting your eating window often results in lower calorie intake. While this may be helpful for weight loss, calorie restriction has a significant impact on your thyroid. 

For example, your inactive thyroid hormone, called reverse T3, increases with dieting and competes with your active thyroid hormone (T3) to bind to your receptors. Both short and long-term dieting can result in up to a 50 percent decrease in your active T3 hormone (7, 8). Increased reverse T3 and reduced active T3 may result in a worsening hypothyroid condition.

Despite these findings, some studies show that your thyroid hormones return to pre-fasting values after breaking your fast with a meal, indicating that some prolonged fasting may not be detrimental to individuals without a hypothyroid condition (9, 10). 

Long periods of fasting can also lead to low blood sugar levels and cause your adrenal glands to release a hormone called cortisol. 

Cortisol is a steroid hormone that has many important functions in the body, like reducing inflammation during an acute injury, helping your body adapt to dangerous situations, and regulating blood sugar levels. Yet, when cortisol is chronically elevated, we can experience negative side effects. For example, chronically elevated cortisol levels can impair thyroid hormone production and function (11). 

If you struggle with adrenal issues, like elevated cortisol levels, intermittent fasting may worsen cortisol balance and subsequently hinder your thyroid function. 

Fasting aside, many Americans do not meet the recommended daily amounts for nutrients like vitamin A and iron. What’s more, a whopping 95 percent of people fail to meet the recommended amounts of fiber (12). Fiber has a vital role in maintaining optimal gut health which then indirectly affects your thyroid health! Skipping meals or fasting for multiple days during the week may create nutrient deficiencies that can ultimately worsen a hypothyroid condition. 

Intermittent fasting is one tool that may be beneficial for some individuals. 

However, fasting with a hypothyroid condition should be carried out carefully. As always, focus on eating an unprocessed, whole-food diet that is rich in fiber. Listen to your body, and stop any practice that may make you feel worse, mentally or physically. Because of the clear connection between low-calorie diets and reduced thyroid function, we usually recommend against full-day fasts. A safer alternative may be to extend your overnight fast by eliminating late-night snacking (which tends to involve low-quality foods anyway!). 

Prolonged fasting is not for everyone. Along with hypothyroid disorders, intermittent fasting is not recommended during pregnancy, for women with eating disorders, or those with unresolved adrenal issues.

If you have a thyroid condition (or suspicions for one), check out our Functional Medicine Membership to help you find healing.