Momentum in the Mountains 2024


Why Fasting Lowers Your Thyroid Hormone T3 a Little Bit

author avatar Dr. Eric Berg 05/29/2024

Fasting has become a popular way to boost health and lose weight, but have you ever wondered how it impacts your thyroid? As someone who's been curious about the effects of fasting on thyroid hormones, I decided to dig deeper and share what I've learned with you.

Whether you're a seasoned faster or just starting out, it's crucial to understand how this eating pattern can influence your thyroid function. So, let's explore the fascinating world of fasting and thyroid health together!

Understanding Thyroid Hormones and Their Functions

Your thyroid gland is a small, butterfly-shaped organ located at the base of your neck. Despite its size, it plays a crucial role in regulating various bodily functions through the production of thyroid hormones.

The two main thyroid hormones are thyroxine (T4) and triiodothyronine (T3). These hormones are essential for maintaining a healthy metabolism, regulating body temperature, and supporting proper growth and development.

Role of T4 and T3

T4 and T3 work together to keep your body functioning optimally. T4, the inactive form of thyroid hormone, is produced in greater quantities by the thyroid gland. In fact, T4 represents about 90% of all thyroid hormones, while T3 accounts for only about 9%.

T3, the active form of thyroid hormone, is primarily responsible for the metabolic effects of thyroid hormones on the body. It regulates heart rate, body temperature, and the breakdown of carbohydrates, fats, and proteins for energy.

Conversion Process

For T4 to exert its effects on the body, it must first be converted to T3. This conversion process primarily takes place in the liver and kidneys through the action of specific enzymes called deiodinases.

Approximately 60% of T4 is converted to T3 in the liver, while the remaining 40% is converted in other tissues such as the kidneys, muscles, and brain.

Interestingly, a small portion of T4 (about 0.9%) is converted to reverse T3, an inactive form of thyroid hormone that helps regulate T3 levels in the body.

Understanding the intricate balance between T4, T3, and reverse T3 is crucial when considering the effects of fasting on thyroid hormones. As we'll explore further, fasting can influence the conversion process and alter the levels of these hormones in the body.

The Impact of Fasting on Thyroid Hormone Levels

Fasting has gained popularity in recent years as a potential way to improve overall health and promote weight loss. However, it's essential to understand how different types of fasting can affect your thyroid hormone levels.

healthy fats fasting clock

Intermittent Fasting

Intermittent fasting involves alternating periods of eating with periods of fasting, typically on a daily or weekly basis. While some studies suggest that intermittent fasting may have benefits for weight loss and metabolic health, it's crucial to consider its impact on thyroid function.

Research has shown that intermittent fasting can potentially lower the levels of the active thyroid hormone T3.

This decrease in T3 may be due to a reduction in the conversion of T4 to T3 during fasting periods, which can lead to symptoms such as fatigue and brain fog in some individuals.

Ramadan Fasting

Ramadan fasting is a religious practice observed by Muslims worldwide, involving abstaining from food and drink from sunrise to sunset for a month. Studies have investigated the effects of Ramadan fasting on thyroid hormone levels, yielding mixed results.

Some research suggests that Ramadan fasting may lead to a decrease in T3 levels, while others have found no significant changes.

It's important to note that individual factors such as age, health status, and medication use can influence how Ramadan fasting affects thyroid function.

Hypothyroid patients should consult with their healthcare provider before participating in Ramadan fasting to ensure their thyroid hormone levels remain stable.

Periodic Fasting

Periodic fasting, also known as prolonged fasting, involves fasting for extended periods, typically lasting several days or weeks. While periodic fasting may have potential benefits for certain health conditions, it can also have a significant impact on thyroid health.

During periodic fasting, the body may adapt by slowing down the conversion of T4 to T3, resulting in lower levels of active thyroid hormone.

Additionally, periodic fasting can lead to higher levels of reverse T3, an inactive form of thyroid hormone that can further suppress thyroid function.

It's crucial to approach periodic fasting with caution, especially if you have a history of thyroid disorders or are taking thyroid medication.

Consulting with a healthcare provider before embarking on any prolonged fasting regimen is essential to ensure the safety and well-being of your thyroid health.

Mechanisms Behind Fasting-Induced Changes in Thyroid Hormones

To fully grasp the impact of fasting on thyroid function, it's essential to understand the underlying biological mechanisms at play.

Let's dive into how fasting influences metabolic efficiency and energy preservation, and how these factors contribute to changes in thyroid hormone levels.

Metabolic Efficiency

During periods of fasting, your body undergoes metabolic adaptations to conserve energy and optimize fuel utilization. One of these adaptations is an increase in metabolic efficiency, which allows your body to make the most of its available resources.

Fasting triggers a shift from glucose to fat as the primary energy source, a process known as ketosis. This metabolic switch helps preserve glucose for vital functions and reduces the overall energy demands on the body.

As a result, the rate and ratio of conversion from T4 to T3 or reverse T3 is determined by the body's metabolic need during fasting, which can lead to a slight decrease in T3 and an increase in reverse T3.

Energy Preservation

In addition to increasing metabolic efficiency, fasting also prompts your body to prioritize energy preservation. During periods of food restriction, the body aims to conserve energy by reducing non-essential processes and focusing on vital functions.

One way the body achieves this is by reserving active thyroid hormone (T3) until refeeding occurs.

By temporarily decreasing the conversion of T4 to T3 and increasing the production of reverse T3, the body can effectively slow down metabolism and conserve energy during fasting.

It's important to note that these fasting-induced changes in thyroid hormone levels are typically temporary and reversible. Once normal eating patterns resume, the body gradually adjusts, and thyroid hormone levels often return to their pre-fasting state.

However, the extent and duration of these changes can vary depending on individual factors and the type of fasting undertaken.

Clinical Significance of Changes in Thyroid Hormone Levels During Fasting

When discussing the effects of fasting on thyroid hormones, it's crucial to address the clinical significance of these changes. Many people may wonder if the fluctuations in thyroid hormone levels during fasting indicate a disease state or have long-term consequences for thyroid health.

Misconceptions About Hypothyroidism and Fasting

One common misconception is that fasting-induced changes in thyroid hormone levels automatically signify hypothyroidism, a condition characterized by an underactive thyroid gland.

However, it's essential to distinguish between temporary, adaptive changes and genuine thyroid dysfunction.

While fasting can lead to a decrease in T3 levels and an increase in reverse T3, these changes are usually within the normal range and do not necessarily indicate hypothyroidism.

In fact, fasting-induced changes in thyroid hormone levels are not typically associated with the classic symptoms of hypothyroidism, such as fatigue, weight gain, and cold intolerance.

Importance of Monitoring Thyroid Function

Although fasting-induced changes in thyroid hormone levels are generally not a cause for concern in healthy individuals, it's still important to monitor thyroid function, especially for those who fast regularly or have pre-existing thyroid conditions.

If you have a history of thyroid disorders or are taking thyroid medication, it's crucial to work closely with your healthcare provider to ensure that your thyroid function remains stable during periods of fasting.

Regular thyroid function tests, such as measuring TSH levels, can help detect any significant changes that may require adjustments to your thyroid hormone replacement medication doses.

It's worth noting that higher levels of reverse T3 can be observed in certain situations, such as recovery from illness, surgery, or significant weight loss. In these cases, the elevated reverse T3 levels are usually temporary and not indicative of a chronic thyroid problem.

Overall, while fasting can influence thyroid hormone levels, it's essential to approach these changes with a balanced perspective and seek guidance from a qualified healthcare professional to ensure the long-term health and well-being of your thyroid.

Practical Tips for Managing Thyroid Health While Fasting

If you're considering incorporating fasting into your lifestyle, it's important to take proactive steps to support your thyroid health. Here are some practical tips to help you navigate the effects of fasting on thyroid hormones and maintain optimal thyroid function.

Tailoring Your Meal Plan

When fasting, it pays off to make your meals count during eating periods by picking nutritious and well-rounded options. Be good to your thyroid by adding iodine-rich seaweed, selenium-packed Brazil nuts, and zinc-laden oysters into the mix.

Consider working with a registered dietitian or nutritionist who specializes in thyroid health to develop a personalized five-day thyroid meal plan that takes into account your specific needs and fasting goals.

They can help you optimize your nutrient intake, provide a grocery list, and ensure that you're meeting your body's requirements for proper thyroid function.

Timing Your Meals

Eating at the right times can boost your thyroid health during fasting. It's smart to break fast with a meal full of protein, allowing for better thyroid hormone production which helps you effectively transform T4 into T3.

Additionally, consider experimenting with different fasting windows to find what works best for your body.

Some individuals may fare better with shorter fasting periods, such as 12-16 hours, while others may tolerate longer fasts without significant impact on their thyroid function.

Be sure to include healthy snack options in your eating schedule to maintain stable blood sugar levels and reduce the risk of disordered eating patterns.

Consulting Healthcare Providers

If you have a pre-existing thyroid condition, such as hypothyroidism or Hashimoto's thyroiditis, it's essential to consult with your healthcare provider before starting any fasting regimen.

They can assess your individual needs, monitor your thyroid function closely, and make necessary adjustments to your thyroid medication dosage if required.

Regular check-ins with your healthcare provider can help ensure that your thyroid health remains stable throughout your fasting journey.

They can also provide guidance on how to safely incorporate fasting into your lifestyle while minimizing any potential risks to your thyroid function, such as the development of insulin resistance or oxidative stress.

Remember, everyone's body responds differently to fasting, and what works for one person may not work for another. By staying attuned to your body's signals, working closely with healthcare professionals, and making informed choices about your fasting practices,

you can support your thyroid health while reaping the potential benefits of intermittent fasting, such as reduced inflammation and improved heart health.

Fasting for Thyroid Health

Delve into the intriguing relationship between intermittent fasting and thyroid health, revealing how strategic fasting can support thyroid function and potentially reduce goiter size.

Understanding how to shrink a goiter naturally involves adopting dietary practices that reduce inflammation and support hormonal balance. Incorporating fasting can enhance these effects, promoting overall thyroid health and offering a natural approach to managing goiters.

By focusing on nutrient-dense foods and mindful eating patterns, you can create a supportive environment for thyroid health and explore effective ways to shrink a goiter naturally.


Fasting can be a powerful tool for improving overall health, but it's essential to be mindful of its potential impact on your thyroid hormones.

By understanding the mechanisms behind fasting-induced changes in thyroid function, you can make informed decisions about your fasting practice.

Remember, everyone's body is unique, so it's crucial to listen to your own needs and consult with a healthcare professional if you have any concerns.

With the right approach and monitoring, you can enjoy the benefits of fasting while maintaining optimal thyroid health.

So, go ahead and explore the world of fasting, armed with the knowledge you need to thrive!

Healthy Keto Guide for Beginner

FREE Keto Diet Plan