The Impact Of Intermittent Fasting On Thyroid Function

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The Impact Of Intermittent Fasting On Thyroid Function

Intermittent fasting continues to be widely accepted as a form of eating that has many health benefits. Yet, some people may have concerns about how it affects thyroid function.

This article explores the impact of intermittent fasting on thyroid function, understanding its effects to help you decide if intermittent fasting is right for you.

Brief Overview Of Intermittent Fasting

Intermittent fasting (IF) is an eating pattern that cycles between periods of fasting and eating1. It doesn’t prescribe specific foods to eat but rather focuses on when you should eat them.

Types of Intermittent Fasting

  1. 16/8 Method: This method involves fasting for 16 hours and eating during an 8-hour window. For example, one could eat from noon to 8 PM and fast from 8 PM to noon the next day.
  2. 5:2 Diet: This diet involves eating normally five days a week and restricting calorie intake to 500-600 calories on the remaining two non-consecutive days.
  3. Eat-Stop-Eat: Involves fasting for 24 hours once or twice a week.
  4. Alternate-Day Fasting: Involves alternating between fasting days (no or very low-calorie intake) and regular eating days.
  5. Warrior Diet: This diet involves eating small amounts of raw fruits and vegetables during the day and one large meal at night, typically within a 4-hour window.


  1. Weight Loss: IF can help reduce calorie intake and boost metabolism, leading to weight loss2.
  2. Improved Metabolic Health: IF can improve various metabolic markers, including blood sugar, insulin sensitivity, and cholesterol levels3.
  3. Cellular Repair: Fasting initiates autophagy, a process where cells remove damaged components, potentially reducing the risk of several diseases4.
  4. Brain Health: IF may enhance brain function and protect against neurodegenerative diseases by reducing inflammation and oxidative stress5.
  5. Longevity: Some animal studies suggest that intermittent fasting may help extend lifespan6.


  • Hormone Regulation: Fasting affects hormone levels that help with weight loss and muscle gain. For example, insulin levels drop, which facilitates fat burning, and growth hormone levels increase, which aids in fat loss and muscle gain7.
  • Autophagy: During fasting, the body’s cells initiate autophagy, removing dysfunctional proteins and other cellular debris8.

Understanding Thyroid Function

What is Thyroid?

The thyroid is a butterfly-shaped gland in the front of the neck, just below the Adam’s apple. It regulates various metabolic processes by producing hormones, primarily thyroxine (T4) and triiodothyronine (T3). These hormones control the body’s metabolism, energy levels, growth, and development9.

The thyroid gland receives signals from the pituitary gland in the brain, which secretes thyroid-stimulating hormone (TSH) to stimulate the thyroid to produce thyroid hormones. The thyroid hormones T3 and T4 are then released into the bloodstream, where they travel throughout the body and exert their effects on various organs and tissues10.

Thyroid Hormones

The thyroid gland produces two main hormones:

  1. Thyroxine (T4): The primary hormone the thyroid gland produces, T4 has four iodine atoms. It is converted into triiodothyronine (T3) in the liver and other tissues11.
  2. Triiodothyronine (T3): The active form of thyroid hormone, T3 has three iodine atoms and is much more potent than T4. It influences almost every physiological process in the body12.

Regulation of Thyroid Hormones

The hypothalamus and the pituitary gland regulate the production and release of thyroid hormones through a feedback loop known as the hypothalamic-pituitary-thyroid (HPT) axis.

  1. Hypothalamus: Releases thyrotropin-releasing hormone13.
  2. Pituitary Gland: In response to TRH, the pituitary releases thyroid-stimulating hormone13.
  3. Thyroid Gland: TSH stimulates the thyroid gland to produce and release T4 and T310.

Thyroid Function Tests

To assess thyroid function, several tests can be performed:

  1. TSH Test: Measures the level of thyroid-stimulating hormone in the blood. High TSH indicates hypothyroidism (underactive thyroid), while low TSH indicates hyperthyroidism (overactive thyroid).
  2. Free T4 Test: This test measures the level of unbound thyroxine, providing insight into how much hormone is available to the body’s tissues.
  3. Free T3 Test: Measures the level of unbound triiodothyronine, the active form of thyroid hormone.
  4. Thyroid Antibodies Test: This test detects antibodies that may indicate autoimmune thyroid diseases such as Hashimoto’s thyroiditis or Graves’ disease.

Thyroid Disorders

Common thyroid disorders include:

  1. Hypothyroidism: An underactive thyroid gland producing insufficient hormones. Symptoms include fatigue, weight gain, and depression. Common causes include Hashimoto’s thyroiditis and iodine deficiency14.
  2. Hyperthyroidism: An overactive thyroid gland producing excess hormones. Symptoms include weight loss, rapid heartbeat, and anxiety. Common causes include Graves’ disease and thyroid nodules15.
  3. Goiter: Enlargement of the thyroid gland can occur in hypothyroidism and hyperthyroidism16.
  4. Thyroid Nodules: Lumps in the thyroid that can be benign or malignant17.
  5. Thyroid Cancer: Malignant tumors in the thyroid gland, though relatively rare, can be treated effectively if detected early18.

The Impact Of Intermittent Fasting On Thyroid Function

1. Thyroid Hormone Levels

  • Thyroxine (T4) and Triiodothyronine (T3): Some studies suggest intermittent fasting may reduce T3 levels, while T4 levels might remain relatively stable. T3 is the active form of thyroid hormone, and its reduction could indicate a slowdown in metabolism, possibly as a response to reduced calorie intake13.
  • Reverse T3 (rT3): An increase in rT3, an inactive form of the hormone, can occur during fasting. This increase is considered a mechanism to conserve energy when the body perceives a reduced energy intake19.

2. Thyroid-stimulating Hormone

  • TSH Levels: The impact of intermittent fasting on TSH levels is unclear. Some research indicates that TSH levels may remain stable, while other studies suggest a potential for slight increases. This variability may be due to differences in study designs, fasting protocols, and individual responses20.

3. Energy Expenditure and Metabolic Rate

  • Basal Metabolic Rate (BMR): Intermittent fasting can decrease BMR due to lower T3 levels. This adaptive response aims to conserve energy during periods of reduced food intake21.
  • Adaptive Thermogenesis: The body’s response to fasting includes adaptive thermogenesis, where the energy expenditure is reduced to match the lower caloric intake, impacting overall metabolism22.

4. Autophagy and Cellular Health

  • Autophagy: Intermittent fasting promotes autophagy, which helps remove damaged cells and regenerate healthy cells. While autophagy benefits cellular health, its direct impact on thyroid function is unclear and requires further research4.

5. Insulin Sensitivity and Inflammation

Optimal Tips For Intermittent Fasting With Thyroid Issues

Intermittent fasting can be approached cautiously if you have thyroid issues, as it may affect your metabolism and energy levels differently. Here are some tips for practicing intermittent fasting with thyroid issues:

  1. Consult Your Doctor: Always seek advice from your healthcare provider before starting intermittent fasting, especially if you have thyroid issues. They can provide personalized guidance based on your specific condition and health history.
  2. Choose the Right Fasting Protocol: There are different intermittent fasting protocols. Start with a less restrictive approach and monitor how your body responds. For example, the 16/8 method (fasting for 16 hours, eating within an 8-hour window) tends to be easier for most people to adapt to.
  3. Monitor Your Symptoms: Pay attention to how you feel during fasting periods. Thyroid issues can affect energy levels and metabolism, so if you feel exhausted or experience dizziness, reconsider your fasting schedule or duration.
  4. Stay Hydrated: Drink plenty of water during fasting periods to stay hydrated, which is essential for thyroid function and overall health. Herbal teas and black coffee (in moderation) can also be consumed during fasting periods, but be mindful of how they affect your energy levels.
  5. Include Nutrient-Dense Foods: When you do eat, focus on nutrient-dense foods that support thyroid health, such as iodine-rich foods (e.g., seaweed, seafood), selenium sources (e.g., Brazil nuts, tuna), and foods rich in vitamins B and D (e.g., eggs, dairy, fatty fish).
  6. Avoid Extreme Caloric Restriction: Intermittent fasting should not be used as a means for extreme caloric restriction, especially with thyroid issues. Ensure you get enough calories to support your energy needs and metabolic function.
  7. Monitor Thyroid Function: As your healthcare provider recommends, regularly check your thyroid function through blood tests. Intermittent fasting should not adversely affect thyroid function in most cases, but monitoring can help catch any changes early.
  8. Consider Adjustments Based on Feedback: If intermittent fasting negatively impacts your thyroid symptoms or overall well-being, consider modifying your fasting schedule or consult your healthcare provider for alternative recommendations.
  9. Manage Stress: Chronic stress can affect thyroid function. Incorporate stress management techniques such as meditation, yoga, or deep breathing exercises to support overall thyroid health while fasting.
  10. Be Patient and Listen to Your Body: Adjusting to intermittent fasting with thyroid issues may take time. Listen to your body’s signals and adjust your fasting schedule and food choices to support your health and well-being.

Final Thoughts

Intermittent fasting involves cycling between periods of eating and fasting, such as the 16/8 method or the 5:2 diet. Research indicates IF may aid weight loss, improve insulin sensitivity, and promote cellular repair, among other benefits. However, its effects on thyroid function and body weight can be complex.

Thyroid hormones like T3 and T4 regulate metabolism, growth, and temperature. T3 levels may temporarily decrease during prolonged fasting, prompting a compensatory increase in thyroid-stimulating hormone to maintain a balanced energy metabolism. These changes are reversible upon resuming regular eating.

Responses to fasting vary based on factors like age, health, thyroid health, and how long you fast. For those with thyroid issues, it’s important to start intermittent fasting carefully, monitor how it affects you, and get regular blood tests to check your thyroid function. This helps ensure that fasting is safe and supports your overall health.


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