The Impact of Fasting on Thyroid Health and Metabolic Function
Many people turn to fasting for better health, only to feel their energy dip or other symptoms over time. Intermittent fasting has become a popular wellness trend, but its effects on thyroid health are often overlooked.
Your thyroid plays a major role in regulating energy, hormones, and metabolism, and fasting can disrupt that delicate balance if it's not done carefully.
This article explains the connection between fasting and thyroid function, how different types of fasting can impact the body, and safer alternatives for individuals with thyroid concerns.
Table of Contents
Why the Thyroid Plays a Central Role in Metabolism
What the Thyroid Actually Does
The thyroid, a butterfly-shaped gland in your neck, is responsible for many body functions. It is controlled by thyrotropin-releasing hormone (TRH), which is released by the hypothalamus, which in turn tells the pituitary gland to release thyroid-simulating hormone (TSH).
TSH then signals to the thyroid to release triiodothyronine (T3) and thyroxine (T4) hormones. These hormones allow the thyroid to regulate many body functions, including:
- Metabolism: The way your body produces and uses energy.
- Temperature: These hormones help you maintain a normal body temperature.
- Heart rate: Your thyroid gland helps regulate the speed at which your heart beats.
- Breathing: T3 and T4 influence how you breathe.
- Digestion: These hormones help regulate the speed at which food moves through your body.
- Mood: Underactive or overactive thyroids can generate feelings of depression, fatigue, anxiety, restlessness, mood swings, and more.
- Bone and muscles: The thyroid helps maintain bone and muscle function.
How the Thyroid Responds to Energy Availability
The thyroid is the hub for energy production and conservation because it is directly responsible for the body’s metabolism. The body’s metabolic rate is adjusted based on food availability; therefore, it is very sensitive to calorie intake.
When calories are restricted, the thyroid lowers the metabolism and reduces the conversion of T3 and T4, essentially putting the body in starvation mode to conserve energy.
The same might happen if you follow a low-carb or low-protein diet because T3 levels are reduced and the overall function of the thyroid is affected. Consistently undereating may cause
lower metabolic function.
Understanding Fasting and Caloric Restriction
Intermittent Fasting vs. Extended Fasting
Various types of fasting may all impact the thyroid.
- Time-restricted eating (TRE): TRE is a type of intermittent fasting in which food intake is limited to a very specific time frame. Food consumption is usually allowed for 8–10 hours, followed by a period of 14–16 hours of fasting.
- Potential positive thyroid impacts include:
- May improve metabolic health
- May help manage hypothyroidism
- May help reduce body weight
- May help lower blood pressure
- Potential negative impacts on the thyroid:
- May produce less T3 and T4, slowing metabolism
- May induce hypothyroidism
- Alternate-day fasting (ADF): ADF is a form of intermittent fasting in which a person will fast for 24–36 hours, followed by 12 hours of normal eating.
- Potential positive thyroid impacts include:
- It is a safer alternative to severely restricting calorie intake.
- Lowering of T3 and T4 hormones is reversible and doesn’t harm the thyroid long-term.
- Potential negative impacts on the thyroid during fasting are:
- It may increase stress on the body.
- People with hypothyroidism may experience fatigue.
- Multi-day fasts: a multi-day fast requires no caloric intake from at least 24 hours to several days. As a result, the body often shifts into ketosis.
- Potential positive thyroid impacts include:
- Thyroid levels return to normal when regular eating habits resume.
- It may be well tolerated by people without thyroid conditions.
- Potential negative impacts on the thyroid may include:
- Reduction of thyroid hormone levels
- Increased stress on the body
- Possible nutrient deficiencies that put stress on the thyroid
- Risky for individuals with thyroid conditions and autoimmune diseases
The length of the fast, the health of the individual, and the intensity of calorie restriction will determine the impact on the thyroid.
HealthierU can help you determine the type of fasting and the nutrition required to keep your body healthy and your thyroid functioning properly. Request a
nutrition consultation for guidance.
The Difference Between Strategic Fasting and Chronic Undereating
Short, intentional fasts done correctly don’t typically create thyroid issues. However, incorrectly fasting or continually undereating, creating long-term caloric deficits, may cause problems, such as:
- Long-term metabolic slowdown
- Possible chronic hypothyroidism
- Inadequate intake of nutrients to support the thyroid
- Increase in stress hormones
- Inconsistent hormone levels
Fasting in moderation is recommended. Be sure to eat nutrient-rich foods when you eat. Additionally, monitor your symptoms, and contact your doctor if you experience fatigue, cold intolerance, or
weight gain, as it could signal a thyroid issue.
3 Ways Fasting Can Affect Thyroid Hormones
#1: Reduced T3 Levels and Metabolic Adaptation
When you’re fasting, a survival response is triggered. The thyroid produces less T3 hormone to reduce the metabolic rate and reserve energy. The conversion of T4 to T3 in the liver also decreases. Therefore, the overall effectiveness of the thyroid hormones is reduced with calorie restriction.
Production of T3 and T4 typically returns to normal when regular, nutritious meals resume because the body is no longer in starvation mode.
#2: Cortisol, Stress, and Thyroid Suppression
Fasting, especially for prolonged periods, may trigger a stress response. The stress hormone, cortisol, increases because of the resulting low blood sugar.
The cortisol increase may also cause sleep issues because your body is on alert. You may find that you are more restless and wakeful, or you may have difficulty falling asleep.
Additionally, elevated cortisol is connected to impaired thyroid signaling. It reduces the conversion of T4 to T3, which slows the metabolic rate. This is the body’s way of handling food deprivation and hormone changes to manage stress and conserve energy.
#3: Reverse T3 and the Body’s Protective Response
Reverse T3 (rT3) is produced when the body converts T4 into an inactive, non-functioning form of T3 rather than active T3 that the thyroid needs to regulate body systems. This reversal happens during times of high stress, illness, or starvation.
This conversion serves to keep the body from spending too much energy because it feels threatened. It slows the metabolic process when the body has a high energy demand.
Who Is Most at Risk for Thyroid Issues While Fasting?
Women With Hormonal Imbalances
Women with hormonal imbalances have a delicate system, and fasting may add thyroid issues to the mix. Fasting releases cortisol to keep the blood sugar consistent, which may chronically elevate cortisol. In turn, it affects the thyroid and slows the metabolic rate.
Fasting stresses the body, and in response, the female body sacrifices reproductive hormones to survive, creating irregular menstrual cycles and low estrogen.
Caloric restriction, elevated stress, and intense exercise may also result in functional hypothalamic amenorrhea (FHA). Calorie restriction affects the hypothalamus, which tells the body to stop menstruating for several months. FHA also signals to the thyroid to decrease T3 levels, which suppresses metabolism.
People With Existing Thyroid Conditions
People with existing thyroid conditions, such as hypothyroidism, subclinical thyroid dysfunction, and Hashimoto’s, are at risk of thyroid issues when fasting because the thyroid already doesn’t function properly. Calorie restriction continues to slow their metabolism. Fasting may also cause a nutrient deficiency, preventing proper thyroid function.
For people who take medication for their thyroid condition, fasting may affect the timing of medication and may impact the stability of their hormone levels.
If you have an existing thyroid condition, it is critical to consult with your doctor before beginning a fasting routine to ensure that you can fast safely.
At HealthierU, we are experts on thyroid health and can guide you through the fasting process to ensure that you are doing it safely. Reach out for a nutrition consultation.
High-Stress, High-Output Individuals
People with high stress and high output, such as athletes, entrepreneurs, caregivers, and people with sleep issues, are at risk of thyroid issues when fasting because their body requires a high metabolic demand. When the body isn’t getting enough calories to fuel that demand, the starvation response is triggered.
The thyroid struggles to function as reverse T3 is produced rather than T4 in these conditions. This results in hypothyroidism-like symptoms, such as cold intolerance, fatigue, and weight gain, as the body tries to conserve energy.
Does Fasting Cause Hypothyroidism?
If an individual is healthy, intermittent fasting is not likely to result in hypothyroidism. As long as the caloric restrictions are not extreme or long-term, thyroid function will return to normal when regular eating habits resume. If fasting is long-term, the thyroid may have reduced function that may not be completely reversed.
For people with pre-existing thyroid issues, the problem could worsen, and a doctor should be consulted before making significant dietary changes.
Does Water Fasting Create Thyroid Issues?
Water fasting affects the thyroid by slowing the metabolism as T3 hormone production is reduced and rT3 takes its place. If fasting is not prolonged or long-term, the effects on the thyroid are typically reversible. However, if water fasting is repeated or prolonged, the risk of thyroid suppression increases. The chance is greater with individuals who already have hormonal or stress-related imbalances.
Can Fasting Ever Support Thyroid Health?
The best way to support thyroid health when fasting is to use a gentle fasting approach. Gentle fasting may look like a half-fast, where you eat dinner and then do not eat again until lunch the next day. This can occur for two or three days in a row, or it can be spaced out with regular eating days in between.
It may also look like restricting calories and only eating small amounts on fasting days instead of a full fast. It is best for your thyroid not to fast daily.
It is also important to eat food that supplies your body with adequate nutrition when you eat. Focus on protein intake and quality calories. This will help your body have vital nutrients to keep your thyroid functioning.
How To Fast Without Compromising Thyroid Function
When you’re fasting, you must not under-fuel your body when you eat. There are vital nutrients you need to keep your thyroid functioning, including:
- Iodine: Found in dairy products, saltwater fish, and sea vegetables like kelp
- Selenium: Found in tuna, eggs, and sunflower seeds
- Zinc: Found in shellfish, beef, chicken, and lentils
- Iron: Found in red meat, liver, lentils, beans, spinach, and fortified cereals
- Vitamin D: Found in fatty fish, eggs, fortified dairy drinks, and mushrooms
HealthierU: A Smarter Approach To Fasting and Hormone Health
HealthierU specializes in women’s health, nutrition, and wellness. We utilize holistic and natural remedies to correct hormonal imbalance, digestive health issues, hot flashes, thyroid dysfunction and more.
We offer nutrition consultations and holistic chiropractic services to heal your body and improve health concerns. Whether you have existing thyroid issues, hormone imbalances, or are just looking for a healthy, safe way to fast, our team at HealthierU can guide you in the right direction.
Reach out today for a nutrition consultation or to schedule an appointment.
