Underactive Thyroid and Fasting: Can Fasting Help with Hypothyroidism?

Approximately 5% of Americans over the age of 12 have underactive thyroid. If you are part of this statistic, you may be exploring options like fasting to help manage underactive thyroid symptoms. This article will guide you through the potential benefits and risks of fasting with underactive thyroid, helping you decide if it could be the right choice for your health.

What Is an Underactive Thyroid?

Underactive thyroid, or hypothyroidism, occurs when the thyroid gland produces insufficient amounts of thyroid hormones, which are essential for regulating metabolism and energy production in the body. This impacts the body’s ability to convert food into energy, where a drop in energy levels and a slowed metabolism can result in weight gain and feelings of fatigue. 

Types of Fasting Protocols

Fasting is the practice of voluntarily abstaining from food and sometimes drinks for a specific period. Over the years, diverse fasting approaches have emerged, enabling individuals to tailor protocols to suit their lifestyles. Take the Fastic quiz to learn which fasting protocol fits best your health, objectives, and everyday routine. 

Here are some of the most common fasting approaches:

• Intermittent fasting: Also known as time-restricted eating (TRE), only allows individuals to eat during specific eating windows. Some variations include 14:10, 16:8 and 20:4. 

• 5:2 method: Requires individuals to eat as usual five days a week, and significantly reduce calorie intake—around 500 to 600 kcal—on the remaining two, non-consecutive days.

• Alternate-day fasting (ADT): Consists of alternating between regular eating days and fasting days. On fasting days you don’t eat or can choose to reduce calorie intake to about 500 kcal. 

• Extended fasting: Involves going without food for more than 24 hours. During a water fast, only water is allowed, while a dry fast restricts all fluids, including water. 

How Does Fasting Impact Thyroid Function?

To understand how fasting can impact thyroid function, it’s important to first learn about the roles the relevant hormones play in the body.

• Triiodothyronine (T3): An active thyroid hormone that regulates metabolism, energy, and heart rate.

• Thyroxine (T4): A thyroid hormone that, when converted into T3 in the body, supports metabolism, growth, and overall energy.

• Thyroid-stimulating hormone (TSH): A hormone released by the pituitary gland that signals the thyroid to produce T3 and T4, maintaining a balanced metabolism.

A recent study on the effects of long-term calorie restriction (CR) with adequate protein and micronutrients on thyroid hormones suggests that long-term CR is associated with lower T3 levels, which may help conserve energy and reduce oxidative stress, potentially contributing to increased lifespan. Calorie restriction, often achieved through fasting, can similarly affect thyroid hormone levels, specifically by lowering T3 without altering T4 or TSH levels. A reduction in T3 levels may support energy efficiency and metabolic adaptation as the body conserves energy during periods of limited caloric intake. This finding suggests that calorie restriction should be coupled with a balanced protein and macronutrient intake to prevent potential negative effects on thyroid function.

A study on alternate-day fasting found that T3 levels decreased; however, as assessed by TSH and T4, thyroid function was not negatively impacted. Another study comparing ADT and CR showed that TSH and T4 levels remained stable. A review of intermittent fasting’s effects on the circadian rhythm found a significant drop in T3 levels during the first 24 hours, although TSH levels remained unchanged. TSH levels must fluctuate to affect thyroid hormone production, as even a small change can affect the synthesis of T3 and T4. These findings suggest that fasting may help maintain stable thyroid function while adjusting levels, potentially leading to improved energy metabolism balance.

Potential Benefits of Fasting for an Underactive Thyroid

Here are some potential benefits of fasting for those with an underactive thyroid. Keep in mind that the effects can vary, and it may not be suitable for everyone.

Cellular renewal and repair

Fasting can trigger autophagy, a process that removes damaged components from cells and promotes their renewal. Autophagic enhancement can support mitochondrial health and energy production, which may benefit individuals with low thyroid function.

Reduced inflammation

Research on fasting has shown to reduce inflammatory markers in the body. Fasting can modulate immune responses, potentially supporting immune health in autoimmune thyroid disorders like Hashimoto’s disease, though its benefits for non-autoimmune hypothyroidism are less direct.

Weight management support

Studies have shown that fasting can help with weight loss by promoting the burning of fat, which is often a challenge for those suffering from underactive thyroid due to slower metabolism. 

Improve insulin sensitivity

Scientific studies have demonstrated how fasting improves insulin sensitivity, which may benefit metabolic balance—a key concern for individuals with an underactive thyroid due to experiencing a slower metabolism.

Reduced oxidative stress

Fasting can boost antioxidant production by inducing a mild rise in cellular reactive oxygen species (ROS) due to calorie restriction. This small rise in ROS activates an adaptive response in cells that increases their antioxidant defenses, helping protect mitochondria from damage and supporting better energy production.

Enhance mental clarity

Fasting promotes ketone production and lowers insulin, promoting brain-derived neurotrophic factor (BDNF) release and supporting neuroplasticity, which enhances the brain’s ability to reorganize neural pathways and generate new neurons. Fasting can help alleviate brain fog, a common symptom of underactive thyroid, by supporting more efficient neuronal function.

People respond differently to fasting. Some may experience effects, while others may not.

Risks of Fasting with an Underactive Thyroid

Fasting can negatively impact individuals with underactive thyroid, potentially worsening symptoms.

• Slower thyroid function: Prolonged fasting—longer than 24 hours—can reduce T3 levels, possibly worsening fatigue and brain fog symptoms. Lower T3 levels can slow metabolism and reduce energy production.

• Increase stress on the body: Fasting can increase cortisol levels, a stress hormone, which can exacerbate symptoms and disrupt thyroid hormone production. 

• Result in hormonal imbalance: Fasting may disrupt the balance of other hormones that influence thyroid function, potentially leading to metabolic issues. This imbalance can affect appetite regulation, energy levels, and thyroid hormone function. (Read about how intermittent fasting impacts female hormones.)

• Strain adrenal glands: Chronic stress from fasting can cause an initial spike in cortisol, but over time this may impair the adrenal glands’ ability to produce sufficient cortisol, contributing to adrenal dysfunction and worsening underactive thyroid symptoms.

• Lead to nutrient deficiencies: Fasting may result in insufficient intake of essential vitamins, minerals, and macronutrients due to lack of or shortened eating windows. A deficiency in nutrients like iodine or zinc, which are critical for thyroid function, can further impair thyroid health. 

Download Fastic to find the fasting approach that best supports your body and health.

Disclaimer: It is advised to speak with a health professional before beginning intermittent fasting or a diet program. There may be side effects for people with certain medical conditions.