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Exploring the Relationship Between Intermittent Fasting and Hypothyroidism

Written by: Ana María Kausel, Endocrinologist at Anzara Health

Published: 06-06-2024

Intermittent Fasting and Hypothyroidism

As an endocrinologist, I’m often asked about the relationship between intermittent fasting and hypothyroidism.

Fasting has become increasingly popular over the last decade as a proposed solution for weight control and insulin regulation.

If you have hypothyroidism, there are some safety guidelines and considerations you need to know before deciding if fasting is best for you. 

I’ve created this guide to help you better understand how fasting impacts your thyroid health, answering common questions about safety, risks, and potential benefits. 

Intermittent Fasting Explained

Intermittent fasting is an eating pattern that alternates between periods of fasting and eating.

Standard methods include the 16/8 method (fasting for 16 hours and eating during an 8-hour window), the 5:2 method (eating normally for five days and restricting calories for two non-consecutive days), and the alternate-day fasting method (fasting every other day).

The proposed purpose of intermittent fasting is to promote metabolic health by cycling between periods of eating and fasting.

People who adopt this eating pattern claim it enhances weight loss, improves insulin sensitivity, and supports cellular repair processes.

Proponents argue that allowing the body to utilize stored fat for energy during fasting periods can reduce the risk of chronic diseases and improve overall well-being.

Intermittent Fasting Types

Intermittent fasting is an umbrella term that covers different fasting and eating schedules. This article focuses on four main ones that you should know. 

  • Alternate-day fasting: You eat one day and fast the next, alternating continuously. This type of fasting puts your body in a large caloric deficit, leading to weight loss, cravings, and stress. 
  • Time-restricted feeding: Also known as the 16/8 method, this regime consists of practitioners limiting their daily food intake to a specific window of time, typically ranging from 4 to 12 hours, and fasting for the remaining hours of the day. This type of fasting is less aggressive regarding weight management but monitors intake within eating windows to avoid overeating.   
  • Periodic fasting: This fasting method typically includes longer fasting intervals, ranging from 24 hours to several days, followed by regular eating periods. Unlike daily fasting schedules, periodic fasting is less frequent and can occur weekly or monthly. Proponents of this schedule report improved mental clarity and focus but can pose risks for people with underlying health conditions. 
  • Long-term fasting: Long-term fasting involves abstaining from all or most caloric intake for extended periods, typically from several days to weeks. This type of fasting goes beyond intermittent fasting and often requires careful planning and supervision, as there’s a potential for severe issues like nutritional deficiencies, muscle mass loss, and physical fatigue. 

Can Intermittent Fasting Cause Hypothyroidism?

There is no direct evidence to suggest that intermittent fasting causes hypothyroidism.

Hypothyroidism is typically caused by underlying factors such as autoimmune thyroid disease (Hashimoto’s thyroiditis), iodine deficiency, or certain medications.

While some studies have shown that fasting can affect thyroid hormone levels in the short term, there is limited evidence to suggest that intermittent fasting directly leads to hypothyroidism when done correctly.

Intermittent Fasting as a Treatment for Hypothyroidism

While this eating pattern may offer various health benefits, it is not a proven treatment for hypothyroidism.

Hypothyroidism is a chronic condition that requires appropriate medical management, usually in the form of thyroid hormone replacement therapy.

Although lifestyle modifications such as diet and exercise can complement medical treatment, they should not be used as a substitute for thyroid hormone therapy.

How Does Fasting Affect Thyroid Function?

Some studies have shown that fasting can temporarily reduce thyroid hormone levels, particularly T3 (triiodothyronine) while increasing reverse T3 (rT3) levels.

However, these changes are often transient and are part of the body’s adaptive response to fasting.

In healthy individuals, thyroid function typically returns to normal once fasting ends and regular eating resumes.

Risks Associated with Intermittent Fasting and Hypothyroidism

People with hypothyroidism should approach intermittent fasting with caution and consult with their healthcare provider before embarking on any fasting regimen.

Fasting may exacerbate symptoms of hypothyroidism, such as fatigue, constipation, and weight gain, mainly if thyroid hormone levels are not adequately controlled.

If you’re planning to fast, you should monitor your symptoms closely and adjust fasting schedules as needed under the guidance of a healthcare professional.

Potential Benefits

Although intermittent fasting does not directly impact thyroid health, it may offer various health benefits such as weight loss, improved metabolic health, and enhanced insulin sensitivity.

Some studies suggest intermittent fasting may also have anti-inflammatory and anti-aging effects, although more research is needed to confirm these findings.

Fasting Safety If You Have Hypothyroidism

Intermittent fasting may not be suitable for everyone, particularly individuals with certain medical conditions, eating disorders, or metabolic disorders. 

Pregnant or breastfeeding women, individuals with a history of disordered eating, and those taking medications that require food intake should consult with a healthcare provider before attempting intermittent fasting.

Effects on Hormone Medication

Thyroid hormone replacement medication is commonly as a therapeutic option for symptom management for people with Hypothyroidism and Hashimoto’s. This treatment helps restore your thyroid hormone levels back to a normal range, helping correct TSH levels. 

Although there is no scientific consensus on whether fasting impacts thyroid medication dose, studies analyzing TSH levels during religious fasting observed an increase in the studied group’s levels that exceeded the normal ranges. 

This suggests that medication dosage must be adjusted by your doctor and hormone levels observed when you decide to undertake an IF regime to avoid the risk of hyperthyroidism. 

Final Thoughts

While intermittent fasting has gained popularity for its potential health benefits, it’s essential to approach this eating pattern with caution, particularly for individuals with hypothyroidism.

Although this practice isn’t a direct cause of hypothyroidism, it may affect thyroid hormone levels temporarily and may not be suitable for everyone.

As always, it’s essential to consult with a healthcare provider before making any significant changes to your diet or lifestyle, especially if you have a pre-existing medical condition like hypothyroidism.

Consulting with a healthcare provider and monitoring symptoms closely allows you to make informed decisions about incorporating fasting into your lifestyle while prioritizing your thyroid health.


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