How to intermittent fast without harming your thyroid

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Fasting diet tips for people with Hashimoto’s and an underactive thyroid 

Intermittent fasting (IF) is a popular diet approach for many people—including those diagnosed and undiagnosed with thyroid conditions. A lot of research has shown that IF might be beneficial for improving health (1-5).

If you have thyroid issues, you should take a cautious approach towards implementing intermittent fasting into your lifestyle. 

  • Talk to your physician
    Nobody can replace the vast knowledge doctors have on your health. They’re qualified to give you advice on whether this diet is safe for your general health.

  • Start slowly
    Begin with short fasting protocols (12:12 hours or 14:8 hours). If you go from being a frequent eater to not eating for 16 hours a day, you’ll activate several molecular mechanisms that put your entire body in a starvation mode. Starvation mode fully changes the pattern by which thyroid hormones are produced, so you might be at risk of gaining weight and worsening your general health (6-9).

  • Be aware of what you are eating
    Your gut microflora will change when you change your dietary habits. Try keeping the components of a healthy diet in (proper ratios of food groups). Sugar enables bad bacteria to prevail so it’s beneficial to have a low intake (10,11). If you have a sweet tooth, try dark chocolate. According to a BOOST Thyroid survey, people who ate around 50g or less of at least 70% dark chocolate a max three times per week didn’t gain weight. Also, dark chocolate is anti-inflammatory (12-16). Consuming dairy or other inflammatory agents is counterproductive when intermittent fasting, as your body will use the time of rest (12-16 hours) to repair the correlated damage instead of resting.

  • Choose your teas
    You can have tea and black coffee while you fast. Some teas (like green tea and oolong tea) are great to jumpstart your metabolism and can enable you to be productive before your first meal. (17-19)

  • Don’t over exercise

Increasing exercise routines  while you’re starting intermittent fasting might quickly deplete your thyroid hormones (20, 21).

  • Don’t take painkillers on an empty stomach

Certain painkillers are better to use than others for people with autoimmune conditions. If you’re taking NSAIDs, make sure you take them after you’ve eaten. 

  • Make sure you’re getting the right vitamins

 Iodine, omega 3, selenium, and zinc are crucial for thyroid health.

  • Don’t stress

Stressing over intermittent fasting is counterproductive—as stress can substantially harm your gut and the damage takes a long time to repair.

  • Be aware of food triggers

If you’re not sure what foods trigger flare-ups, try to avoid making a full meal out of dairy or wheat-based products. According to our survey, 7 in 10 people with Hashimoto’s have a problem with dairy, wheat, or both. If you eat foods that trigger your immune system during IF you might feel increasingly tired and unable to concentrate on your daily tasks.

  • Look out for signs of thyrotoxicity

If you notice you are losing weight and have shaky hands and palpitations, it may mean you are taking more thyroid hormones than your body needs. This is a state of thyroid hormone overtreatment (thyrotoxicity) and you should check in with your physician as soon as you can.

If you intend to do the 16:8 fasting protocol, work your way up.

Increase the number of fasting hours every day by about 20 minutes and do it five days per week. Leave weekends or any other two days without an increase. This way you will increase the number of fasting hours by 6.5 hours a month, and if you started with 8 hour fasting (while you are asleep) you can expect to be on a full fasting schedule in 1.5 months.

If you intend to do a 5:2 fasting schedule, start similarly to 16:8 and slowly eat less food from one day to the next.

Track diet and symptoms in BOOST Thyroid, and observe how your health improves through time.


  1. Challet E. Circadian clocks, food intake, and metabolism, 2013

  2. Longo VD, et al. Fasting, circadian rhythms, and time-restricted feeding in healthy lifespan, 2016

  3. Xu Z, et al. Dietary effects on human gut microbiome diversity, 2015

  4. Bhutani S, et al. Alternate day fasting and endurance exercise combine to reduce body weight and favorably alter plasma lipids in obese humans, 2013

  5. Yamaguchi M, et al. Relationship of dietary factors and habits with sleep–wake regularity, 2013

  6. Duyon L, et al. Effect of obesity and starvation on thyroid hormone, growth hormone, and cortisol secretion, 2002

  7. Sesmilo G, et al.Role of thyroid hormones on serum leptin levels, 1998

  8. Feldt-Rasmussen U. Thyroid and leptin, 2007

  9. Zimmermann-Belsing T, et al. Circulating leptin and thyroid dysfunction, 2003

  10. Biesel CL, et al. Rethinking the Hierarchy of Sugar Utilization in Bacteria, 2016

  11. Dukowicz AC, et al. Small Intestinal Bacterial Overgrowth, 2007

  12. Katz DL, et al. Cocoa and Chocolate in Human Health and Disease, 2011

  13. Jenkins DJ, et al. Effect of cocoa bran on low-density lipoprotein oxidation and fecal bulking, 2000

  14. Sternberg FM, et al. Keen CL. Cocoa and chocolate flavonoids: implications for cardiovascular health, 2003

  15. Court R, et al. Cocoa and cardiovascular health, 2009

  16. Selma C, et al. The anti-inflammatory properties of cocoa flavanols, 2006

  17. Hursel R, et al. The effects of green tea on weight loss and weight maintenance: a meta-analysis, 2009

  18. Gahreman D, et al. Green Tea, Intermittent Sprinting Exercise, and Fat Oxidation, 2015

  19. Rumpler W, et al. Oolong tea increases metabolic rate and fat oxidation in men, 2001

  20. Ciloglu F et al. Exercise intensity and its effects on thyroid hormones, 2005

  21. Al-Hashem F et al. Exhaustive exercise and vitamins C and E modulate thyroid hormone levels at low and high altitudes, 2012