How Intermittent Fasting Impacts Insulin, Your Thyroid and Cortisol (Adrenals): new study review

by Mike Mutzel


Intermittent fasting and prolonged fasting decrease insulin but also impact your thyroid, cortisol and other hormones. In this show discuss this research and considerations when deciding how often or long to fast for your health.




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Time Stamps:

01:39 Prolonged fasting impacts hormones and your circadian rhythm. Hormones oscillate with your circadian rhythm.

02:22 Fasting may not beneficial in general for metabolically healthy individuals who are physically active. They may see changes in thyroid hormone and increases in counter-regulatory hormones like cortisol and adrenaline. This can impact sleep and sense of wellbeing.

02:52 Fasting benefits those of us who have significant body fat to lose.

03:37 Insulin drops about 35% over a 24 hour fast. It is halved after 72 hours. Insulin should be elevated only in the post meal window, not during fasting.

04:22 Create a fasting plan that is sustainable and consistent.














As figure discusses how Intermittent Fasting impacts insulin and the circadian rhythms of various hormones.

05:27 Your metabolic rate is reduced during fasting. Thus, there is a somewhat significant reduction in free T3 levels and a reduction in TSH with fasting. With hypothyroidism, TSH can increase.

06:07 Autoimmune hypothyroidism is characterized by leptin interfering and changing immune system tolerance.

07:07 If you are lean and have thyroid issues, perhaps fasting is not of benefit to you.

07:17 Early time restricted feeding pushes your circadian rhythm into an early robust cortisol waking response.

07:52 Fasting, eating, and waking up causes a transient increase in cortisol.

08:37 The primary benefit to fasting is a reduction in glucose and reduction in insulin.

08:52 Early time restricted feeding that Mike recommends consists of breaking your fast at about 10 a.m. and ending by 6 p.m. This way, you do not go to bed on a full stomach.



1.Kim, B. H. et al. Effects of Intermittent Fasting on the Circulating Levels and Circadian Rhythms of Hormones. Endocrinol Metab 36, 745–756 (2021).

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