Fasting, Hypothyroidism, and a Gentler Way to Support Autophagy

Fasting is having a real moment, celebrated for its role in autophagy, gut health, weight management, and cellular renewal. But it has actually been part of every major religion for hundreds of years. In East Asia, monks fasted to reach mental clarity and spiritual enlightenment. It is worth noticing, though, who was doing that fasting: men.

Why does that distinction matter? Because over the years, many women have told me the opposite of enlightenment happened when they started fasting. Their hair thinned, their focus slipped, and they wanted to sleep all the time. That is not the kind of clarity any of us are looking for. So what is going on?

The women who described this to me had one thing in common: low thyroid function, or hypothyroidism. Fasting and hypothyroidism are generally not a good match, and this is one reason men often tolerate fasting better than women, since hypothyroidism is roughly ten times more common in women.

How does fasting create its benefits? Hormesis explained

Fasting works largely through hormesis, the process by which a manageable stress produces a benefit. When fasting temporarily starves cells of nutrients, the body responds by clearing out less efficient components like damaged proteins and mitochondria, then regenerating new cellular material from the recycled parts. This is a core part of how fasting supports autophagy and cellular renewal.

That hormetic stress is beneficial for many people. For those with hypothyroidism or low adrenal reserve, however, the same nutrient stress can be counterproductive.

How does fasting affect thyroid function?

In people with hypothyroidism, fasting can further down-regulate the thyroid, tipping it into a kind of quasi-hibernation that leaves you tired and brain-fogged. This is an ancient survival mechanism. When food was scarce for our ancestors, often with no prospect of more soon, the body sensibly slowed its metabolism (governed by the thyroid) to conserve energy through lean months.

The thyroid makes its raw hormone, T4, from nutrients including iodine, iron, zinc, selenium, and B vitamins. When those nutrients seem unlikely to be reliably available, the thyroid slows its conversion of T4 into the active hormone T3. It also produces Reverse T3, which occupies cells' T3 receptors and blocks active T3 from getting in, conserving it for later.

Think of it this way: your thyroid tends your body's internal furnace (metabolism) by chopping trees (T4) into logs (T3) that fit the fire. When the body senses no more trees are coming, it shuts the furnace door (Reverse T3). That conserves scarce resources, but it also leaves you cold, sluggish, and foggy, and yes, it can contribute to hair shedding, since the body treats hair as non-essential to survival.

Should people with hypothyroidism avoid fasting altogether?

General medical guidance suggests that people with hypothyroidism are often better off not fasting, since it can compound already-low thyroid hormone status. This is general information, not personal medical advice, so anyone with a thyroid condition should talk with their own doctor or endocrinologist before changing how they eat.

If some form of fasting is appropriate for you, a gentler option is intermittent fasting, where you eat within an 8 to 12 hour window and rest the digestive system the rest of the time. That break lets the body focus on repair and tissue regeneration rather than digestion. Longer fasts and the popular keto diet tend to be poorer choices with hypothyroidism, because very low carbohydrate intake drops insulin, prompting the adrenal glands to raise cortisol.

A morning cortisol rise is helpful for alertness, but staying in a high-cortisol state can drive chronic inflammation. High cortisol at night also suppresses melatonin, the sleep hormone, which can leave you tossing and turning instead of getting the deep sleep that itself supports autophagy.

Is there an alternative to fasting for thyroid patients?

Yes. There are gentler ways to pursue autophagy's benefits without the nutrient deprivation that can strain the thyroid, ranging from modified intermittent fasting to spermidine from food and supplements. Which approach suits you is worth discussing with your healthcare provider, especially if you manage a thyroid condition.

A gentler intermittent fasting approach

One option is to skip breakfast or start the day with a high-fat coffee, such as a Bulletproof-style coffee blended with grass-fed ghee or butter and MCT oil like C8. The clean fat gives your brain ketones for energy without technically breaking your overnight fast, letting your eating window start around 10am to noon. Aim to finish with an evening meal that includes a slow carbohydrate such as taro, sweet potato, or cassava, so insulin does not dip too low and interfere with nighttime melatonin. Even so, people with hypothyroidism often still feel tired on intermittent fasting, which is where another route comes in.

How to support autophagy without fasting

You can support autophagy and mimic some of the benefits of calorie restriction without fasting at all, by eating spermidine-rich foods like Japanese natto and tempeh and by taking a food-derived spermidine supplement. This may offer a gentler way to pursue autophagy's benefits while you are still eating normally, without the nutrient deprivation that can burden the thyroid and metabolism.

Research also associates spermidine with several of the hallmarks of aging, the biological pathways through which the body ages, which is part of why it has become such a focus in longevity science.

Primeadine: our food-derived spermidine supplement

Primeadine is a food-derived spermidine supplement made from plant-derived Japanese ingredients and tested in both Japan and the USA. It provides a consistent daily amount of spermidine alongside its companion polyamines.

Customers often share the changes they notice over time. These vary from person to person and are not guaranteed outcomes, but commonly mentioned experiences include clearer-looking skin, healthier-feeling hair, more settled sleep, steadier daytime energy, and sharper focus. For many, it is simply a way to feel more vibrant as they age, and a practical alternative when fasting is not the right fit.

If you would like a gentler path to autophagy that works alongside your meals rather than against your thyroid, spermidine may be worth exploring.

References

1. Hypothyroidism (underactive thyroid), National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK). https://www.niddk.nih.gov/health-information/endocrine-diseases/hypothyroidism (Accessed: 22 May 2024).
2. Bulletproof Coffee Recipe, Bulletproof. https://www.bulletproof.com/recipes/bulletproof-diet-recipes/bulletproof-coffee-recipe/ (Accessed: 22 May 2024).