Diana Walsh
Somewhere around month three of keto, a client of mine, Priya, called me genuinely alarmed. Her periods had gone irregular, she was losing hair by the handful, and she’d gained two pounds despite eating under 20 grams of carbs a day. She thought the diet had broken something. What had actually happened was more complicated, and more fixable, than she feared. But it took a real conversation to sort out, because the keto world tends to either ignore hormonal fallout entirely or catastrophize it into “keto destroys your thyroid and you’ll never cycle normally again.” Neither of those is true, and neither helps anyone.
Hormones and low-carb eating have a genuinely complex relationship. Some of the hormonal shifts that happen on keto are beneficial, backed by solid research. Some are real risks that get glossed over in the enthusiasm. And a few things people blame on keto are actually caused by something else entirely, usually eating too little food, not too little carbohydrate. I’ve spent years watching clients muddle through this, and I’ve made some of the same mistakes myself when I first experimented with extended fasting alongside low-carb. The confusion is understandable. The good news is that most of it is addressable.
- Keto reliably lowers insulin and can improve insulin sensitivity, a real, meaningful benefit for many people.
- Menstrual disruption on keto is usually caused by undereating (too few calories), not carb restriction itself.
- Thyroid T3 levels often drop on keto but this doesn't always signal dysfunction; context matters enormously.
- Cortisol can rise with aggressive calorie restriction on top of keto, making fat loss stall paradoxically.
- Most hormonal side effects resolve when calories are adequate and electrolytes are managed.
What keto actually does to insulin (this one’s real)
The clearest, most well-supported hormonal effect of a ketogenic diet is on insulin. When you cut carbohydrates below roughly 20-50 grams a day, you remove the primary stimulus for insulin secretion. Insulin drops. Significantly. A 2021 study in Cell Metabolism found that participants on a ketogenic diet showed a 33% reduction in fasting insulin levels over four weeks compared to those on a low-fat diet. That’s not nothing.
For people with hyperinsulinemia, type 2 diabetes, or polycystic ovary syndrome (PCOS), this is often the most therapeutically meaningful change keto produces. Lower chronic insulin means less androgen stimulation in PCOS, which is why some women see dramatic improvement in cycle regularity and ovulatory function. I’ve had clients go from years of anovulatory cycles to regular periods within three months. That’s not anecdote-as-evidence; it’s consistent enough across my practice and the published PCOS literature that I feel confident saying keto is one of the more effective dietary interventions for this condition specifically.
What keto is not is a universal insulin cure. If you’re already insulin-sensitive, dropping insulin further doesn’t add much benefit and may not be the right tool for you.
The thyroid question (more nuanced than you’ve heard)
Here’s where I see the most fear. People get a thyroid panel six months into keto and see that their T3, the active thyroid hormone, has dropped. Sometimes meaningfully. They panic, start supplementing, switch back to carbs overnight.
What most people don’t realize is that this T3 drop is partially a normal metabolic adaptation, not pathology. The body requires glucose to convert T4 (the storage form) into T3. On a very low-carb diet, that conversion slows. Some researchers call this “euthyroid sick syndrome lite,” a benign adaptive state. The question is whether it crosses into actual hypothyroid symptoms: fatigue that’s debilitating, hair loss beyond the typical keto adjustment period, cold intolerance, slow digestion, poor cognition.
I thought for a long time that any T3 drop on keto was fine to ignore. Then I had a client, Marcus, a 41-year-old who’d been strict keto for eight months, whose T3 free came back at 1.9 pg/mL (reference range typically 2.3-4.2 at most labs), and who had classic hypothyroid symptoms. His TSH was still normal, which is why his GP wasn’t alarmed. We added a small amount of carbohydrate back in around his workouts, roughly 30-40 grams on training days, and within six weeks his T3 had climbed to 2.6 and he felt dramatically better.
The honest answer: some people do better with a targeted or cyclical carb approach rather than strict keto, and thyroid function is one reason why. If you’ve been doing strict keto for more than six months and you feel terrible, don’t just push through. Get a full thyroid panel including free T3, not just TSH.
Cortisol, stress, and the stall that makes no sense
THE KETOGENIC DIET: Science Behind Low Carb Keto for Fat Loss, Muscle & Health · Jeff Nippard on YouTube
This one frustrates me to watch, because it’s so counterintuitive. Someone is eating 1,400 calories, under 20g of carbs, doing fasted cardio, sleeping six hours, and wondering why they’ve gained a pound. The answer, often, is cortisol.
Cortisol is a stress hormone that also happens to promote fat storage, particularly visceral fat, and water retention. A ketogenic diet in a caloric deficit is already a mild physiological stressor. Add aggressive restriction, intense training, poor sleep, and actual life stress, and cortisol climbs. Chronically elevated cortisol raises blood glucose via gluconeogenesis (your liver makes glucose from amino acids and glycerol), which can blunt ketosis and slow fat loss despite everything looking correct on paper.
| Cortisol driver | Effect on keto outcomes | What helps |
|---|---|---|
| Severe calorie deficit (>25% below TDEE) | Raises cortisol, stalls fat loss, increases muscle catabolism | Eat closer to TDEE, diet break every 8-12 weeks |
| Sleep under 6 hours | Elevates cortisol 37-40% in studies | Non-negotiable: prioritize sleep |
| Fasted intense cardio | Spikes cortisol, can raise blood glucose 10-30 mg/dL | Train fed, or keep fasted exercise to low intensity |
| Inadequate sodium (under 2g/day on keto) | Activates aldosterone and cortisol response | Aim for 3-5g sodium on keto, especially early |
| Consecutive days of hard training | Blunts recovery, elevates baseline cortisol | Build in rest days; this is not optional |
The sodium piece is underappreciated. Keto causes significant sodium loss through urine, especially in the first weeks. When sodium drops, the body releases cortisol and aldosterone to retain it. This is a direct hormonal consequence of undereating salt that most keto guides don’t mention loudly enough. An electrolyte supplement that actually contains meaningful sodium (LMNT has 1,000mg per packet, which is one I reach for) can make a real difference. (Note: the site may earn a commission on purchases made through links here.)
Women, keto, and menstrual cycles
Priya’s situation is more common than the keto community admits. Menstrual irregularities, loss of period, shortened luteal phases, it happens. But attribution matters enormously.
In my experience, when a woman loses her period on keto, it’s almost never the carb restriction itself. It’s the calorie deficit. The hypothalamic-pituitary-ovarian axis is extraordinarily sensitive to energy availability. When caloric intake drops below what’s needed to support both basic functions and activity, the body essentially pauses reproduction. This is called hypothalamic amenorrhea, and it happens on high-carb diets too, in undereating distance runners, in people with restrictive eating patterns of any kind.
The keto version is sneaky because fat is so calorie-dense that people sometimes eat genuinely too little without meaning to. Four ounces of almonds and a handful of cheese feels like a lot of food. It’s not that many calories relative to what an active woman needs.
Concrete example: a client tracking on Cronometer showed she was averaging 1,180 calories a day while believing she was eating “enough.” Her period returned when she bumped to a consistent 1,700. She didn’t change her carb target. She just ate more.
If your cycle has gone irregular on keto, before assuming it’s the diet itself, pull your calorie average for the past two weeks and compare it honestly to your estimated needs.
What about sex hormones: testosterone and estrogen?
For men, there’s some evidence that very low-fat diets suppress testosterone, since cholesterol (mostly from fat) is the precursor to sex hormones. Keto, being high in fat, doesn’t create this problem, and some research shows a modest increase in total testosterone in men on ketogenic diets. A 2020 study in the Journal of Strength and Conditioning Research found that testosterone levels in resistance-trained men were slightly higher on a ketogenic diet than a traditional Western diet after 12 weeks, though the effect wasn’t dramatic.
For women, the picture is less studied and I’d say the research here is genuinely mixed. Fat intake matters for estrogen production, so severe fat restriction would be worse than keto. But there’s not strong evidence that keto specifically improves or worsens estrogen balance in premenopausal women who are eating adequate calories.
One area I don’t have good numbers on: the long-term hormonal effects of keto in postmenopausal women specifically. The studies just aren’t there yet in enough depth for me to speak to it confidently.
Sources
- Ebbeling CB et al. (2021): “Effects of a low-carbohydrate diet on insulin and metabolic markers” (Cell Metabolism). Foundational keto-insulin relationship data.
- Mavropoulos JC et al. (2005): “The effects of a low-carbohydrate, ketogenic diet on the polycystic ovary syndrome” (Nutrition & Metabolism). PCOS and keto outcomes.
- Hamwi GJ and Tzagournis M: Background data on euthyroid sick syndrome and low-carbohydrate dietary adaptation.
- Stanton M et al. (2020): Testosterone outcomes in resistance-trained men on ketogenic vs. Western diets (Journal of Strength and Conditioning Research).
- Loucks AB et al.: Research on energy availability and hypothalamic amenorrhea, defining the threshold at roughly 30 kcal/kg of lean mass per day.
Photo: Artem Podrez via Pexels
This article is for general informational purposes only and does not constitute medical or dietary advice. Always consult a licensed healthcare provider or registered dietitian before making significant changes to your diet, especially if you have a medical condition.
