Intermittent fasting does affect your hormones, and the changes are wide-ranging. Some shifts are beneficial, like improved insulin sensitivity and a surge in growth hormone. Others are potentially problematic, particularly for women’s reproductive health and stress hormones. The impact depends on the type of fasting you do, how long your fasting windows last, and your sex.
Cortisol Rises During Fasting
Your body treats fasting as a mild stressor, and it responds accordingly. Cortisol, your primary stress hormone, begins to increase almost immediately after fasting starts. Short fasts of a few days elevate cortisol levels and can shift your natural cortisol peak from the morning (where it normally belongs) to the afternoon. Even a moderate protocol like early time-restricted feeding, where you eat between 8 AM and 2 PM, slightly but significantly increases morning cortisol after just four days.
This doesn’t necessarily mean you’ll feel stressed. A temporary cortisol bump is part of how your body mobilizes energy from stored fat. But chronically elevated cortisol can interfere with sleep, raise blood sugar, and break down muscle tissue. If you’re already dealing with high stress levels, stacking a demanding fasting protocol on top of that may not work in your favor.
Growth Hormone Gets a Major Boost
One of the most dramatic hormonal responses to fasting is the spike in human growth hormone, which helps preserve muscle mass, burn fat, and repair tissue. During a 24-hour water-only fast, growth hormone increases roughly 5-fold in men and up to 14-fold in women. People who start with very low baseline levels see the most dramatic jumps, with some experiencing increases above 1,000%.
This surge is one reason fasting advocates argue it protects muscle during weight loss. Your body ramps up growth hormone to shift energy use toward stored fat rather than breaking down protein. The effect is temporary: growth hormone drops back toward baseline once you eat again.
Thyroid Hormones Slow Down
Fasting consistently lowers levels of T3, the active thyroid hormone that drives your metabolism. Even a single 24-hour fast reduces free T3 by about 6% while increasing reverse T3 (an inactive form) by 16%. Longer fasts amplify this effect. T4 and TSH, the other key thyroid markers, tend to stay roughly the same or dip slightly.
This drop in T3 is your body’s way of conserving energy. It slows your resting metabolic rate, which is a survival mechanism but not ideal if you’re fasting specifically to lose weight. Over time, a persistently suppressed metabolic rate can make weight loss plateau or make you feel sluggish and cold. This is more of a concern with extended or aggressive fasting protocols than with a standard 16:8 schedule.
Insulin Sensitivity Improves
Insulin is arguably the hormone most positively affected by intermittent fasting. Every time you eat, your pancreas releases insulin to shuttle glucose into cells. Fasting gives your body extended breaks from insulin production, which helps your cells become more responsive to insulin over time. This is particularly relevant for people carrying extra weight, who often have elevated fasting insulin levels and reduced sensitivity to it.
Improved insulin sensitivity has cascading benefits for other hormones too. High insulin drives fat storage, increases androgen production, and interferes with hunger signaling. Bringing insulin down can help correct several hormonal imbalances at once, which is why fasting has shown particular promise for conditions linked to insulin resistance.
Hunger Hormones Shift With Body Composition
Ghrelin, the hormone that triggers hunger, rises during fasting and normally drops after eating. But this response varies by body composition. In lean, healthy people, ghrelin drops reliably after a meal regardless of how many calories it contains. In people with obesity, this postprandial ghrelin suppression is blunted, meaning the “full” signal is weaker.
Leptin, the hormone that signals satiety, behaves differently too. People with obesity typically have very high leptin levels, but their brains have become resistant to it, similar to how cells become resistant to insulin. Leptin and insulin levels are closely correlated in both lean and overweight individuals. By improving insulin sensitivity, fasting may help restore some leptin signaling over time, though the research on this is still evolving.
Effects on Women’s Reproductive Hormones
This is where intermittent fasting gets complicated. Your reproductive system is tightly linked to your energy balance through a brain signaling molecule called kisspeptin. When your body senses insufficient energy, kisspeptin production drops, which reduces the hormones that drive ovulation: luteinizing hormone (LH) and follicle-stimulating hormone (FSH). Animal studies show that fasting directly suppresses kisspeptin gene expression and lowers circulating LH in female subjects.
For women who are lean and active, this sensitivity to energy restriction can be especially pronounced. The reproductive system essentially interprets fasting as a signal that conditions aren’t favorable for pregnancy. This can lead to irregular periods, missed ovulation, or other menstrual disruptions. The risk appears higher with longer fasting windows and more severe caloric restriction.
A Different Picture for Women With PCOS
For women with polycystic ovary syndrome, intermittent fasting may actually improve hormonal balance rather than disrupt it. PCOS is driven largely by insulin resistance and excess androgens, and fasting targets both of these. In a study of women with PCOS who followed an 8-hour time-restricted feeding window for six weeks, total testosterone dropped from a median of about 53 to 33 ng/dL. DHEAS, another androgen, fell from roughly 253 to 188 µg/dL. LH, FSH, estradiol, and prolactin all decreased significantly as well.
These reductions reflect a correction of the hormonal excess that characterizes PCOS rather than a suppression of healthy function. By lowering insulin, fasting reduces the signal that drives the ovaries to overproduce androgens. This is why some researchers consider time-restricted feeding a strong candidate for first-line PCOS management.
Testosterone Drops in Lean, Active Men
Men’s reproductive hormones respond to intermittent fasting too, though less dramatically. In lean, physically active young men, intermittent fasting has been shown to reduce testosterone levels. Sex hormone-binding globulin (SHBG), a protein that binds testosterone and reduces its availability, was not significantly affected. This means the drop in testosterone likely reflects a real decrease in the hormone’s activity, not just a change in how it’s measured.
For men who are overweight, the picture may differ. Since excess body fat converts testosterone to estrogen and drives insulin resistance (which further suppresses testosterone), fasting-related fat loss and improved insulin sensitivity could offset or reverse this effect. The context of your starting metabolic health matters.
IGF-1 Drops With Time-Restricted Eating
Insulin-like growth factor 1 (IGF-1) is a growth-promoting hormone linked to aging and cell proliferation. In a study of obese young women following time-restricted feeding for 20 days, IGF-1 levels dropped dramatically, from a median of about 199 ng/mL to 104 ng/mL. Interestingly, alternate-day fasting in the same study did not change IGF-1 at all, suggesting the type of fasting protocol matters for this particular hormone.
Lower IGF-1 is generally considered favorable for longevity. High IGF-1 levels promote cell growth indiscriminately, which accelerates aging at the cellular level. The reduction from time-restricted feeding is one of the mechanisms researchers believe underlies fasting’s potential benefits for long-term health.
Sleep Hormones Stay Mostly Stable
One concern people have is whether fasting disrupts melatonin and sleep quality. Under controlled conditions, where light exposure, meal composition, and sleep schedules were held constant, intermittent fasting did not significantly shift the circadian pattern of melatonin. Peak melatonin still occurred around 2 AM and the lowest point at 11 AM across fasting and non-fasting periods.
There was one notable finding: melatonin levels at 10 PM were slightly lower during fasting compared to baseline. Most studies on fasting and melatonin report some decrease in nocturnal melatonin concentrations, but the overall rhythm and timing of the hormone remain intact. If you’re sleeping well on your current fasting schedule, your melatonin cycle is likely fine.