Intermittent fasting affects nearly every major hormonal system in your body, from insulin and growth hormone to stress hormones and reproductive signals. Some of these shifts are beneficial and drive the metabolic advantages people associate with fasting. Others, particularly with longer or more aggressive fasting schedules, can work against you. The specifics depend on the type of fast, how long you go without food, and your sex.
Insulin Drops Quickly During Fasting Windows
The most immediate hormonal change during a fast is a drop in insulin. When you stop eating, your blood sugar gradually falls, and your pancreas slows its insulin output in response. This lower-insulin state is what allows your body to shift from burning incoming food to burning stored fat. Over weeks of consistent intermittent fasting, this repeated cycling can improve your overall insulin sensitivity, meaning your cells respond to insulin more efficiently even during your eating windows.
This is one of the primary reasons intermittent fasting has shown benefits for people with prediabetes or metabolic syndrome. The repeated periods of low insulin essentially give your cells a break from constant insulin signaling, which can reduce insulin resistance over time.
Growth Hormone Surges During Extended Fasts
Your pituitary gland releases growth hormone in pulses throughout the day, but fasting dramatically amplifies this output. In a study published in the Journal of Clinical Endocrinology & Metabolism, healthy adults who fasted for 33 hours experienced a 3.7-fold increase in their average growth hormone concentration compared to their fed state. That effect kicks in well before the 33-hour mark: growth hormone secretion ramps up most noticeably from late afternoon through the first few hours after midnight during a fast.
This matters because growth hormone helps preserve lean muscle mass while your body taps into fat stores for energy. It’s one of the reasons short-term fasting doesn’t cause the rapid muscle loss you might expect. The hormone also supports tissue repair and cell regeneration. However, these dramatic spikes are most pronounced during longer fasts. A standard 16:8 eating pattern still increases growth hormone output, but the effect is more modest than what you’d see with a full-day fast.
Cortisol and Adrenaline: The Stress Response
Fasting is a mild stressor, and your body responds accordingly. Norepinephrine (your body’s alertness chemical) rises significantly during fasting. One study found urinary norepinephrine concentrations increased from about 17.8 to 27.8 micrograms per milliliter during short-term fasting, a roughly 56% jump. Adrenaline more than doubled. This is actually what gives many people that sharp, energized feeling during their fasting window, and it contributes to a modest bump in metabolic rate during short fasts.
Cortisol, your primary stress hormone, follows a more nuanced pattern. A study on an 18:6 fasting protocol (six-hour eating window) found that evening cortisol levels dropped significantly, while morning cortisol trended slightly higher. This suggests fasting may actually reinforce your body’s natural cortisol rhythm, which peaks in the morning and tapers off at night. People who skip breakfast, on the other hand, showed lower morning cortisol but elevated midday levels, a somewhat less ideal pattern. The takeaway: when you place your eating window may shape how fasting influences your stress hormones.
Thyroid Hormones Slow Down
Your thyroid is sensitive to energy availability, and fasting consistently nudges it toward conservation mode. The key change happens in how your body processes thyroid hormones. Normally, your liver converts the storage form of thyroid hormone (T4) into the active form (T3), which drives your metabolism. During fasting, this conversion slows. In animal studies, the enzyme responsible for this conversion dropped by 54% during fasting. Instead of producing active T3, your body shunts more T4 into an inactive form called reverse T3.
What’s unusual is that this drop in active thyroid hormone doesn’t trigger the normal feedback response. You’d expect your brain to sense low thyroid levels and ramp up its signaling to the thyroid gland, but that doesn’t happen. Instead, TSH (the brain’s signal to the thyroid) stays low or even decreases. A 60-hour fast reduced average 24-hour TSH concentration by lowering the strength of each TSH pulse rather than the number of pulses. Your brain essentially resets its thermostat, accepting the lower thyroid output as appropriate for a fasting state.
For most people doing a daily 16:8 fast, this effect is temporary and reverses during the eating window. But if you’re already dealing with an underactive thyroid or are chronically undereating, aggressive fasting schedules could compound the problem.
Hunger Hormones Adapt Over Time
Ghrelin, your primary hunger hormone, is what makes the first week or two of intermittent fasting feel difficult. It surges at the times your body expects food. But a Ramadan fasting study that tracked both ghrelin and leptin (your satiety hormone) over several weeks found no significant change in average ghrelin concentrations across the entire fasting period compared to baseline. Your body didn’t produce more hunger signaling overall; it simply shifted when those signals occurred.
Leptin levels did drop at specific nighttime time points, falling from about 194 to 133 at 10 PM during fasting. But the overall daily average didn’t change significantly. This suggests that intermittent fasting reorganizes the timing of your hunger and satiety signals rather than fundamentally increasing or decreasing them. Most people find that after two to three weeks of a consistent schedule, the hunger pangs during fasting windows become noticeably easier to manage.
A Hormone That Improves Metabolic Health
Adiponectin is a hormone released by fat cells that improves insulin sensitivity and has anti-inflammatory effects. Higher levels are associated with lower risk of type 2 diabetes and cardiovascular disease. Intermittent fasting has been shown to significantly increase adiponectin levels, particularly in people who already have metabolic risk factors. One study of men with risk factors for type 2 diabetes found a highly significant rise in adiponectin after a prolonged fasting period, alongside a modest (though not statistically significant) decrease in an inflammatory marker called TNF-alpha.
Effects on Reproductive Hormones
This is where intermittent fasting diverges sharply between men and women, and where caution is most warranted.
In Men
A review of human trials found that intermittent fasting reduced testosterone levels in lean, physically active young men. This is worth noting if you’re a young male athlete adopting an aggressive fasting schedule. The reduction doesn’t appear to affect sex hormone-binding globulin (SHBG), which means the drop reflects an actual decrease in testosterone production rather than just a change in how the hormone is transported.
In Women
The female reproductive system is particularly sensitive to energy availability signals. In animal models, alternate-day fasting reduced levels of three critical reproductive hormones: GnRH (the master signal from the brain), LH, and FSH (the hormones that drive ovulation and cycle regularity). In human studies, women with polycystic ovary syndrome showed fewer LH pulses during short-term fasting. Large-scale human trials on how intermittent fasting affects menstrual cycles in healthy women are still lacking, but the existing evidence suggests that the reproductive axis in women responds more dramatically to fasting than in men.
A study of premenopausal women during Ramadan fasting found significant decreases in estrogen levels. Postmenopausal women in the same study actually saw a small but significant increase in estrogen. Progesterone levels didn’t change meaningfully in either group. These findings reinforce a pattern: women of reproductive age appear more hormonally sensitive to fasting than other groups.
If you’re a premenopausal woman and notice changes to your menstrual cycle after starting intermittent fasting, that’s a signal your body is interpreting the fasting stress as a threat to reproductive function. Shorter fasting windows (12 to 14 hours rather than 16 or more) may be a more sustainable starting point.