Petra Scott exercisingDieting? Exercising? Still not losing weight? Whether you have tried it all or are just embarking on your weight loss journey, finding out what caused your weight gain in the first place is key. You see, weight gain isn’t the problem. Weight gain is the symptom of an unbalanced body/unbalanced hormones.

Permanent weight loss can only occur with balanced hormones. It might surprise you, but understanding your hormones is absolutely essential for long-term weight loss. There are several hormones that can literally make or break your ability to gain or lose weight. If you struggle to get and stay lean, despite all the dieting and exercises you’ve tried, I promise hormones are to blame.

Some of the cues that hormones have begun to turn against you are hunger, cravings, low energy levels, moodiness, and lack of sleep. If only one hormone is off balance, it will inevitably affect the other.

In this article, I focus on 17 weight controlling hormones. I share what they are, how they affect weight, and what exactly you can do to bring them back into balance and lose the weight you want.

Hormones and Endocrine Organs

1. Insulin – The Fat-Storing Hormone

Insulin is a hormone released by the pancreas. Its main job is to maintain safe and stable glucose levels after a meal. When you eat food, it’s broken down into basic nutrients (glucose, amino acids, and fatty acids). Insulin helps  control blood glucose levels by instructing the liver, muscle, and fat cells to take in the glucose and use it for energy. If there’s more glucose in the blood than the liver or muscle cells can use, insulin signals to the fat cells to take up all the remaining glucose and store it as fat. That’s why insulin has been dubbed as the “fat-storing hormone”.

In addition to instructing fat cells to store glucose as fat, insulin also inhibits the breakdown of fat. Fat cells contain an enzyme called hormone-sensitive lipase (HSL), which helps break down body fat. Insulin suppresses the activity of HSL.

However, there’s another side of insulin – its potential to build muscle. While insulin doesn’t directly build muscle, it decreases the rate at which muscle proteins break down.

To positively affect insulin levels:

  1. Avoid refined and simple carbohydrates: refined and simple carbohydrates (such as white bread, white rice, pasta, sugary cereals, cookies, or sugary drinks) lead to high spikes of insulin. Repeatedly high levels of insulin circulating in the system cause your body to become resistant to the insulin message. That means that glucose builds up in the blood instead of being absorbed by the cells. With nowhere else to go, the body turns this extra glucose into fat and stores it for later. (1)
  2. Don’t overeat: the bigger the meals, the larger the insulin spike at the beginning of every meal, and the bigger the drop in glucose afterward. That’s because the amount of the first phase insulin release is dependent on the amount of insulin needed for the previous meal. (2)
  3. Exercise: physical activity of any kind is beneficial for insulin sensitivity in healthy as well as insulin-resistant individuals. (3)

Petra Scott exercising

2. Glucagon – The Fat-Burning Hormone

Glucagon is a hormone released by the pancreas. It’s released when the concentration of glucose in the blood becomes too low. Insulin and glucagon are like yin and yang of blood glucose maintenance: insulin lowers the concentration of glucose in the blood; glucagon raises the concentration of glucose in the blood. Glucagon also signals to the body to release stored fat and use it as fuel. Essentially, insulin promotes storing energy (glucose and fat) while glucagon promotes the release of stored energy (glucose and fat).

To stimulate glucagon release and burn fat, you need to keep your blood sugar and insulin levels low. Your body prefers burning sugar over burning fat so as long as your body has access to glucose, it won’t start burning fat. Glucose levels are generally the lowest between meals and when exercising.

To positively affect glucagon levels:

  1. Avoid refined and simple carbohydrates: refined and simple carbohydrates (such as white bread, white rice, pasta, sugary cereals, cookies, or sugary drinks) create blood glucose levels spikes.
  2. Don’t overeat: glucose levels rise after every meal. However, the larger the meals, the greater the glucose spikes. (2)
  3. Exercise: muscle contractions during exercise allow your cells to take up glucose and use it for energy. (4)

3. Leptin – The Satiety Hormone

Leptin is an appetite-regulating hormone that’s produced by fat cells. It acts on the hypothalamus, the brain’s hunger center, to turn off the hunger switch. Leptin is also very effective at monitoring fat storage. The more fat your fat cells contain, the more leptin they produce and the more your appetite shrinks.

When you lose weight, leptin levels decrease. That’s one of the reasons it’s so hard to maintain weight loss. The brain thinks you are starving and pushes you to eat more.

Now, you might be thinking that overweight people shouldn’t have any problems with overeating since they typically carry around more fat and therefore more leptin. However, many overweight people are leptin resistant (5). In other words, the message to stop eating doesn’t get through to the brain. The brain erroneously interprets leptin resistance as starvation and activates multiple mechanisms to increase fat stores, rather than burn excess fat.

Leptin resistance is most often caused by habitual eating patterns. If you constantly overeat, you can damage the receptor signal. According to Dr. Kent Holtorf, “leptin resistance also stimulates the formation of reverse T3, which blocks the effects of thyroid hormone on metabolism.” (6) What results is slow metabolism.

To positively affect leptin levels:

  • Consume little to no simple and refined carbohydrates: high-carbohydrate diets tend to increase leptin resistance (6).
  • Eat plenty of protein and healthy fatshealth fats promote satiety and give the body the building blocks to make hormones. Protein may also improve leptin sensitivity. (7)
  • Get plenty of sleep: inadequate sleep has been implicated in problems with leptin. (8)

4. Ghrelin – The Hunger Hormone

Ghrelin is a hormone released primarily by the stomach. Its main function is to increase appetite and store fat by acting on the brain’s pleasure centers and encouraging the production of NPY. In addition to being an appetite-stimulating hormone, ghrelin also affects food intake, chewing time, and taste preferences. Have you ever had to finish the whole tub of ice cream or empty the entire bag of chips just because you remembered how good it tasted? Well, blame ghrelin!

If you want to optimize ghrelin for weight loss, dieting is not the way to go. If you under-eat or go on a diet, your ghrelin levels increase, making it hard to lose weight and keep it off. Also, the longer you diet, the higher ghrelin levels rise. According to one study, ghrelin levels of bodybuilders who went on a 6-month low-calorie diet increased by 40%. Surprisingly, overweight people have actually lower ghrelin levels than lean people; they are just more sensitive to ghrelin’s effects.

Apart from food, mindset has a surprisingly great impact on ghrelin as well. According to research, believing that the food you have just consumed is satisfying, drives your hunger hormone down significantly. (9)

To positively influence ghrelin levels:

  • Avoid dieting: chronic dieting disrupts key metabolic hormones, including ghrelin. Maintain stable weight or at least avoid weight extremes for optimal ghrelin levels.
  • Eat a high-water, high-fiber, high-protein foods: those types of foods fill you up quickly and help you stay full longer.
  • Get plenty of sleep: inadequate sleep (less than 7 hours) can increase hunger and weight gain. (10)

5. Thyroid – The Metabolism Hormone

Thyroid hormones are produced by the thyroid gland in the base of your neck. The thyroid gland releases two hormones: thyroxine (T4) and triiodothyronine (T3). These hormones control the rate at which your body burns calories. According to Dr. Keoni Teta, the thyroid hormones are highly sensitive to “sleep (how long and how deep), nutrition (types and amounts of food), stress (acute or chronic), and exercise (longer moderate intensity or shorter high intensity), etc. It influences every system in your body by sensing your internal and external environment”. (11)

What negatively affects thyroid hormones is dieting. The production of reverse T3 seems to be the primary method by which the body tries to regain any weight lost as a result of dieting. As soon as you restrict calories, your body thinks “starvation” and increases levels of reverse T3 while decreasing levels of T3. That’s why it’s so difficult to prevent regaining any lost weight achieved by dieting.

To positively influence T4 and T3 levels:

  • Avoid severe calorie restriction and dieting. (12)

6. Cortisol – The Stress Hormone

Cortisol is a hormone produced by the adrenal glands located above the kidneys. Its main role is to prime your body to “fight or flight” from danger (stress). This mechanism used to be crucial in the earlier days of dinosaurs and cavemen, when running away from predators and seeking protection for survival was common. The extra surge of cortisol (together with adrenalin) provided extra physical energy from stored carbohydrates and fats.

While nowadays lifestyles are extremely stressful, they aren’t nearly as physically demanding as those of our ancestors. Whether you are running for life or stuck in a traffic jam, your body doesn’t react any differently. It can’t distinguish between physical and psychological stress. And because you’re not burning this energy by running for your life, this additional energy is stored as fat.

Cortisol is both a fat-storing hormone (due to its activation of a fat-storing enzyme lipoprotein lipase) and a fat-releasing hormone (due to its activation of a fat-releasing enzyme hormone sensitive lipase). Cortisol’s fat burning potential is activated during exercise when other fat burning hormones – adrenalin and human growth hormone – are released into the system as well. Cortisol’s fat-storing potential is activated in a fed state, especially after a carbohydrate-rich meal, when fat-storing hormone insulin is released into the system.

Therefore, cortisol itself is not the problem. It’s the combination of cortisol and lack of exercise that causes weight gain. What’s interesting is that abdominal fat cells have special receptors for cortisol, which is why high stress levels have been linked to abdominal obesity. Also, women seem to react to stress with a greater increase in cortisol than men.

To positively influence cortisol levels:

  • Exercise: according to Dr. Jade Teta, “short intense exercise, or exercise that’s weight training dominant, and slow relaxing exercise are best for cortisol. (…) With longer-duration moderate and intense exercise, cortisol can easily dominate over the growth promoting hormones and be associated with more post-workout hunger and cravings”. Exercise also tricks your body into thinking that you’re escaping from the source of your stress. Your blood starts to circulate faster and cortisol is flushed out of your system.
  • Don’t diet: constant dieting can raise the negative effects of cortisol by as much as 18%. Dieters may not even realize that calorie restriction produces a stress response because they don’t feel psychologically stressed. (13)
  • Manage your stress: since cortisol is released in response to stress, it only makes sense to reduce stress as much as possible. Try hot baths, massage, yoga, or meditation. Do anything that relaxes your body and mind.
  • Get plenty of sleep: lack of sleep causes your body and mind to remain in a constant state of alertness, requiring high levels of cortisol. (14) The National Sleep Foundation recommends 7 to 9 hours.
  • Ditch the coffee or choose decaf: the combination of stress and caffeine raises cortisol levels much higher than just stress alone.

7. Adrenaline – The Fight or Flight Hormone

Adrenaline is a hormone released by the adrenal glands. It has many different roles, but its main function is to prepare the body for the “fight or flight” response in reaction to stress. Adrenaline causes an increase in strength and energy, as well as heightened senses and better focus.

When it comes to weight loss, adrenaline is responsible for the release of glucose into the blood, which a fight-or-flight response would use. It also initiates breaking down of fat to access another source of energy. Essentially, adrenaline frees up energy for muscle and brain use in response to a perceived stressor.

This is extremely beneficial during exercise. When your body is under physical stress – from lifting weights or doing cardio – it responds by producing more adrenaline, which in turn helps to increase the amount of energy available to use. A chain reaction then stimulates the release of other hormones, such as cortisol, testosterone, and HGH, which send your body into a fat-burning mode.

While acute (short-term) stress can be quite beneficial, chronic (long-term) stress is a different story. When your nervous system increases adrenaline levels, it also decreases leptin levels. This can easily lead to overeating, sometimes called “stress eating”. When you are under constant stress, not only do your leptin levels stay low, but your adrenaline levels stay chronically high, which can desensitize your cells and cause adrenaline resistance. Once the resistance sets in, you become poor at releasing stored fat into the blood stream, but you are still overeating.

To positively influence adrenaline levels:

  • Exercise: strength training and high intensity interval training (H.I.I.T.) cause a much quicker adrenaline release than cardio exercise. That’s because cardio requires slower energy release than strength training or H.I.I.T.
  • Don’t over-exercise: exercise is a stressor. Over-exercising can contribute to adrenaline resistance so it’s essential to pay attention to your body and constantly evaluate how you feel. The right amount of exercise will vary from person to person.
  • Manage your stress: even just 10 minutes a day of quiet breathing or meditation can do wonders for your adrenals.

8. Human Growth Hormone (HGH) – The Anti-Aging Hormone

Human Growth Hormone (HGH) is a naturally occurring hormone made by the anterior pituitary gland located at the base of the brain. Its primary function is to fuel growth and development. Its levels peak in adolescence and dramatically decline with aging.

In the fitness community, HGH is one of the most frequently cited hormones for weight loss. It’s associated with reduced body fat, increased muscle mass and strength, and tissue-repairing effects on the musculo-skeletal system. That’s why many people turn to HGH injections and supplements to “reverse” the HGH decline that comes with aging. However, acquiring HGH requires a prescription from a medical doctor and its use comes with possible side effects.

That being said, there are many things you can do to achieve HGH optimal levels without supplements.

To positively influence HGH levels:

  • Get enough high-quality sleep: sleep promotes the release of HGH (15), which is one of the reasons sleep helps us recover and repair.
  • Exercise: the major factors that determine how much HGH increases are load and frequency. Lifting heavier loads at a greater frequency (less rest time) leads to greater amounts of HGH. Endurance training, on the other hand, can cause a decreased effect on HGH release. (16)
  • Try intermittent fasting: fasting can raise HGH by 1,300% in women and 2,000% in men! (17) It’s important to note that intermittent fasting is more about the timing of meals rather than starving yourself for several days in a row. The most common is abstaining from food for 14-18 hours.
  • Eat enough protein: protein helps support healthy levels of HGH.

9. Testosterone – The Male Hormone

Testosterone is a hormone secreted by the testicles of males, and to a lesser extent by the ovaries of females and adrenal glands of both sexes. It’s primarily responsible for the proper development of male reproductive tissues as well as secondary sexual characteristics. Although testosterone is crucial for both men and women, it affects men differently than women.

Testosterone in Men

Testosterone is wonderful for fat loss (not just weight loss). The higher the testosterone levels, the great the fat loss. Testosterone inhibits the creation of fat cells and prevents obesity. Contrary to popular belief, testosterone is unlikely to significantly affect muscle growth.  The bad news for men considering dieting is that restricting calories lowers testosterone levels because muscle mass helps keep testosterone levels in a normal range.

Testosterone in Women

The story of testosterone and weight loss for women is a little more complex. Weight loss for women comes down to the correct ratio of estrogen, progesterone, and testosterone rather than just high levels of testosterone. It’s true that without testosterone women could exercise all day and never gain muscle mass. However, too high levels of testosterone come with negative effects including PCOS, deepening of the voice, facial hairiness, loss of hair on the head, acne, and irregular menstruation.

To positively influence testosterone levels:

  • Exercise: resistance training, such as weight lifting, is the best type of exercise to boost testosterone. (18) High intensity interval training (H.I.I.T.) is also very effective. (19)
  • Get plenty of restful, high-quality sleep: according to a few studies, for every additional hour of sleep you get, testosterone levels rise on average 15% higher. (20)
  • Eat the right fats: your body synthesizes testosterone from the fats you eat. The best type of fats for this purpose is monounsaturated and saturated fats. Think avocados, almonds, olive oil, and whole eggs. (21)
  • Have a healthy sex life: sex plays an important role in regulating testosterone levels. (22)

10. Estrogen – The Female Hormone

Estrogen is a hormone produced primarily in the ovaries. Adrenal glands and fat tissue also make some estrogen, which is why men have small amount of estrogen as well. Estrogen is responsible for the development and regulation of the female reproductive system and secondary sex characteristics.

There are actually three forms of estrogen: estradiol, estriol, and estrone. Most often, when people talk about “estrogen”, they really mean estradiol, which is the most potent out of the three.

Estrogen acts as both a fat-storing and fat-burning hormone. It increases the number of receptors that slow fat release in the lower body of women, contributing to the pear-shape fat distribution in women. It has the opposite effect on abdominal fat, which is why menopausal women (who don’t produce as much estrogen anymore) see their fat distribution switch to the abdominal area.

An excessive estrogen/progesterone ratio is more generally involved in weight gain than either a simple excess of estrogen or a deficiency of progesterone. Estrogen and progesterone work in synchronization with each other as checks and balances to achieve hormonal balance. With the gradual drop in estrogen but severe drop in progesterone during the perimenopausal and menopausal years, there’s insufficient progesterone to counteract the amount of estrogen in the body. This state is called estrogen dominance, which is one of the main causes of weight gain in menopausal women.

It’s a catch-22 because excessive estrogen turns more calories into fat and fat cells make more estrogen. Your body has an enzyme that turns adrenal steroids to estrogen. Therefore, the higher the fat intake, the higher the conversion of fat to estrogen.

To positively influence estrogen levels:

  • Decrease exposure to estrogens in the diet. This includes:
    • Soy, which contains high levels of phytoestrogens.
    • Non-organic produce, which is sprayed with pesticides, herbicides, and fungicides. (23)
    • Conventionally raised farm animal protein, which is overloaded with steroids, antibiotics, and toxins from their feed and the way they’ve been raised. When you eat this type of meat or dairy products, those substances are released into your body and can behave like estrogen. (24)
  • Beware of environmental estrogens through plastics, canned food, household products, and skincare. (25, 26) Although the amount may seem insignificant in each, the additive effect from years of chronic exposure can lead to estrogen dominance.
  • Increase the intake of insoluble fiber, cruciferous vegetables (broccoli, cabbage, cauliflower, collard greens, and kale), water, and green tea to help your body naturally detoxify excess estrogen. (27, 28)
  • Limit caffeine and coffee intake: both caffeine and coffee increase estrogen levels. (29)
  • Eat more plants (plant-based diet): plants contain over 5,000 known sterols that have progestogenic effects.

11. Progesterone – The Feel Good Hormone

Progesterone is a hormone produced mainly in the ovaries, but also in smaller amounts by the adrenal glands in both sexes and by testes in males. It’s the raw material (precursor) from which your body makes several hormones, including cortisol, estrogen, and testosterone. It’s also the primary hormone of fertility and pregnancy.

Progesterone plays an important role in weight loss in several ways. First, it encourages the use of fat for energy, thus opposing the estrogenic tendency to store fat. Second, it balances blood sugar levels by enhancing the sensitivity of the body to insulin and reducing the amount of insulin circulating in the blood. This is very important for managing sugar cravings. Third, progesterone acts as a natural diuretic due to its ability to increase sodium loss in the urine. Studies investigating this effect have shown the ability of progesterone to ease symptoms of water retention, including swelling, bloating, and “false fat”. (30)

An important fact is that weight loss actually increases progesterone levels. The more body fat you have, the more estrogen you tend to produce, but not progesterone. Therefore, if you’re overweight, your progesterone levels can’t offset the amounts of estrogen in your body.

To positively influence progesterone levels:

  • Manage stress: stress decreases the production of progesterone. In simple terms, the adrenal glands “steal” the precursor to progesterone and instead use it to produce cortisol, the stress hormone. (31)
  • Decrease estrogen levels: estrogen counteracts progesterone. High levels of estrogen throw off the delicate estrogen/progesterone balance.
  • Increase your vitamin and mineral intake:
    • Vitamin B6 helps maintain optimal levels of progesterone. It’s also necessary for the liver to break down estrogen. One study has shown that taking vitamin B6 at doses of 200-800 mg/day can reduce blood estrogen levels and increase progesterone levels. (32) Good sources of vitamin B6 include sunflower seeds, walnuts, whole grains, bananas, spinach, beans, potatoes, and fortified cereals.
    • Vitamin C increases progesterone levels. Taking 750 mg/day can increase progesterone levels by 77%. (33) Good sources of vitamin C are raw fruits and vegetables, particularly bell peppers, broccoli, oranges, kiwi, or kale.
    • Magnesium breaks down excess estrogen. Good sources of magnesium include black beans, spinach, raw plantain, halibut, whole grain cereals, pumpkin and squash seeds, okra, and nuts.
    • Zinc prompts the pituitary gland to release follicle stimulating hormones, which in turn promotes ovulation and stimulates the ovaries to produce progesterone. Good sources of zinc include shellfish, pumpkin, squash seeds, dark chocolate, wheat germ, and chickpeas.
    • Amino Acid L-Arginine increases blood flow to the corpus luteum, which produces progesterone. One study showed that 71% of women who took 6 g/day of L-arginine had improved progesterone levels. (34)

12. Adiponectin – The Starvation Hormone

Adiponectin is a hormone produced by the fat cells. Its main function is to increase the rate at which the body breaks down fat, boost metabolism, enhance muscle’s ability to use carbohydrates for energy, and regulate glucose levels.

The more adiponectin you have, the more fat you burn. Also, the more fat you carry around, the harder it is for your body to produce adiponectin. So as you lose weight and your percentage of body fat decreases, your adiponectin levels rise, helping you to lose even more fat and maintain the loss more easily than before.

When adiponectin levels are low, the body stores excess fat in fat cells to protect against possible starvation during difficult times… hence its nickname, the “starvation hormone”.

To positively influence adiponectin levels:

  • Take fish oil daily: daily intake of fish oil or omega-3 supplementation can increase adiponectin levels by 14-60%. (35)
  • Exercise: both low and high intensity exercise have a positive impact on adiponectin levels. However, shorter duration higher intensity exercise is perhaps the most effective form of exercise as it has a more pronounced impact on metabolism and other hormones. When combined with a healthy diet, exercise can boost adiponectin levels by 18-48%. (35, 36)
  • Increase fiber intake: fiber can increase adiponectin by up to 60-115%. (35)
  • Add monounsaturated fats into your diet: common sources include avocados, nuts, and olive oil.

13. Incretins – The Food Tasters

There are two incretin hormones: Glucagon-Like Peptide (GLP-1) and Glucose Dependent Insulinotropic Peptide (GIP). Both hormones are released from the small intestine in response to a meal to help the body sense the macronutrient ratio of the incoming food. Protein and fiber stimulate release of more GLP-1 relative to GIP; fat and sugar stimulate release of more GIP relative to GLP-1.

High levels of GLP-1 result in prolonged periods of decreased desire to eat (increased satiety) and lessened sensations of hunger by inhibiting gastric emptying and stimulating the satiety center in the hypothalamus.

GIP, on the other hand, promotes fat storage and is a contributing factor in the development of obesity.

To positively influence GLP-1 and GIP levels:

  • Diet is the single most important thing when it comes to increasing GLP-1:
    • Eat lots of leafy greens. (37)
    • Eat plenty of protein. (38)
    • Include probiotics. (39)
  • Avoid sugary and fatty foods, which release GIP. (40)

14. Cholecystokinin (CKK) – The Digestion Hormone

Cholecystokinin (CKK) is a hormone produced by the cells in the small intestine and by some neurons in the brain in response to a meal.

CCK improves digestion by slowing down the emptying of food from the stomach and by stimulating the production of bile and enzymes to help break down food. CCK also reduces appetite by increasing a short-term sensation of fullness (that is, during a meal rather than between meals).

To positively influence CKK levels:

  • Increase fat intake: fat stimulates the CCK release. (41)
  • Load up on fiber: fiber prolongs CCK release by slowing the rate of fat absorption. (41)
  • Eat protein at every meal: protein is the most satiating of all macronutrients and directly stimulates CCK production. (42)

15. Peptide Tyrosine Tyrosine (PYY) – The Appetite-Reducing Hormone

Peptide Tyrosine Tyrosine (PYY) is a hormone produced by the cells of the intestine and colon. It’s released after a meal and remains elevated for about 1 hour after eating.

PYY suppresses appetite, increases energy expenditure, and reduces the motivation to seek high-fat food. PYY works by slowing down gastric emptying and absorption of nutrients. Because of its effects, PYY plays a major role in decreasing the risk of obesity.

To positively influence PYY levels:

  • Eat plenty of protein: high-protein meals increase PYY levels. (43)
  • Limit your consumption of carbohydrates: carbohydrates tend to decrease PYY levels. (43)
  • Increase your fiber intake: fiber increases PYY levels. (44)

16. Neuropeptide Y (NPY) – The Fat-Amplifying Hormone

Neuropeptide Y (NPY) is a hormone produced and secreted by neurons in the sympathetic nervous system and in various areas of the brain, particularly the hypothalamus.

NPY increases appetite, with a preference for carbohydrates, and promotes fat storage. Its levels rise under conditions of stress and during times of severely restricted calorie intake.

To positively influence NPY levels:

  • Reduce your stress to the minimum: try stress management techniques that relax your body and mind, such as hot baths, massage, meditation, yoga, or walks in the nature.
  • Eat enough protein: low-protein diets increase NPY. (45)
  • Don’t fast or drastically diet. (46, 47)

17. Irisin – The Exercise Hormone

Irisin is a recently discovered hormone that is produced by the skeletal muscles. It helps the body burn calories by increasing metabolism and converting white fat (which is used to store excess calories) to brown fat (which is used to burn calories to generate heat).

To positively influence Irisin levels:

  • Exercise: levels of irisin rise during exercise and in some cases, even cold weather. Researcher still need to find out what kind of exercise raises irisin, but data suggest that high intensity training with weights is particularly effective. (48) However, cardio also helps secrete this hormone.