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synergy · 1
High urate and the cluster of problems in metabolic syndrome (high blood glucose, abnormal blood lipids, high blood pressure, abdominal obesity, fatty liver) often occur together; each is a risk factor for the other, and the mechanisms run in both directions rather than one simply causing the other.
cofactor · 2
Iodine is the raw material of thyroid hormone: the thyroid builds it into T4 (four iodine atoms) and T3 (three). In other tissues a selenium-dependent deiodinase removes one iodine from T4 to make active T3. Too little iodine means too little hormone, and the pituitary pushes thyroid-stimulating hormone (TSH) up.
In pregnancy, placental hormones make the body less responsive to insulin, while contracting muscle can move glucose into cells without insulin. In a randomized trial (Wang 2017), 300 overweight or obese pregnant women started before week 13 and cycled on a stationary bike 3 times a week for at least 30 minutes; gestational diabetes occurred in 22.0%, against 40.6% in the control group. It is a single trial and needs confirming.
regulates · 19
Hashimoto's thyroiditis is an autoimmune disease in which the immune system attacks the thyroid, and one of the most common causes of an underactive thyroid; it plays out along the hormone axis of hypothalamus, pituitary and thyroid. Seen along that axis, it becomes clear why thyroid-stimulating hormone (TSH) is the test to watch, and why levothyroxine is taken on an empty stomach and kept apart from acid-suppressing drugs and iron supplements.
The kidney is also an endocrine organ: it releases erythropoietin (EPO), which directs red blood cell production, converts vitamin D into its active form, and releases renin, which starts the renin-angiotensin-aldosterone system that regulates blood pressure. When kidney function falls, anemia, bone disease and blood pressure problems often appear together.
The reproductive system is governed by a hormone axis of hypothalamus, pituitary and gonads: the hypothalamus sends a signal, the pituitary releases gonadotropins, the ovaries or testes release estrogen or testosterone, and those hormones in turn hold back the upstream signals. Polycystic ovary syndrome (PCOS) is a classic case in which this axis, insulin resistance and excess androgens are tangled together.
The hypothalamus connects the nervous system to the endocrine system: the three hormone axes for the stress response (cortisol), the thyroid and reproduction all take their orders from here. So long-term stress may pull on several hormone axes at once through the hypothalamus.
→Bone
The breakdown and building of bone is directed by several hormones: when blood calcium falls, parathyroid hormone draws calcium out of bone, and calcitonin works the other way; estrogen protects bone density; and cortisol that stays too high for a long time (including long-term glucocorticoid drugs) causes secondary osteoporosis.
The gut releases many hormones: GLP-1, released after eating, promotes insulin release and curbs appetite; GIP also promotes insulin release; and ghrelin from the stomach makes you feel hungry when it is empty. The way gut hormones amplify the insulin response after a meal (the incretin effect) is the physiological basis of GLP-1 receptor agonist drugs.
Muscle also releases hormone-like signaling molecules (myokines); IL-6 released during exercise, for example, prompts the body to make anti-inflammatory molecules. This is thought to be one reason exercise benefits the whole body, but the effects of most myokines come from animal and mechanism studies, and the role of one of them, irisin, in people is still unsettled.
When blood glucose rises, the pancreas's beta cells release insulin, the liver and muscles store glucose as glycogen, and fat tissue stores fat. When blood glucose falls, alpha cells release glucagon, the liver breaks down glycogen and starts gluconeogenesis, and fat is mobilized. These two hormones switch the body back and forth between storing and releasing.
←GERD
More abdominal fat raises the pressure inside the belly and pushes up on the junction between the stomach and esophagus, making reflux easier. In a study of more than 10,000 women, the higher the BMI, the more women had reflux symptoms every week, and even in normal-weight women a rise in BMI went with more symptoms. That is an observed association; how much weight loss relieves symptoms is a question for intervention studies.
Insulin works like a key that lets glucose into cells, and insulin resistance means the cells respond to that key less and less. The body makes more and more insulin to compensate, until the pancreas's beta cells can no longer keep up; only then does blood glucose really rise, and type 2 diabetes appears.
In the liver, fructose goes through the enzyme ketohexokinase (KHK), bypassing a brake in glucose metabolism, and when it arrives in large amounts it is turned into fat; as fat builds up in the liver and muscle, insulin works less well. In a 10-week trial, overweight adults drinking fructose-sweetened drinks gained visceral fat and lost insulin sensitivity.
Mechanism · An enzyme without a brake
When glucose enters a cell, phosphofructokinase (PFK) acts as a brake: once the cell has enough energy, glycolysis slows down. Fructose goes through a different enzyme, ketohexokinase (KHK), which has no such feedback: whatever arrives, the liver processes, and the extra carbon is turned directly into fat (de novo lipogenesis). As fat builds up in the liver and muscle, cells respond less to insulin. Insulin resistance has many starting points, and a large, fast flow of fructose into the liver can be one of them.
Evidence · Fructose versus glucose drinks
Stanhope 2009 randomized overweight or obese adults aged 40–72 to drinks sweetened with fructose or with glucose, supplying 25% of their daily energy needs, for 10 weeks. Weight gain was similar in both groups, but only the fructose group gained visceral fat, made more fat in the liver, and saw after-meal triglycerides and several blood lipid measures rise and insulin sensitivity fall. The Tappy 2010 review gives a more cautious overall picture: in rodents, high-fructose feeding causes insulin resistance and obesity; in humans the evidence is weaker, but high intakes cause abnormal blood lipids and impair the liver's insulin sensitivity, and there is no clear evidence that moderate amounts of fructose are directly harmful.
In practice · Which fructose to cut
The fructose in whole fruit is wrapped in fiber and cell structure, so it reaches the liver at a completely different speed. What matters here is the liquid, fast-arriving share, such as sugary drinks and juice, not the apple. Insulin resistance also has many other routes, including visceral fat, poor sleep and sitting too much; fructose is one of them, not the only switch. Clearing the extra fat from the liver can restore insulin's effect, and none of this means giving up fruit.
Inflammatory signals such as IL-6 suppress the thyroid hormone axis, so in serious illness blood T3 can run low while the thyroid itself is healthy (euthyroid sick syndrome); they also stimulate the stress axis and raise cortisol. Chronic inflammation and hormone disturbance are often bound up with each other.
The liver plays several endocrine roles: it converts thyroxine (T4) into the more active T3, makes most of the body's IGF-1 on orders from growth hormone, and is one of the main organs insulin acts on. So obesity, fatty liver and hormone disturbances are often several sides of the same metabolic problem.
←Bone
In mice, undercarboxylated osteocalcin released by bone-building cells enters the blood and promotes insulin release and insulin sensitivity, which is why bone is regarded as an endocrine organ. How much this pathway matters in people is still unsettled.
Skin is not only where vitamin D is made; it can also make and convert cortisol and sex hormones locally. Some of the eczema, hair loss and acne that appear under long-term stress may involve the skin's local stress-hormone pathway and the enzyme aromatase; these mechanisms come mainly from laboratory research.
The brief rise in growth hormone, IGF-1 and testosterone after training contributes little to muscle growth. Long-term hormone levels matter somewhat more, but muscle growth depends mainly on the tension of training itself and on eating enough protein. So claims that a workout sharply boosts testosterone mean little for building muscle.
High-intensity interval training (HIIT) briefly drives up hormones such as growth hormone and adrenaline during a session, but these short peaks are thought to contribute little to long-term adaptation. HIIT's main gains are in the heart, lungs and mitochondria: in the controlled trials pooled by Milanović 2015, HIIT raised VO2max about as much as continuous endurance training, possibly slightly more.
After puberty, men's blood testosterone is more than 15 times women's (Handelsman 2018), the main reason women build less muscle in absolute terms and do not easily turn bulky. The menstrual cycle is often said to make women stronger in the follicular phase and weaker in the luteal phase, but pooled across many people the difference is small, so adjusting to how you feel is enough.
DIM affects how the liver processes estrogen that has already been used: it shifts the mix of estrogen breakdown products toward the milder 2-hydroxyestrone. In a 12-month trial in women taking tamoxifen, that ratio did rise, but breast density did not change; and the active metabolites of tamoxifen were lower in the DIM group, and whether that weakens the drug is not known.