Hormones run on feedback loops and clocks, which is why one test can mislead

Key takeaways

• In 1902, William Bayliss and Ernest Starling injected an extract of a dog’s intestinal lining into a vein and watched the pancreas respond; Starling introduced the word “hormone” in 1905.
• Hormones reach almost every tissue through the blood, but only cells with a matching receptor respond, and feedback loops pull levels back toward a set range.
• Hormones keep time: insulin comes out in pulses every 5 to 10 minutes, testosterone runs highest in the early morning, and one large study suggests some hormones shift with the seasons.
• Binding proteins, pregnancy, birth control pills, illness and medicines can all move a hormone result, so a useful test starts with a clear question and the right time of day.
• At least 1% of people take steroid medicines long term, which can quiet the body’s own adrenal glands, one reason hormone medicines come with follow-up.

On a single afternoon in 1902, two London physiologists ran an experiment that rewrote how doctors understood the body. William Bayliss and his brother-in-law, Ernest Starling, were working on an anesthetized dog¹².

They tied off a loop of intestine and cut away its nerves, so it was linked to the body only by blood vessels. When they put weak acid into the loop, the pancreas still released digestive juice².

Then they took a piece of intestine, rubbed its lining with sand and weak acid, filtered the mix and injected the liquid into a vein. Within moments, the pancreas poured out more juice than before².

No nerve could explain that. A chemical had carried the order through the blood. They named it secretin¹².

The finding challenged Ivan Pavlov, the Russian scientist who believed nerves ran digestion. Pavlov had students repeat the work and reportedly agreed they were right¹.

In June 1905, in lectures at the Royal College of Physicians, Starling gave these messengers a name: hormones, from a Greek word meaning to set in motion¹².

Today, “balance your hormones” sells supplements, saliva kits and clinic plans, and our article on those claims sorts them out. This one is the owner’s manual: what hormones are, how glands talk, why levels rise and fall on schedules, and what that means when you get tested.

Hormones travel everywhere, but only some cells are listening

A hormone is a chemical messenger released into the blood³⁴. The Endocrine Society, a professional group of hormone specialists, says hormones work slowly, over time, shaping growth, metabolism, sexual function, reproduction and mood³.

Nerves carry their signals as impulses along nerve cells. Hormone effects last longer and spread more widely, according to the National Cancer Institute’s anatomy course⁵. The two systems also work together: some glands release their hormones when a nerve tells them to⁴.

A hormone drifting past a cell does nothing unless that cell has a receptor, a docking site that fits it. Cells with the right receptor are called target cells. They can sit in a single organ or be scattered across the body⁴.

Hormones come in two broad families. Most are proteins or built from proteins. They latch onto receptors on a cell’s surface and act fairly quickly⁴. Steroid hormones, such as the sex hormones and the hormones of the outer adrenal gland, slip inside the cell and change which proteins it makes, so they act more slowly⁴.

Tiny amounts do big jobs, so the body keeps tight control over how much it releases⁴. And hormones don’t come only from classic glands. Secretin itself comes from the lining of the small intestine¹.

The hormone map: who makes what, and what can skew the test

Use this map to see how the major glands split the work. If you’re about to be tested, the last column matters most.

Gland	Main hormones	Main jobs	Common test	What can throw off the result
Hypothalamus (base of the brain)	Releasing hormones such as GnRH	Tells the pituitary when to act315	Doctors test the pituitary hormones it controls, such as LH and FSH15	Its signals come in bursts, not a steady stream6
Pituitary (the “master gland”)	TSH, LH, FSH and others	Directs the thyroid, adrenals, ovaries and testes3915	TSH is usually checked alongside thyroid hormone9	Read it together with the hormone of the gland it controls9
Thyroid	T4 and T3	Sets the pace of metabolism3	TSH plus free T49	Pregnancy, birth control pills, steroids and severe illness89
Adrenals	Cortisol and aldosterone	Steroid hormones, plus raw material the body turns into sex hormones3	Morning blood cortisol14	Time of day; steroid pills, inhalers, creams or joint shots14
Pancreas	Insulin and glucagon	Help turn food into energy for cells3	Blood sugar tests (see our blood glucose guide)	Insulin itself comes out in pulses every 5 to 10 minutes6
Ovaries	Estrogens and progesterone	Puberty, periods, fertility and menopause3	Estradiol, FSH and LH11	Near menopause, tests may need repeating while periods still come11
Testes	Testosterone	Puberty, sperm, bone and muscle37	Total testosterone, drawn between 7 and 10 a.m.7	Time of day, age and some medicines7

The parathyroid glands make a hormone that controls calcium³. Several rows of the map are links in one chain, which is where feedback comes in.

Feedback loops are the body’s own thermostat

Most hormones are controlled by negative feedback. A gland senses the level of what it regulates and pushes the other way when that level drifts⁴.

The thyroid shows how it works. The pituitary makes thyroid-stimulating hormone, or TSH, which tells the thyroid how much hormone to release. When thyroid hormone runs low, TSH climbs. When thyroid hormone runs high, the pituitary eases off⁹.

That’s why a doctor reads TSH and thyroid hormone together. Our article on thyroid function explains what each pattern means.

Cortisol runs on a similar chain from the brain to the adrenal glands³. Steroid medicines plug into that chain.

A joint 2024 guideline from the Endocrine Society and the European Society of Endocrinology says the risk of adrenal suppression starts with steroid pills taken for about three to four weeks or longer, at more than the equivalent of 15 to 25 milligrams of hydrocortisone a day¹⁴. Sensing plenty of cortisol-like signal, the body turns its own production down⁴¹⁴. Our article on the stress response covers cortisol’s normal clock.

The reproductive chain adds a twist. The hypothalamus sends GnRH to the pituitary in pulses, and the pulse speed matters. Faster pulses favor LH, and slower pulses favor FSH⁶. Drugs that mimic GnRH can end up suppressing LH and FSH, and doctors use them to pause puberty that starts too early¹⁵.

If you value the body’s ability to regulate itself, feedback loops are the strongest proof of it. Respecting that system means two things. Don’t override it with products that contain real hormones, and don’t mistake its normal swings for disease.

Hormones keep time by the minute, the day and the season

Hormone levels aren’t meant to hold still. They move on several clocks at once.

By the minute and the hour. Your pancreas releases insulin in pulses every 5 to 10 minutes⁶. LH, which drives the ovaries and testes, comes in pulses roughly once an hour⁶. Researchers have found that pulse size and speed can change in disease. Insulin pulses shrink in early type 2 diabetes, and persistently fast GnRH pulses are linked to polycystic ovary syndrome⁶.

By the day. Testosterone is typically highest between 7 and 10 a.m.⁷. Cortisol also follows a daily curve, and our stress-response article explains it. Melatonin has its own guide in our sleep series.

By the month. The menstrual cycle layers a monthly rhythm on top. Our upcoming article on the cycle walks through its phases.

By the season. In a 2021 study in the Proceedings of the National Academy of Sciences, Israeli researchers analyzed hormone tests from health records covering 46 million person-years¹⁰. Hormones from the target glands, such as the thyroid and adrenals, peaked in winter and spring. Most of the pituitary hormones that drive them peaked months later, in summer¹⁰.

Known delays in these hormone chains last hours, not months. So the team proposed something surprising: the glands themselves grow over months under the pull of the hormones that drive them, which could set up a yearly cycle¹⁰. The data came from one country, so the idea needs testing elsewhere.

Why timing, binding proteins and lab ranges change your result

Most hormone in the blood isn’t free to act. Testosterone, for example, is mostly bound to carrier proteins such as SHBG and albumin. Bound hormone is harder for the body to use⁷.

A total testosterone test counts both kinds. Free testosterone tests measure only the unbound part and are less common⁷.

Thyroid hormone works the same way. A protein called thyroxine-binding globulin (TBG) carries it, and TBG rises in pregnancy and with estrogen, including birth control pills⁸. Steroids, testosterone, a serious illness or surgery can lower it⁸.

When TBG shifts, total T4 and free T4 can tell different stories⁸. Medical experts consider the free T4 test the more accurate of the two, and it’s the one used more often⁹.

Then there’s the reference range. MedlinePlus, the National Library of Medicine’s consumer site, explains that each range is built from test results in big groups of healthy people. A healthy person commonly lands outside a range now and then¹². Labs use different methods, so compare a result only with your own lab’s range, and stick with one lab if you’re tracking a trend¹².

Put those facts together and you can see the problem with a broad “hormone panel” ordered without a clear question. Each number swings with the clock and the season, and some have to be read in pairs. In a healthy person, one or two may still fall outside the range¹². Our articles on “balance your hormones” claims and on out-of-range results explain where that can lead.

Hormone problems are common, and outside forces push on the system

The Endocrine Society says diseases can develop when glands make too much or too little hormone³. One of them is very common.

CDC estimates that 40.1 million Americans, or 12.0% of the population, had diabetes in 2023¹³. About 11.0 million adults with diabetes didn’t know they had it. That’s 27.6% of adults with the disease, more than 1 in 4¹³.

Other hormone problems are rarer but real. Early puberty affects no more than about 1 in 100 people in the U.S., according to the National Institute of Child Health and Human Development (NICHD)¹⁵. Our article on thyroid function covers how common thyroid disease is.

Plenty of everyday forces push on hormone levels, too. The Endocrine Society lists puberty, aging, pregnancy, genetics, the environment, some diseases and medicines, naming herbal supplements, prescription opioids and steroids³. Malnutrition and a sudden illness can lower the protein that carries thyroid hormone⁸.

Sleep matters as well. Our article on sleep and metabolism shows how short nights shift appetite hormones. Chemicals that may interfere with hormones get their own article, which explains what scientists mean by “endocrine disruptor.”

Hormones are also medicines, so they come with follow-up

Several common drugs are hormones or close copies of them. They include insulin, levothyroxine for an underactive thyroid, steroid medicines such as prednisone, birth control pills and testosterone. Because hormones are potent and live inside feedback loops, these drugs need monitoring.

Steroid medicines. At least 1% of the population uses steroid medicines long term¹⁴. The 2024 guideline says prescribers should teach patients about their effects on the adrenal glands¹⁴. It also flags high-dose or long-term inhaled or skin steroids, and recent joint injections, as reasons to watch for adrenal effects¹⁴. Doctors check whether the adrenals have recovered with a morning blood cortisol test¹⁴.

Thyroid medicine. Changes in the carrier protein, such as those in pregnancy or with estrogen, can change the right levothyroxine dose⁸. That’s a reason to ask about rechecking thyroid levels at those times.

Insulin. Our article on insulin’s history covers how the first injections changed diabetes care.

Testosterone. Some medicines can affect the test, and levels normally fall with age⁷. Our upcoming testosterone article covers what treatment involves.

One rule applies to all of them. Don’t stop or change a hormone medicine on your own; MedlinePlus advises against stopping any medicine unless your provider says so⁷⁹. Bring your questions to the clinician who prescribed it.

How strong is the evidence?

Claim	Rating	Why
Hormones act only on cells with matching receptors, kept in range by feedback	Strong evidence	Core physiology, taught in NCI’s anatomy course4
Timing matters for some hormone tests	Strong evidence	Testosterone is drawn early in the morning; adrenal recovery is checked with morning cortisol714
Free T4 is more reliable when carrier proteins shift	Strong evidence	TBG changes with pregnancy, estrogen and illness; experts favor free T489
Long-term steroid medicines can quiet your own adrenal glands	Strong evidence	2024 Endocrine Society and European guideline14
Hormone levels shift with the seasons	Promising but early	One large Israeli records study, not yet confirmed elsewhere10
A broad hormone panel with no clear question helps you	Not enough evidence	Healthy people often land outside reference ranges, and levels swing with time712

When to talk to a clinician

• A result surprised you. Ask whether the time of day, a carrier-protein shift, a recent illness or a medicine could explain it before acting on it⁸⁹¹².
• You take steroid pills, or long-term inhaled or skin steroids, and have surgery or a procedure coming. Ask about adrenal effects; the 2024 guideline calls for extra steroid cover around major stress in some patients¹⁴.
• You take levothyroxine and are pregnant or starting or stopping estrogen. Ask whether your dose needs checking⁸.
• A child shows signs of puberty before age 8 in girls or 9 in boys. That’s NICHD’s definition of early puberty, which a pediatrician can evaluate¹⁵.
• Adrenal crisis is an emergency. The 2024 guideline says it needs injected steroids and fluids¹⁴. Our article on hormone claims lists the warning signs; get emergency care, by calling 911, if they appear.

Questions readers ask

The bottom line

Hormones are the body’s slow, far-reaching messengers. They act only where a receptor waits, and feedback loops keep them in range. They also pulse and cycle, and may even drift with the seasons. That’s normal.

That’s why a good hormone test starts with a question, a time of day and the right measurement, free or total. When hormones are used as medicines, the same feedback loops are the reason for follow-up tests and for never stopping a prescription on your own.

Related reading

• Real hormone problems have names and tests, which “balance your hormones” claims lack
• The stress response saves you in minutes, and viral cortisol fixes misread it
• Why being out of range often means nothing at all
• What scientists mean by endocrine disruptor and what labels mean
• Babies go through a mini-puberty, and hormones keep shifting for the rest of your life

Sources

1. Markel H. “A Hormonal Happy Birthday.” PBS NewsHour, June 20, 2013. pbs.org
2. Pillai C. “One Hundred Years of Secretin, The First Hormone Discovered.” Indian Journal of Physiology and Pharmacology, 2002;46(4). ijpp.com
3. Endocrine Society. “Hormones and Endocrine Function.” Endocrine Library, January 24, 2022. endocrine.org
4. National Cancer Institute, SEER Training Modules. “Characteristics of Hormones.” Accessed October 2026. training.seer.cancer.gov
5. National Cancer Institute, SEER Training Modules. “Review: Introduction to the Endocrine System.” Accessed October 2026. training.seer.cancer.gov
6. Nunemaker CS, Satin LS. “Episodic hormone secretion: a comparison of the basis of pulsatile secretion of insulin and GnRH.” Endocrine, 2014;47(1):49–63. pmc.ncbi.nlm.nih.gov
7. MedlinePlus, National Library of Medicine. “Testosterone Levels Test.” Updated June 5, 2025. medlineplus.gov
8. MedlinePlus Medical Encyclopedia, National Library of Medicine. “TBG blood test.” Reviewed January 25, 2026. medlineplus.gov
9. MedlinePlus, National Library of Medicine. “Thyroxine (T4) Test.” Updated December 12, 2024. medlineplus.gov
10. Tendler A, Bar A, Mendelsohn-Cohen N, et al. “Hormone seasonality in medical records suggests circannual endocrine circuits.” Proceedings of the National Academy of Sciences, 2021;118(7):e2003926118 (abstract via University of Haifa research portal). pubmed.ncbi.nlm.nih.gov
11. MedlinePlus Medical Encyclopedia, National Library of Medicine. “Menopause.” Reviewed July 3, 2025. medlineplus.gov
12. MedlinePlus, National Library of Medicine. “How to Understand Your Lab Results.” Updated September 4, 2025. medlineplus.gov
13. Centers for Disease Control and Prevention. “National Diabetes Statistics Report” (at a glance). Updated September 16, 2026. cdc.gov
14. Beuschlein F, Else T, Bancos I, et al. “Diagnosis and therapy of glucocorticoid-induced adrenal insufficiency.” European Society of Endocrinology and Endocrine Society joint clinical guideline, Journal of Clinical Endocrinology & Metabolism, 2024 (guideline summary page). endocrine.org
15. Eunice Kennedy Shriver National Institute of Child Health and Human Development. “Puberty and Precocious Puberty.” NIH, accessed October 2026. nichd.nih.gov