How a Pea-Sized Gland Under the Brain Controls So Many Others

Nerves send fast, precise signals along fixed paths. Hormones work differently: glands release chemical messengers into the blood, and those messages reach every organ but act only on cells that carry the right receptors.

This lesson covers the endocrine glands with the hypothalamus and pituitary, the hormones of other glands, how hormones act, and hormonal disorders.

The major endocrine glands are the hypothalamus, pituitary and pineal in the brain, the thyroid and parathyroids in the neck, the thymus in the chest, the adrenals above the kidneys, the pancreas near the stomach, and the gonads; the hypothalamus controls the pituitary, which in turn controls several other glands.

The hypothalamus:

- Lies at the base of the forebrain
- Its neurosecretory cells make releasing hormones, such as GnRH, which stimulate the pituitary, and inhibiting hormones, such as somatostatin, which suppress it

The pituitary:

- Anterior pituitary (adenohypophysis) — secretes GH for growth, PRL for milk production, TSH for the thyroid, ACTH for the adrenal cortex, LH and FSH for the gonads, and MSH for skin pigmentation
- Posterior pituitary (neurohypophysis) — stores and releases oxytocin and vasopressin (ADH), which are made in the hypothalamus

An everyday example. A child whose height falls far behind at a paediatric clinic may be tested for growth hormone, because too little of it in childhood slows growth.

The substance. The posterior pituitary makes no hormones of its own — it only stores and releases the oxytocin and ADH produced by hypothalamic neurons.

Each gland secretes hormones with specific jobs — controlling metabolism, calcium balance, stress responses, blood sugar, digestion and reproduction.

Thyroid:

- Thyroxine (T4) and triiodothyronine (T3) — regulate the basal metabolic rate, support red blood cell formation and control carbohydrate, protein and fat metabolism
- Thyrocalcitonin — lowers blood calcium

Parathyroid. Parathyroid hormone (PTH) raises blood calcium by releasing calcium from bone and increasing its reabsorption in the kidneys.

Adrenal:

- Medulla — adrenaline and noradrenaline, the emergency hormones that raise heart rate, breathing rate and blood glucose
- Cortex — glucocorticoids such as cortisol, which raise blood glucose and suppress immune responses, and mineralocorticoids such as aldosterone, which retain sodium and water

Pancreas (islets of Langerhans):

- Insulin from beta cells — lowers blood glucose by helping cells take it up and store it as glycogen
- Glucagon from alpha cells — raises blood glucose by breaking down glycogen

Gastrointestinal tract. Gastrin stimulates gastric juice; secretin stimulates water and bicarbonate from the pancreas; CCK stimulates pancreatic enzymes and bile; GIP inhibits gastric secretion.

Gonads:

- Testes — androgens such as testosterone, controlling sperm formation and male secondary sexual characters
- Ovaries — oestrogen for female secondary sexual characters and progesterone to support pregnancy

An everyday example. Iodised salt sold across India supplies the iodine the thyroid needs to make thyroxine.

The substance. Insulin and glucagon work as a pair — one lowers and the other raises blood glucose, holding it within a narrow range.

Hormones act by binding specific receptors on or inside target cells — protein hormones use membrane receptors and second messengers, while steroid and thyroid hormones enter the cell and act on genes — and their levels are held steady by negative feedback on the hypothalamus and pituitary.

Membrane receptors:

- Protein and peptide hormones such as insulin and glucagon, and amino-acid derivatives such as adrenaline, cannot cross the cell membrane
- They bind receptors on the cell surface, which trigger second messengers such as cyclic AMP or calcium ions inside the cell

Intracellular receptors:

- Steroid hormones such as cortisol and testosterone, and iodothyronines such as thyroxine, pass through the membrane
- They bind receptors inside the cell, and the hormone-receptor complex regulates gene expression in the nucleus

Feedback control of tropic hormones:

- A tropic hormone stimulates another endocrine gland — TSH, for example, stimulates the thyroid
- The hypothalamus releases TRH, the pituitary then releases TSH, and the thyroid then releases thyroxine
- Rising thyroxine inhibits both the hypothalamus and the pituitary, lowering TRH and TSH — negative feedback

An everyday example. An air conditioner with a thermostat switches off once a room reaches the set temperature — hormone feedback works the same way.

The substance. Hormones act only where receptors exist — a hormone reaches every tissue, but only cells with matching receptors respond.

Hormonal disorders arise when a gland secretes too little (hyposecretion) or too much (hypersecretion) of its hormone, disturbing growth, metabolism, water balance or blood sugar.

Pituitary:

- Too little GH in childhood — pituitary dwarfism
- Too much GH in childhood — gigantism; in adults — acromegaly, with enlarged hands, feet and face
- Too little ADH — diabetes insipidus, with large volumes of dilute urine

Thyroid:

- Iodine deficiency — goitre, an enlarged thyroid
- Too little thyroid hormone during pregnancy or infancy — cretinism, with stunted growth and intellectual disability
- Too little in adults — myxoedema, with a low metabolic rate and puffy skin
- Too much — Graves' disease (exophthalmic goitre), with protruding eyes, weight loss and a fast heartbeat

Adrenal cortex:

- Too little — Addison's disease, with weakness, low blood pressure and darkened skin

Pancreas and parathyroid. Too little insulin, or cells that do not respond to it, causes diabetes mellitus, with high blood glucose; too little PTH lowers blood calcium and causes tetany.

An everyday example. A swelling at the front of the neck in people whose diet lacks iodine is goitre, which is why salt in India is iodised.

The substance. Diabetes insipidus and diabetes mellitus are unrelated — both cause heavy urination, but one is an ADH problem and the other an insulin problem.