Why it failed. Chemicals were discovered that behave exactly like hormones but violate one or more of the three classical requirements. The atrial wall of the heart secretes atrial natriuretic factor and the juxtaglomerular cells of the kidney secrete erythropoietin — neither the heart nor the kidney is an endocrine gland. Growth factors are secreted by several non-endocrine tissues. And some messengers act on cells close by rather than on a distantly located organ. A definition that excluded all of these would have to call them something other than hormones, without any functional justification.

The current definition: hormones are non-nutrient chemicals which act as intercellular messengers and are produced in trace amounts. It keeps three essentials — the chemical is not a nutrient, it carries a message between cells, and it works at trace concentration — while dropping the requirements about source, route and distance.

The lesson: definitions in biology follow function, not anatomy. The new definition covers a number of new molecules in addition to the hormones secreted by the organised endocrine glands.

Anterior pituitary hormones. Their secretion would fall drastically or become uncontrolled. The hypothalamic releasing and inhibiting hormones reach the pituitary gland through a portal circulatory system and regulate the functions of the anterior pituitary. Cut that route and the gland no longer receives its instructions, so GH, PRL, TSH, ACTH, LH and FSH can no longer be appropriately regulated. Downstream glands — thyroid, adrenal cortex and gonads — would in turn lose their trophic stimulation.

Posterior pituitary hormones. Their release would also fail, but for a different reason. Oxytocin and vasopressin are actually synthesised by the hypothalamus and are transported axonally to the neurohypophysis, which merely stores and releases them. Cutting the stalk severs those axons, so the hormones can no longer arrive. The posterior pituitary is under the direct neural regulation of the hypothalamus.

The difference: in the anterior lobe the loss is of a chemical signal to a gland that still makes its own hormones; in the posterior lobe the loss is of the hormones themselves, which were never made there. Clinically, one would expect dehydration from lack of ADH (diabetes insipidus) alongside failure of the thyroid, adrenal cortex and gonads.

Hormone: growth hormone (GH), from the pars distalis of the pituitary.

The child — gigantism. Over-secretion of GH stimulates abnormal growth of the body. In a child whose bones can still lengthen, the whole frame grows excessively tall.

The man — acromegaly. Excess secretion of growth hormone in adults, especially in middle age, can result in severe disfigurement (especially of the face). Since the bones can no longer lengthen, the tissues that remain responsive thicken and enlarge instead — hence the coarsening face and enlarged hands.

Why the difference: a hormone does not decide the outcome by itself; the outcome depends on what the target tissue is capable of doing when the hormone arrives. This is also why acromegaly is clinically dangerous: NCERT notes it may lead to serious complications and premature death if unchecked, and that it is hard to diagnose in the early stages and often goes undetected for many years until changes in external features become noticeable.

Diabetes insipidus. The defect is an impairment affecting the synthesis or release of ADH (vasopressin). Vasopressin acts mainly at the kidney and stimulates resorption of water and electrolytes by the distal tubules, thereby reducing loss of water through urine. Without adequate ADH the kidney has a diminished ability to conserve water, so large volumes of dilute urine are passed, leading to water loss and dehydration. Blood glucose is normal and no glucose appears in the urine.

Diabetes mellitus. The defect lies in insulin, from the β-cells of the Islets of Langerhans of the pancreas. Prolonged hyperglycemia leads to this complex disorder, which is associated with loss of glucose through urine and the formation of harmful compounds known as ketone bodies. Here the large urine volume is a consequence of glucose being lost in the urine and carrying water with it.

The distinction in one line: insipidus is a water problem caused by a pituitary hormone; mellitus is a glucose problem caused by a pancreatic hormone. The shared name reflects only the shared symptom of copious urine.

The pattern. TSH stimulates the thyroid gland; ACTH stimulates the adrenal cortex; LH and FSH stimulate gonadal activity and are hence called gonadotrophins; prolactin acts on the mammary glands. Growth hormone is the one that acts chiefly on the somatic tissues of the body at large, stimulating growth. This is why the pars distalis hormones are described as trophic hormones — each one drives the growth or activity of another tissue.

Advantages of a hierarchy.

(i) Coordination. A single controller can adjust several glands together, so the body's responses to a situation (growth, stress, reproduction) are consistent rather than contradictory.

(ii) Amplification. A trace of a hypothalamic hormone releases more pituitary hormone, which releases still more of the peripheral gland's hormone. A small signal can thus produce a large, body-wide effect — which is what allows hormones to work at trace amounts.

(iii) Regulation at more than one level. With the hypothalamus above the pituitary and the pituitary above the peripheral glands, control can be exercised at either step, and the hypothalamus's ability to send inhibiting as well as releasing hormones lets output be turned down as well as up.