Chemical Coordination and Integration · Lesson 3 of 11
Pituitary Hormones: Growth, Reproduction, Milk and Water Balance
“Explain pars distalis, pars intermedia and pars nervosa hormones, their targets, and classic hypo-/hypersecretion disorders.”
• Identify GH, PRL, TSH, ACTH, LH, FSH and MSH and describe principal targets. • Explain oxytocin and ADH synthesis, release and effects. • Distinguish pituitary dwarfism, gigantism and acromegaly. • Compare milk synthesis with milk ejection, and diabetes insipidus with diabetes mellitus.
Why is one tiny pituitary gland associated with so many body systems?
The pituitary contains several specialised populations of endocrine cells. Their products act on different tissues rather than making one general 'growth fluid'. Some hormones directly change tissue activity, such as prolactin supporting milk synthesis. Others stimulate entire peripheral endocrine organs, such as TSH acting on thyroid, ACTH acting on adrenal cortex and gonadotrophins acting on testes or ovaries. The activity of the pituitary reflects hypothalamic control and feedback from the targets; the pituitary is not an all-powerful independent controller.
Growth hormone: childhood height and adult tissue growth
An anterior pituitary protein hormone that influences body growth and metabolism, substantially through growth mediators including IGF-1.
GH supports longitudinal growth during childhood while growth plates remain active, as well as protein synthesis and metabolic regulation in multiple tissues. Much of its effect on skeletal growth involves insulin-like growth factor 1 (IGF-1), produced particularly by the liver and also in local tissues. Inadequate GH secretion during childhood can contribute to pituitary dwarfism, typically characterised by markedly reduced height when untreated. The resulting stature problem should not be confused with every possible cause of short height, which has numerous genetic and nutritional explanations.
Excess GH before the long-bone growth plates close can cause gigantism, with excessive overall height. When excess GH develops after epiphyseal plates have fused, it generally cannot lengthen the long bones in the same manner, but it enlarges hands, feet, facial bones and soft tissues, causing acromegaly. Acromegaly may develop gradually; changes in rings, shoes, facial features and systemic complications can become evident over many years. These are different manifestations of excessive GH depending on developmental timing.
| Condition | Typical GH problem | Key distinction |
|---|---|---|
| Pituitary dwarfism | Insufficient GH during childhood | Impaired longitudinal growth |
| Gigantism | GH excess while growth plates are open | Excessive height and body growth |
| Acromegaly | GH excess usually in adulthood | Enlargement of hands, feet, facial structures and soft tissues |
Prolactin controls milk synthesis, not the physical ejection reflex
An anterior pituitary hormone that promotes mammary development and milk production after birth.
Prolactin acts on mammary secretory cells, encouraging production of milk components. Oxytocin, by contrast, acts on myoepithelial cells around mammary secretory units to contract and expel milk through ducts. This is an essential distinction: making milk is not the same physiological action as letting it flow out during suckling. Both hormones operate as parts of a coordinated maternal response and are influenced by sensory and brain signals.
Tropic hormones connect pituitary to other endocrine glands
Thyroid-stimulating hormone (TSH, thyrotrophin) stimulates thyroid follicular cells to synthesise and secrete T₃ and T₄. Adrenocorticotrophic hormone (ACTH, corticotrophin) stimulates steroid hormone synthesis, particularly glucocorticoids, from adrenal cortex. Both participate in hypothalamus–pituitary–target gland axes and are restrained by negative feedback. A rise in a tropic hormone does not automatically mean the target hormone is high: a failing target gland may produce too little hormone despite strong pituitary stimulation.
LH and FSH regulate the reproductive organs
Pituitary hormones LH and FSH that act on the gonads and help regulate reproductive function.
In testes, luteinising hormone (LH) acts on Leydig cells and stimulates androgen production, especially testosterone. Follicle-stimulating hormone (FSH) acts on Sertoli cells, supporting conditions required for spermatogenesis alongside testosterone. Testosterone production and sperm formation are connected but distinct processes; stating that FSH directly produces mature sperm by itself would omit essential interactions among cells and hormones.
In ovaries, FSH supports growth and maturation of ovarian follicles. A surge in LH triggers ovulation of a mature follicle and supports the corpus luteum formed afterwards, which produces progesterone. The pituitary gonadotrophins therefore participate in a recurring sequence of follicular development, ovulation and luteal function. The timing and feedback mechanisms are more complex than a permanent steady flow of identical amounts.
MSH and the pars intermedia
Melanocyte-stimulating hormone (MSH) acts on pigment-containing melanocytes and can affect pigmentation pathways. The source associates MSH with the pars intermedia, an intermediate region of the pituitary. In adult humans this region is reduced and largely merges with the anterior lobe; the physiology of pigmentation is more complicated than a single gland output. At this level, recognise MSH's broad pigment-related function and its anatomical source convention.
Posterior hormones: oxytocin
Hypothalamic neurosecretory neurons synthesise oxytocin, which reaches the posterior pituitary through axonal transport. When released, oxytocin stimulates smooth-muscle-associated contractions. During childbirth, uterine stretch and neural feedback can strengthen oxytocin-associated contractions until delivery. During breastfeeding, suckling triggers neural signals that promote oxytocin release and contraction of mammary myoepithelial cells, causing milk ejection. Oxytocin does not replace prolactin's main role in milk synthesis.
Posterior hormones: ADH and kidney water conservation
A hypothalamic peptide released by posterior pituitary that increases kidney water reabsorption and helps conserve body water.
Vasopressin responds to conditions such as increased plasma osmolarity or reduced circulating volume. Its principal water-conserving action is on the late distal nephron and especially the collecting ducts, where it promotes water permeability through aquaporin regulation. More water is returned to the blood, producing a smaller volume of more concentrated urine when the kidneys are functioning normally. The original source describes increased water and electrolyte resorption in distal tubules; the collecting-duct emphasis clarifies the major modern physiological mechanism.
If vasopressin synthesis, transport or release is severely impaired, kidneys may fail to conserve water adequately, causing large volumes of dilute urine, marked thirst and dehydration risk. This is central diabetes insipidus; defective kidney response can produce nephrogenic diabetes insipidus with a similar urinary phenotype. Unlike diabetes mellitus, diabetes insipidus does not principally result from abnormally high blood glucose. The shared word 'diabetes' reflects excessive urine, not identical causes.
| Hormone | Region normally associated with secretion | Main effect |
|---|---|---|
| GH | Anterior | Growth and metabolic regulation |
| Prolactin | Anterior | Milk synthesis |
| TSH | Anterior | Thyroid stimulation |
| ACTH | Anterior | Glucocorticoid stimulation |
| LH and FSH | Anterior | Gonadal steroid function and gamete development |
| MSH | Intermediate region convention | Pigmentation regulation |
| Oxytocin | Released by posterior | Uterine contraction and milk ejection |
| ADH | Released by posterior | Kidney water conservation |
GH excess before versus after growth-plate closure has different manifestations. Prolactin produces milk whereas oxytocin ejects it. Diabetes insipidus is a water-balance disorder, whereas diabetes mellitus involves glucose regulation.
Quick Quiz
Quiz
GH excess in an adult most characteristically produces:
A child with severe untreated GH deficiency may show:
Which pituitary hormone stimulates thyroid hormone secretion?
Which hormone promotes glucocorticoid release from adrenal cortex?
Which hormone triggers ovulation of a mature ovarian follicle?
Which pituitary hormone supports milk synthesis?
Which hormone primarily causes mammary milk ejection?
What is a principal ADH effect?
Which disorder is linked to inadequate ADH action?
Practice Problems
- Distinguish gigantism, acromegaly and pituitary dwarfism by timing and GH secretion.
- List the six major pars-distalis hormones and their targets.
- Explain why LH and FSH are called gonadotrophins, with one action in each sex.
- Differentiate prolactin and oxytocin during lactation.
- Explain how reduced ADH release changes urine and why the disorder is not identical to diabetes mellitus.
Excess GH before epiphyseal growth plates close can cause gigantism, whereas excess after closure usually causes acromegaly, with enlarged acral/facial tissues. GH deficiency in childhood can cause pituitary dwarfism and inadequate height gain.
Key Takeaways
• Anterior pituitary GH, PRL, TSH, ACTH, LH and FSH act on distinct target tissues. • Childhood GH deficiency causes pituitary dwarfism; GH excess before versus after maturity causes gigantism versus acromegaly. • LH and FSH regulate gonadal functions; prolactin enables milk synthesis. • Oxytocin stimulates uterine contraction and milk ejection, whereas ADH conserves water at kidneys. • The posterior pituitary releases oxytocin and ADH synthesised in hypothalamic neurons.