Lecture notes Year 1 MBChB: Introduction to Medical Sciences (IMS)

ENDOCRINOLOGY
OVERVIEW

Main endocrine glands
 Hypothalamus/pituitary, thyroid, parathyroid (behind the thyroid), pancreas, adrenal, ovaries/testicles

Anterior pituitary - produces various hormones
 Growth hormone (GH) –skeletal growth- and excess leads to acromegaly in adults and gigantism in children
 Adrenocorticotrophic hormone (ACTH) - stimulates the adrenals to produce steroids
 Gonadotrophins (FSH and LH) - stimulate the testicles or ovaries to produce sex hormones
 Thyroid stimulating hormone or thyrotrophin (TSH) - Stimulates the thyroid to produce TH’s
 Prolactin (PRL) - stimulates breast milk production

Posterior pituitary - stores the hormones produced in the hypothalamus
 Antidiuretic hormone (ADH) - stimulates water reabsorption by the kidneys
 Oxytocin - helps uterine contractions during labour

How the pituitary is controlled
 Corticotrophin releasing hormone (CRH): stimulates ACTH secretion
 Growth hormone releasing hormone (GHRH): stimulates GH secretion
 Thyrotropin releasing hormone (TRH): stimulates TSH secretion
 Gonadotrophin releasing hormone (GnRH): stimulates FSH and LH secretion
 Secretion is on and off
 Prolactin releasing hormone does not exist and prolactin is under the inhibitory effect of the hypothalamus

Switching off pituitary hormones – NEAGTIVE FEEDBACK
 Cortisol switches off ACTH and CRH
 Growth hormone, switches off GH and GHRH
 Thyroid hormones switch off TSH and TRH
 Sex hormones switch off FSH/LH and GnRH

Pituitary controls: thyroid, part of adrenal cortex (for corticosteroids and androgens), tests/ovaries, and skeletal
growth

Glands not controlled by the pituitary
 Adrenal medulla- produces adrenaline and noradrenaline
 Parathyroid- controls calcium levels
 Pancreas- controls sugar levels
 Gut hormones

Thyroid gland
 Composed of midline isthmus (below cricoid cartilage), right lobe, left lobe
 Thyroid cells are arranged in follicles and produce TH’s
 Contains C cells, which produce calcitonin (calcium metabolism)
 Thyroid hormones interact with their receptors in various organs - regulates gene
expression and aspects of organ function
 T3 is active, T4 is inactive – gets converted to T3 from different organs, catalysed by
deiodinase

Calcium metabolism
 Mainly controlled by 4 parathyroid glands sitting behind the thyroid
 Kidneys: calcium excretion and production of active vitamin D
 Gut: absorption of calcium
 Bone: storage of calcium

Adrenal gland
 Adrenal cortex (90% of gland) – corticosteroids (cortisol), androgens, mineralocorticoid (aldosterone)
 Adrenal medulla (10% of gland) – catecholamines (adrenaline, noradrenaline and dopamine)
 Catecholamine secretion is not controlled by the pituitary (related to blood pressure)

,  Mineralocorticoid secretion is not controlled by the pituitary (related to renin-angiotensin system, which controls
the BP)
 Low BP – increase in renin – increase in aldosterone
 ACTH causes cortisol production and androgens

Ovaries
 Situated either side of the uterus, in the pelvis
 Contain follicles, which contain oocytes, at different stages of maturation during
reproductive life
 Oestrogen, more in first half of menstrual cycle, controlled by FSH
 Progesterone, more in second half of menstrual cycle, controlled by LH




Testes
 The testes are found in the scrotum, except in a minority with testicular maldescent
 Composed of:
o Interstitial or leydig cells - produce testosterone
o Seminiferous tubules – made up of germ cells producing sperms
o Sertoli cells – help in sperm production and produce inhibin
 FSH controls sperm production
 LH controls testosterone production

Clinical abnormalities of the various glands
 Hormonal over secretion – primary/secondary
 Hormonal under secretion – primary/secondary

Testing for hormonal abnormalities
 Static tests: can diagnose abnormalities of thyroid, sex glands, prolactinoma
 Primary hyperthyroidism (TH overproduction), test for T3, T4 and TSH
 Stimulation tests: suspected hormonal under-secretion where a static test isn’t enough (equivocal results)
 E.g. giving ACTH to test for adrenal insufficiency
 If an individual fails to respond to a stimulation test, gland failure is diagnosed
 GST and IST for pituitary failure (tests for ACTH and GH response)
 Suppression tests: some hormonal over-secretion
 Giving steroids and testing for endogenous steroid production (external steroids should switch off internal
steroid production)
 Giving glucose and testing GH secretion (glucose switches off GH secretion in normal individuals)
 Secondary hyperthyroidism – pituitary gland causes the problem

Diseases of the endocrine glands
 Over secretion (usually benign tumours)
 Under-secretion: gland destruction due to inflammation, infarction, other
 Tumours/nodules with normal hormone production

Prolactin oversecretion
 Usually due to a pituitary tumour secreting prolactin – prolactinoma
 Symptoms:
o Galactorrhoea (breast milk production), amenorrhoea in women and decreased sex drive in men
o Headaches and visual field problems in large tumours as the optic nerve is close to pituitary
 Static test is enough for diagnosing prolactinoma

Mildly raised prolactin
 May be due to: sex, nipple stimulation, stress, large number of drugs (including antipsychotics and
antidepressants), non-functioning pituitary tumour (compressing the hypothalamus and interfering with the
inhibitory effect on prolactin secretion)

Only over-secreting pituitary tumours can be treated medically as they very rarely require surgical intervention