Endocrine Flashcards
describe the structure of the thyroid gland
2 lobes connected via the isthmus
where is the thyroid gland located
in the anterior of the neck between C5 and T1
wrapped around the cricoid cartilage and the superior rings of the trachea
describe the blood supply and drainage around the thyroid
- arterial supply =
- superior thyroid artery
- 1st branch of the external carotid artery
- supplies superior and anterior porteions of the gland
- inferior thyroid artery
- arises from the subclavian artery
- supplies the inferior portion of the gland
- superior thyroid artery
- venous drainage
- drained by the superior, middle and inferior thyroid veins
- The superior and middle veins drain into the internal jugular vein and the inferior empties directly into the brachiocephalic vein.
how is the thyroid gland innervated
via the sympathetic trunk in the cervical ganglion BUT do not control secretion of hormones
why is the thyroid gland highly vascularised
it secretes hormones directly into the blood
what controls hormone release from the thyroid
the HPT axis
the anterior pituitary gland via TSH, which is stimulated by TRH from the hypothalamus
label
what are the major hormones of the thyroid
- T3 - triiodothyronine
- T4 - thyroxine
- 3 and 4 indicate the number of iodine molecules in each hormone
- Calcitonin
where are thyroid hormones produced
in the follicle cells which are arranged in lobules with a lumen called colloid
how do thyroid hormones take effect
via nuclear receptors in target tissues to increase metabolic rate
give examples of processes affected by thyroid hormones
- Basal Metabolic Rate
- Gluconeogenesis
- Glycogenolysis
- Protein synthesis
- Lipogenesis
- Thermogenesis
describe the process of thyroid hormone synthesis
ATE ICE
-
Active transport
- the sodium-iodide symporter [NIS] actively transports iodide into the follicular cell
- the iodide molecules pass into the colloid by pendrin
-
Thyroglobulin production
- the RER of the follicular cells produce thyroglobulin and it’s packaged by the golgi body
-
Exocytosis
- the packaged thyroglobulin is transported out of the cell by exocytosis into the follicular lumen = colloid
-
Iodination
- the iodide molecules are converted → iodine by thyroid peroxidase.
- the tyrosine molecules on the thyroglobulin have 1 or 2 iodine molecules attached to them → monoiodotyrosine MIT or diiodotyrosine DIT.
-
Coupling
- MIT + DIT = T3
- DIT + DIT =T4
- this occurs while still attached to thyroglobulin
-
Endocytosis
- the iodinated thyroglobulin molecules are endocytosed back into the follicular cells
- thyroglobulin then undergoes proteolysis → free T3 and T4.
which is more active?
which is more abundant?
T3 or T4
T3 is 10 times more active than T4
T4 is more abundant as it is more stable and has a longer half-life than T3.
T4 is converted into T3 at the target tissue
how is T4 converted to T3
via deiodinase
describe the HPT axis [hypothalamic-pituitary-thyoid axis]
- the hypothalamus releases TRH
- TRH stimulates the anterior pituitary to release TSH
- TSH acts on the thyroid gland to synthesise and secrete T3 and T4
- T4 is converted to T3 at target tissue
- T3 inhibits the hypothalamus and stops the release of TRH
when do the thyroid thyroid glands start producing thyroxine in utero
18-20 weeks
overall actions of the thyroid hormones
increases metabolic rate
important in brain maturation
how many parathyroid glands are there
where are they located
4
2 superior and 2 inferior
posterior lateral aspect of the thyroid glands
how are the parathyroid glands innervated
by the sympathetic trunk
this does not control hormone secretion
describe the blood supply to the parathyroid glands
the inferior thyroid artery - branch of the subclavian
drained by the thyroid plexus
what hormone does the parathyroid secrete
what does it do
parathyroid hormone
it regulates phosphate and calcium levels
what triggers release of PTH
- low calcium
- high phosphate
function of calcitonin and which gland releases it
- regulates calcium levels, when present in large amounts it decreases calcium and vice versa
- released by the thyroid gland
the anterior and posterior pituitary glands develop from the same tissue
T/F
False
they develop from different tissues grow next to each other then join together
what is the anterior pituitary gland formed from
the upward protrusion of ectoderm from Rathke’s pouch = adenohypophysis
what is the posterior pituitary form from
it is a neuronal extension of the neural components of the hypothalamus
how does the hypothalamus communicate with the anterior pituitary
- using hypophysiotropic hormones
- these hormones reach the anterior pituitary via the hypothalamo-hypophyseal portal vessels
- their action in the anterior pituitary is to stimulate or inhibit release of 6 hormones
how many hypophysiotropic hormones are there,
what are they
what do they do
5
-
corticotropin releasing hormone [CRH]
- stimulates release of adenocorticotropic hormone ACTH → increased cortisol produced in the adrenal cortex - zona fasiculata
-
Growth hormone releasing hormone [GHRH]
- stimulates release of growth hormone → growth and protein synthesis
- inhibited by somatostatin [which is produced in the hypothalamus and pancreas]
-
thyrotropin-releasing hormone [TRH]
- stimulates release of TSH from the anterior pituitary → stimulates synthesis and secretion of T3 and T4
-
Gonadotropin-releasing hormone [GnRH]
- stimulates release of LH and FSH → production of oestrogen, progesterone and testosterone in the gonads
-
dopamine
- inhibits release of prolactin
6.
- inhibits release of prolactin
how does the hypothalamus communicate with the anterior pit gland
via the hypothalamo-hypophyseal portal vessels
they share a blood network which allows the hormones released by the hypothalamus to reach and affect the APG
function of hypophysiotropic hormones
control the secretion of anterior pituitary hormones hormones
general role of the hypothalamus
- Important for homeostasis + primitive functions :–
- appetite, thirst, sleep, temperature regulation
- Control of autonomic function via brainstem autonomic centres
- Control of endocrine function via pituitary gland
how many hormone producing cell types are there in the APG
and how many hormones are produced
- 5 cell types
- gonadotropes cells → FSH and LH
- corticotropes cells → adrenocorticotropic hormone ACTH
- somatotropes → growth hormone (GH);
- Lactotropes → prolactin (PRL)
- thyrotropes → TSH
- 6 hormones in total
which APG hormones have a negative feedback loop
all of them except for prolactin feedback negatively
which hypothalamic hormone is not involved in the 1st step of the 3 hormone sequence
dopamine
it is an inhibitory regulator of prolactin
what is short loop negative feedback
which hormones exhibit this
- where the anterior pituitary hormone act on the hypothalamus for negative feedback
- prolactin acts on hypothalamus → secretion of dopamine which inhibits the release of prolactin from APG
- GH acts directly on the APG and hypothalamus to inhibit GH and GHRH
- IGF stimulates somatostatin which acts on the APG to inhibit release of GH from the APG
what is long loop -ve feedback
which hormones exhibit this
- where the endocrine gland hormone → negative feedback on the hypothalamus or the APG
- all but PRL and GH
- cortisol
- T3, T4
- oestrogen, progesterone, testosterone
function of growth hormone
stimulates growth and protein synthesis
released in a pulsatile manner throughout life
effect of cortisol
- regulates stress response
- increased energy mobilisation
- salt/water balance
- reduced immune response
- reduced growth
function of prolactin
- lactation
- breast development
- increased testosterone production
whaich hormones are produced by the posterior pituitary gland
- oxytocin
- vasopressin
where is vasopressin synthesised
in the supraoptic nucleus
what stimulates release of vasopressin
- increased osmotic pressure in the blood
- decreased blood volume
- trauma/ stress
- increased PCO2
- decreased PO2
what effects does vasopressin have
- it increases the expression of aquaporin 2 on the apical membrane of the primary cells in the collecting ducts of the kidney
- vasopressin binds to V2 receptors in primary cells of renal collecting duct
- triggers an intracellular cascade →
- upregulation of aquaporin 2 proteins which are inserted on the apical membrane → increased permeability of collecting duct cells
- Water is reabsorbed from the renal collecting duct and returned to the blood stream, decreasing the plasma osmolality
describe the -ve feedback system in high blood volume and low plasma osmolality
define osmolality
osmolality = the amount of solutes per kilo of solvent
describe the -ve feedback system in high plasma osmolality
define osmolality
osmolality = the amount of solutes per kilo of solvent
where is oxytocin produced
in the paraventricular nucleus
what triggers oxytocin release
triggered by nipple stimulation
what are the effects of oxytocin
- milk let down = constriction of muscles of the breast to promote milk ejection
- uterine smooth muscle contraction
- promotion of labour
describe the feedback mechanism of oxytocin
positive feedback
- uterine contraction stimulates the paraventricular nucleus further until labour
- suckling stimulates the paraventricular nucleus further until milk ejection
what is the bodies response to high calcium levels
releases calcitonin from the thyroid gland
where is calcitonin produced
in the parafollicular cells [c cells]
how does T3 and T4 travel in the blood
they are insoluble therefore they travel via transport proteins:
- albumin
- TBG
- tranthyretin
where are the adrenal glands located
supramedial aspect of the kidneys
retroperitoneal
describe the arterial blood supply of the adrenal glands
- Superior adrenal artery – arises from the inferior phrenic artery
- Middle adrenal artery – arises from the abdominal aorta.
- Inferior adrenal artery – arises from the renal arteries.
describe the venous drainage of the adrenal glands
- right kidney drains directly into the IVC
- left kidney drains into the left renal vein then into the IVC
what is the innervation of the adrenal glands
splanchnic nerve [sympathetic]
what are the hormones produced by the anterior pituitary gland
FLAGTOP
- Follicle stimulating hormone
- Leutenising hormone
- Adrenocorticotropic hormone
- Growth hormone releasing hormone
- Thyroid stimulating hormone
- O- melanocyte stimulating hormone
- Prolactin
describe the anatomy of the adrenal glands
- capsule
- cortex
- zona Glomerulosa
- mineraolcorticoids synthesis
- Zona Fascicularis
- Glucocorticoids synthesis
- Zona Reticularis
- androgens synthesis
- zona Glomerulosa
- medulla
- catecholamine synthesis
describe adrenal gland vasculature
- arteries x3
- superior artery = branch of the inferior phrenic artery
- middle artery = branch of the abdominal aorta
- inferior artery = branch of the renal artery
- one vein drains the gland = the adrenal vein
- the right adrenal vein drains directly into the IVC as its is in close proximity
- the left adrenal vein drains into the left renal vein first
give examples of catecholamines
adrenaline and noradrenaline
how is the adrenal gland innervated
the splanchnic nerve
where is ADH produced
hypothalamus
which cells does ADH take effect in
principal cells of the collecting duct
what effect does ADH have
- it binds to the V2R receptors of the principal cells →
- upregulation of aquaporin channels in the collecting tubule walls →
- increased H2O reabsorption and less urine produced
where is erythropoietin produced
what is its function
in the kidneys
increases production of RBCs in the bone marrow
what hormones are produced in the adrenal glands
- Aldosterone
- cortisol
- corticosterone
- DHEA
- androstenedione
1= glomerulosa
2+3=fasciculata
4+5 = reticularis
what is the role of the adrenal medulla
- part of the autonomic nervous system
- production of catecholamines
- adrenaline 80% and noradrenaline20%
- main site for adrenaline synthesis
- production of catecholamines is dependent on cortisol levels [permissive effect]
what is the effect of catecholamine release
- lipolysis in adipocytes
- gluconeogenesis in liver and muscles
- tachycardia and increased cardiac contractility
- redistribution of circulatory volume
where are corticosteroids produced
in the adrenal cortex
how do corticosteroids work
- they are lipid soluble so can pass through the lipid membrane and:
- act on intracellular receptors
- alter gene transcription directly or indirectly
what are steroids and steroid hormones made from
cholesterol
role of ACTH
ACTH stimulates the synthesis of corticosteroids and cortisol acutely
where are glucocorticoids produced
in the zona fasciculata and reticularis
(more-so in fascicularis)
glucocorticoids are essential to life
T/F
true
- glucocorticoids are essential to life
what stimulates the production of glucocorticoids
- hypothalamus releasing CRH - corticotropin releasing hormone
- CRH→ APG causing release of ACTH
- ACTH acts on the zona fasciculata in the adrenal cortex to produce cortisol
what are the effects of cortisol and glucocorticoids
- Increase glucose mobilisation
- Augment gluconeogenesis
- Amino acid generation
- Increased lipolysis Important during “stress”
- Maintenance of circulation
- Vascular tone
- Salt and water balance
- Immunomodulation
- Dampen immune response
how are glucocorticoids transported
- In the circulation glucocorticoids are mostly bound to proteins –
- 90% bound to Corticosteroid-Binding Globulin (CBG)
- 5% bound to albumin
- 5% “free”
what fraction of glucocorticoids in blood are bioavailable
the free glucocorticoids are bioavailable thus ~5%
when stressed level of binding reduces so there is more free cortisol as CBG is cleaved
how is cortisol production regulated
via negative feedback
cortisol has a diurnal rhythm - what does this mean
the the secretion follows a daily cycle
cortisol secretion is higher in the morning and early afternoon
what factors stimulate cortisol secretion
stress
diurnal / circadian rhythm
cytokines
what is calssified as stress
- “The sum of the bodies responses to adverse stimuli”
- Infection
- Trauma
- Haemorrhage
- Medical illness
- Psychological
- Exercise/exhaustion
how does the HPA axis change in acute illness
- there is less negative feedback so more cortisol is produced
- less CBG synthesised and more broken down → more free cortisol
what are mineralocorticoid -give example
where are they produced
aldosterone
produced in the zona glomerulosa
function of aldosterone
- critical for salt and water balance in the:
- kidneys
- pancreas
- colon
- salivary glands
- sweat glands
what triggers aldosterone secretion
renin from the juxtaglomerular cells
effects of aldosterone secretion
- salt balance in th kidneys, colon, pancreas, salivary glands and sweat glands.
- in the kidneys it acts on the distal convoluted tubule and collecting duct
- increases epithelial Na+ channels and Na/K ATPase expression
where are adrenal androgens produced
in the zona reticularis
what androgens are produced in the adrenal glands
- DHEA
- androstenadione
what is the most abundant adrenal steroid
DHEA
which androgen has 1/10th adrongenic activty as testosterone
androstenedione
what is a major source of androgens in women
androstenedione and DHEA
what stimulates production of DHEA anandrostenedione
ACTH [not ghrh]
effect of high cortisol on sex hormones
- High cortisol levels inhibit the gonadotropin releasing hormone (GnRH) leading to
- decreased progesterone and oestrogen in females.
- Decreased testosterone and sperm count in males
