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