Hypothalamus, pituitary and thyroid function Flashcards
Describe the anatomy of the hypothalamus, the posterior pituitary and the anterior pituitary
The hypothalamus sends neurones from the hypothalamus to form a posterior pituitary, which releases nueropeptides like oxytocin and ADH. The anterior pituitary is very close but not apart of the brain.
Describe the process of the transmission of a hormone from the hypothalamus to the anterior pituitary.
The neurones in the hypothalamus send axons to the median eminence. Therre is no BBB there,, so the hormones enter the portal vessels. They then bind to their complementary receptors.
State the 5 hormones released from the hypothalamus
TRH (Thyrotropin releasing hormone) Prolactin CRF (Corticotrophin releasing factor) GnRH (Gonadotrophin releasing hormone) GHRH (Growth hormone releasing hormone)
State the 6 hormones released from the anterior pituitary
TSH (Thyroid stimulating hormone) ACTH (Adrenocorticotrophin Hormone) LH (Luteinising Hormone) FSH (Follice Stimulating Hormone) GH( Growth Hormone )
Name 2 inhibitory molecules that affect the release of hormones and state the systems they target
Somatostatin halts GH/TRH
Dopamine halts Prolactin release
Starting at the hypothalamus, describe how LH and FSH are released
Hypothalamus - Releases GnRH via median eminence and portal vessels to the anterior pituitary. The anterior pituitary releases LH and FSH via gonadotrophs.
Describe the complications that arise in children that lack FSH or LH and suggest how this can be solved.
These children will not be able to undergo puberty and may lose their reproductive function. GnRH, FSH and LH can be used to manipulate reproductive function, as they are even used in IVF.
Starting at the hypothalamus, describe how GH is released
Hypothalamus - Releases GHRH via median eminence and portal vessels to the anterior pituitary. The anterior pituitary releases GH via somatrophs.
State what happens if individials have a lack of GH
These individuals tend to have short statures. This can be treated by GH treatment.
State what happens if individials have excess GH
Too much growth hormone may result in giantism.
What is acromegaly?
Too much growth hormone in adults- usually do to a pituitary adenoma (a type of cancer). Somatotrophs produce extra GH resulting in the ones in the face, fusing together.
Starting at the anterior pituitary, describe how prolactin is released
The anterior pituitary releases prolactin via mammotrophs. Too much prolactin in males can result in gynaecomastia.
Starting at the hypothalamus, describe how TSH is released
Hypothalamus - Releases TRH via median eminence and portal vessels to the anterior pituitary. The anterior pituitary releases TSH via thyrotrophs.
Describe the histology of the thyroid
The thyroid is composed of smaller components known as thyroid follicles. These are balls of epithelial cells that surround proteinaceous (non-cellular) colloid where the thyroid hormones are sent.
How much more potent is T3 than T4?
10x
Describe the synthesis of thyroid hormones
TSH binds to target receptor –> opens Na+/i- symporter in the follicular cells.I- (AKA chloride trapping). Throglobulin is synthesised in the thyroid follicle cell. I- oxidised by throif peroxidase to make iodous ions. Thyroglobulin and iodous ions move to the throid follice and T3/T4 are bound to thyroglobulin. The thyroglobulin is endocytosed in to the thyroid follicle cell. Lysosomes catalyse the breakdown of thyroglobulin to form T3/T4.
Where in the blood is T3/T4 found? Relation of T3 to T4?
Bound to plasmas serum proteins including thyroid binding globulin. T4 thought to be a prohormone of T3.
What is the MOA of T3/T4?
Free T3 an T4 bind to to target receptors in the cell nucleus and are of the super family of nuclear receptors. They act as transcription factors. The T3 binds to the thyroid hormone receptor and in the presence of 9-cis retinoic acid and the RXR receptor, both hormone-receptor complexes dimerise and act as transciription factors ——-> regulation of protiens.
Name 4 roles of the thyroid hormone
Increases 02 consumption
Increases catecholamine sensitivity in the CVS
Increases nervous system excitability
The thryoid system has permissive effects on growth and development
How is the negative feedback system controlled?
The free t3 and T4 turn off more TRH and TSH release
Name 4 causes of hypothryoidism
Lack of iodide in diet
Congenital reasons
Hashimoto’s throiditis
Circulating antibodies against Throid peroxidase
Name 5 symptoms of hypothyroidism
Cold sensitivity Depression Bradycardia Weight gain Throid Goitres
Why do throid goitres arise?
They arise due to the excess release of TSH and not enough T3/T4 stopping the release of TSH
Name 2 causes of hypethryoidism
Excess iodide in diet
Grave’s Disease
Name 6 symptoms of hyperthyroidism
Heat sensitivity Axiety Tachycardia Weight loss Toixc Goitres Exophthalmos
Describe the treatment and management for hypothyroidism
Levothyroxine (T4) and Lio-thyronine (T3) - should be cautioned in the elderly as they can increase the heart rate in the elderly Levthryoxine dose 50-100mcg a day life long. Monthly tests until euthyroid and then once established, test every 6 months. Take t4 on an empty stomach. Take into account skiiny patients.
What are the side effects associated with hypothyroid treatment ?
Patients may experience thyrotoxicosis -symptoms of hyperthroidism.
What drug- drug interactions need to be monitored?
PPIs, calcium containing drugs, multivitamins and iron will lower T4 absorption.
Carbamazepine, phenytoin and rifampicin can increase the clearence of t4
Describe the treatment for hyperthyroidism and note any major side effects
Carbimazole or propylthiouracil or radioactive iodine 13 and Beta blockers.
Carbimazole - prodrug which is metabolised to methimazole.
Methimazole prevents coupling of iodine on tyrosine residues.These actions lower the production of thyroid hormones. SE: burising without provocation or sore thorats, mouth ulcers and fever. Agranulocytosis is dangerous.
Propylthiouracil- inhibiting the enzyme- thyoid peroxidase, lowering the productionof thyroid hormone production. SE: Hepatic impairment- noticedd from jaundice, itchiness and dark urine.
I-131 - destroys the overactive throid cells
Beta blocker- lowers the sympathetic side effects of hyperthyoidism
Describe the management for hyperthyroidism
The treatment will last for 18 months. It can take 2 months for the patient to become euthyroid. Once euthyroid, titrate the dose or supplement with levothyroxine. Pateints may become lethargic and may gain weight due to treatment, Patients should have regualr TFTs (Thryoid function tests).
Describe the cause of Grave’s disease
Antbody stimulating the TSH receptor without TSH stimulating the recepors on the thyroid follicle- results in excess thyroid production.
How does amiodarone cause hyperthyroidism?
Amiodarone - Class 3 anti-arrhythmic drug. Similar sturcture to T3. Normal ioidine daily intake is 6-12 mg/day. If on amiodarone, it is redued to 0.15-0.30mg/kg if the patient is on amiodarone.It can cause Amindoarone induced thyrotoxicosis (AIT) as excess iodine builds up.
How does amiodarone cause hyperthyroidism?
Amiodarone - Class 3 anti-arrhythmic drug. Similar sturcture to T3. It can cause less iodine being taken up by the thyroid follicle and reduces T4 —-> T3 convesion.
How can lithium cause hypothyroidism?
Lithium is thought to lower the concentrating capacity of iodine. Lithium may inhibit the secretion of thyroid hormones by stabilising the follicular cell microtubule.
