55. Integrative Role of Hypothalamus Flashcards
State some of the effects of hypothyroidism.
Reduced basal metabolic rate – everything slows down Cold intolerance Deep voice Weight gain Loss of appetite Depression Lethargy Speech slows down Bradycardia Constipation
What are the main thyroid hormones? Which is more active?
T3 and T4 T3 is more active but most of the thyroid hormone released by the thyroid gland is in the T4 form
What converts T4 to T3?
Deiodinase
Describe the mechanism of action of thyroxine.
Thyroxine enters the target cell and is converted to T3 by deiodinase T3 then binds to a thyroid hormone receptor in the nucleus and then heterodimerises with a retinoid X receptor This complex then binds to a thyroid response element, which causes a change in gene expression
What are the two main drugs that are used as thyroxine and T3 replacement?
T4 replacement – Levothyroxine Sodium T3 replacement – Liothyronine Sodium
What is thyroxine replacement used to treat?
Autoimmune primary hypothyroidism Iatrogenic primary hypothyroidism
How often is the tablet taken and what measurement is taken toguide the dose?
Once daily TSH is measured and the aim is to use thyroxine replacement to suppress TSH so that it is within the reference range
Describe the levels of thyroxine and TSH in someone with primary thyroid failure.
Thyroxine = LOW TSH = HIGH
What is secondary hypothyroidism? What measurement is used to guide the dose in this case?
This is a problem with TSH production by the adenohypophysis There is no problem with the thyroid gland itself As there is no TSH production, thyroxine replacement therapy is monitored by measuring free T4 (fT4) levels and keeping it within the reference range
What is the clinical use of liothyronine sodium?
Treatment of myxoedema coma (very rare complication of hypothyroidism) You give IV liothyronine sodium because the onset of action is faster than levothyroxine sodium
Why would you give a patient combined thyroid hormone replacement (T3+T4)?
Some patients don’t feel better with T4 replacement alone though their TSH may be normal
What is the problem with giving T3 replacement?
T3 is very potent so it is difficult to get the dose right Too high a dose can lead to patients complaining of thyrotoxicosis type symptoms: palpitations, tremor, anxiety
Describe some adverse effects of thyroid hormone over-replacement.
Skeletal Increased bone turnover Reduced bone mineral density Risk of osteoporosis Metabolic Increased energy expenditure Weight loss Cardiac Tachycardia Risk of dysrhythmia Beta-adrenergic activity Tremor Nervousness
What are the half-lives of T3 and T4?
T3 = 2-5 hours T4 = 6 days
What plasma protein is T3 and T4 mainly bound to?
Thyroxine binding globulin
What can cause an increase in the production of plasma proteins?
Pregnancy Prolonged treatment with oestrogen and phenothiazines
What can cause a decrease in the amounts of the plasma proteins?
Liver failure (most plasma proteins are produced by the liver) Severe malnourishment
How many carbon atoms does cholesterol have?
27
What determines which steroids the different parts of the adrenal gland produce?
The combination of enzymes present in the different parts of the adrenal gland – hydroxylation in different positions of the cholesterol molecule gives different products.
How many carbon atoms does cholesterol have?
27
Which sets of enzymes are switched on by ACTH?
Cortisol synthesis enzymes Sex steroid synthesis enzymes
The pituitary gland produces ACTH in response to what?
Stress
Which enzyme converts corticosterone to aldosterone?
Aldosterone synthase (includes 18-hydroxylase)
State three causes of adrenocortical failure.
Tuberculous Addison’s Disesae – most common cause worldwide Autoimmune Addison’s Disease – most common cause in the UK Congenital Adrenal Hyperplasia
State some consequences of adrenocortical failure.
Hypotension (due to lack of aldosterone) Hyperkalaemia Loss of salt (sodium) in urine Fall in blood glucose (due to lack of cortisol) High ACTH –> pigmentation Vitiligo Eventual death due to severe hypotension
Why do Addison’s patients get vitiligo?
Vitiligo is an autoimmune disease where you have antibodies against melanin Autoimmune diseases tend to go hand-in-hand
Why does Addison’s cause skin pigmentation?
The lack of cortisol stimulates the production of huge amounts of ACTH. ACTH is formed from the cleavage of POMC (pro-opiomelanocortin) to ACTH and alpha-MSH High ACTH also means high alpha-MSH –> skin pigmentation
State some tests for Addison’s disease.
9 am cortisol (this should be high in a normal person) You could measure ACTH and this should be high in Addison’s Inject synthetic ACTH (synacthen) and if they have functioning adrenals, they should start to produce cortisol NOTE: 250 mg IM synacthem
What is the most common cause of congenital adrenal hyperplasia (CAH)?
21-hydroxylase deficiency
What are the two degrees of CAH?
Partial or Complete (absence of the enzyme)
Why are foetuses with CAH normally fine in utero?
In utero, the foetus’ will have maternal cortisol and aldosterone so don’t need to rely on their own endogenous production.
Which hormones are absent in complete 21-hydroxylase deficiency?
Cortisol and Aldosterone
What effect does 21-hydroxylase deficiency have on sex steroid synthesis?
It funnels the precursors towards the steroid synthesis pathway so you get an increase in adrenal sex steroids.
What will happen to a baby with complete 21-hydroxylase deficiency after they are born?
They will have a salt losing Addisonian crisis due to the lack of aldosterone.
What are the two main features of 21-hydroxylase deficiency?
Hypotension Virilisation (female babies will be born with ambiguous genitalia)
Describe the presentation of partial 21-hydroxylase deficiency.
They may present much later because they don’t have a salt losing Addisonian crisis. They will present once they start to see the effects of the excess adrenal sex steroids e.g. hirsuitism and precocious puberty They will also be a little hypotensive
Describe and explain the presentation and explanation of complete 11-hydroxylase deficiency.
With 11-beta hydroxylase deficiency there will be a build up of 11-deoxycorticosterone 11-deoxycorticosterone has mineralocorticoid effects so they don’t have a salt losing Addisonian crisis – they will behave as if they have HIGH aldosterone They will be HYPERTENSIVE and HYPOKALAEMIC There will still be some funnelling towards the sex steroid synthesis pathway so the child will also be VIRILISED NOTE: boys may be missed because they tend to look normal
Describe and explain the presentation of 17-hydroxylase deficiency.
They will have high levels of aldosterone meaning that they are hypertensive and hypokalaemic They are missing cortisol and sex steroids so they will not go through puberty They will usually present around pubertal age – they will be hypertensive, have borderline hypoglycaemia and absent puberty NOTE: they will also have a lot of infections because you need cortisol to cope with the stress of infections
What determines which steroids the different parts of the adrenal gland produce?
The combination of enzymes present in the different parts of the adrenal gland – hydroxylation in different positions of the cholesterol molecule gives different products.
Which enzyme converts cholesterol to pregnenolone?
Cytochrome P450 (short chain cleavage)
Which sets of enzymes are switched on by ACTH?
Cortisol synthesis enzymes Sex steroid synthesis enzymes
The pituitary gland produces ACTH in response to what?
Stress
Describe and explain the presentation of 17-hydroxylase deficiency.
They will have high levels of aldosterone meaning that they are hypertensive and hypokalaemic They are missing cortisol and sex steroids so they will not go through puberty They will usually present around pubertal age – they will be hypertensive, have borderline hypoglycaemia and absent puberty NOTE: they will also have a lot of infections because you need cortisol to cope with the stress of infections
Describe and explain the presentation and explanation of complete 11-hydroxylase deficiency.
With 11-beta hydroxylase deficiency there will be a build up of 11-deoxycorticosterone 11-deoxycorticosterone has mineralocorticoid effects so they don’t have a salt losing Addisonian crisis – they will behave as if they have HIGH aldosterone They will be HYPERTENSIVE and HYPOKALAEMIC There will still be some funnelling towards the sex steroid synthesis pathway so the child will also be VIRILISED NOTE: boys may be missed because they tend to look normal
Describe the presentation of partial 21-hydroxylase deficiency.
They may present much later because they don’t have a salt losing Addisonian crisis. They will present once they start to see the effects of the excess adrenal sex steroids e.g. hirsuitism and precocious puberty They will also be a little hypotensive
What are the two main features of 21-hydroxylase deficiency?
Hypotension Virilisation (female babies will be born with ambiguous genitalia)
What will happen to a baby with complete 21-hydroxylase deficiency after they are born?
They will have a salt losing Addisonian crisis due to the lack of aldosterone.
What effect does 21-hydroxylase deficiency have on sex steroid synthesis?
It funnels the precursors towards the steroid synthesis pathway so you get an increase in adrenal sex steroids.
Which hormones are absent in complete 21-hydroxylase deficiency?
Cortisol and Aldosterone
Why are foetuses with CAH normally fine in utero?
In utero, the foetus’ will have maternal cortisol and aldosterone so don’t need to rely on their own endogenous production.
What are the two degrees of CAH?
Partial or Complete (absence of the enzyme)
What is the most common cause of congenital adrenal hyperplasia (CAH)?
21-hydroxylase deficiency
State some tests for Addison’s disease.
9 am cortisol (this should be high in a normal person) You could measure ACTH and this should be high in Addison’s Inject synthetic ACTH (synacthen) and if they have functioning adrenals, they should start to produce cortisol NOTE: 250 mg IM synacthem
Why does Addison’s cause skin pigmentation?
The lack of cortisol stimulates the production of huge amounts of ACTH. ACTH is formed from the cleavage of POMC (pro-opiomelanocortin) to ACTH and alpha-MSH High ACTH also means high alpha-MSH –> skin pigmentation
Why do Addison’s patients get vitiligo?
Vitiligo is an autoimmune disease where you have antibodies against melanin Autoimmune diseases tend to go hand-in-hand
State some consequences of adrenocortical failure.
Hypotension (due to lack of aldosterone) Hyperkalaemia Loss of salt (sodium) in urine Fall in blood glucose (due to lack of cortisol) High ACTH –> pigmentation Vitiligo Eventual death due to severe hypotension
State three causes of adrenocortical failure.
Tuberculous Addison’s Disesae – most common cause worldwide Autoimmune Addison’s Disease – most common cause in the UK Congenital Adrenal Hyperplasia
Which enzyme converts corticosterone to aldosterone?
Aldosterone synthase (includes 18-hydroxylase)
Which enzyme converts cholesterol to pregnenolone?
Cytochrome P450 (short chain cleavage)
Describe the effects of excess cortisol on protein and fat synthesis.
Decrease protein synthesis Increase fat synthesis
Explain why people with Cushing’s disease get stretch marks.
They are putting on a lot of fat quickly, which stretches the skin. Because protein synthesis is switched off, you can’t make the protein required for skin growth so the skin tears.
Describe the clinical features of Cushing’s syndrome.
Moon face Interscapular fat pad (buffalo hump) Proximal myopathy Easy bruising Striae Thin skin Osteoporosis Diabetes Centripetal adiposity (lemon on sticks) Hypertension and hypokalaemia
Why does Cushing’s syndrome cause hypertension and hypokalaemia?
At high concentrations, cortisol can have mineralocorticoid effects –> increased sodium absorption and potassium excretion
State four causes of Cushing’s syndrome.
Pituitary adenoma Ectopic ACTH Oral glucocorticoid drugs Adrenal adenoma
What are the three main tests used to diagnose Cushing’s syndrome?
24-hour urine free cortisol Blood diurnal cortisol levels (or midnight serum cortisol) Low dose dexamethasone suppression test
Describe the results you’d expect from a normal subject and a patient with Cushing’s syndrome in the 24-hour urine free cortisol and blood diurnal cortisol tests.
You would expect lower cortisol at night in a normal subject and high cortisol in the morning. In someone with Cushing’s syndrome they would have high cortisol all the time. NOTE: a problem with this test is that the cortisol levels are affected by stress.
Explain the scientific basis of the low dose dexamethasone suppression test.
Dexamethasone is a glucocorticoid so by giving this extra glucocorticoid, it should suppress ACTH and reduce cortisol production. So in a normal subject undertaking the dexamethasone suppression test, you would expect zero cortisol. In a Cushing’s patient, cortisol will remain high despite the presence of dexamethasone.
State some surgical treatments for Cushing’s syndrome.
Treatment is dependent on cause Transsphenoidal Hypophysectomy (for Cushing’s disease) Bilateral adrenalectomy Unilateral adrenalectomy for adrenal mass
State two drugs that are used to treat Cushing’s syndrome before surgery.
Metyrapone Ketoconazole
Draw the adrenal steroid synthesis pathway.
-
Which enzyme is inhibited by metyrapone?
11-Beta-hydroxylase
What effect does metyrapone have on the steroid synthesis pathway?
It prevents the conversion of: 11-deoxycorticosterone –> corticosterone 11-deoxycortisol –> cortisol This means that no corticosterone or cortisol is produced
State two uses of metyrapone.
Preparation of a Cushing’s patient for surgery (improves their healing abilities and makes them better surgical candidates) Treatment of Cushing’s syndrome symptoms following radiotherapy (radiotherapy has a delayed effect)
State two negative aspects of metyrapone.
Metyrapone causes the accumulation of 11-deoxycorticosterone (because it doesn’t have any negative feedback effects on the ACTH axis) 11-deoxycorticosterone has mineralocorticoid effects so causes SALT RETENTION and HYPERTENSION. Metyrapone inhibits two limbs of the steroid synthesis pathway so it funnels the precursors towards the sex steroid synthesis pathway. This leads to increased adrenal androgens, which has effects such as hirsuitism.
State some unwanted effects of metyrapone.
Nausea, vomiting, dizziness Hypertension Hirsuitism Sedation, hypoadrenalism
What is ketoconazole and why is it no longer used?
Ketoconazole is an anti-fungal, which had an effect on the steroidsynthesis pathway. It is no longer used because of its hepatotoxicity.
What are the effects of ketoconazole on steroid production?
Ketoconazole inhibits cytochrome P450 short chain cleavage enzyme. This enzyme converts cholesterol –> pregnenolone This means that it inhibits the production of glucocorticoids, mineralocorticoids and sex steroids.
State some unwanted actions of ketoconazole.
Nausea, vomiting, abdominal pain Alopecia Gynaecomasia, oligospermia, impotence, decreased libido Ventricular tachycardias LIVER DAMAGE (could be fatal) These unwanted effects are due to loss of sex steroids, glucocorticoids and mineralocorticoids
What is Conn’s syndrome?
Aldosterone secreting adenoma of the adrenal gland (zona glomerulosa)
What are the two main features of Conn’s syndrome?
Hypertension Hypokalaemia
What is primary hyperaldosteronism?
Hyperaldosteronism caused by an adrenal adenoma.
What can you test to exclude secondary hyperaldosteronism?
Check for suppression of the renin-angiotensin system Measure aldosterone and if that’s high, measure the renin and that should be low because if would be suppressed by the high blood pressure.
What is the usual treatment plan for someone with Conn’s syndrome?
Medical management (spironolactone) Surgery to remove the tumour
What is spironolactone and how does it work?
Spironolactone is an aldosterone receptor antagonist It reduces the effects of aldosterone so it decreases sodium reabsorption and decreases potassium excretion.
What is the active metabolite of spironolactone that acts as a mineralocorticoid receptor antagonist?
Canrenone
What treatment would you give for someone with bilateral adrenal hyperplasia?
Long-term spironolactone You don’t want to remove both adrenals because then they wouldn’t produce any cortisol or aldosterone so you give them long-term spironolactone to reduce the effects of excess aldosterone.
Describe the pharmacokinetics of spironolactone.
Orally active Given daily in single or divided doses Highly protein-bound and metabolised in the liver
State some unwanted effects of spironolactone.
Spironolactone is very non-specific so it has several side effects Progesterone receptor agonist –> menstrual irregularities Androgen receptor antagonist –> gynaecomastia GI tract irritation Contra-indications: Renal and hepatic disease