Endocrine Physiology - Part 2 Flashcards
What is the Thyroid gland?
- A small gland located in the upper third of the neck, anterior to the trachea and just below the larynx in mammals
- 2 lobes connected by a narrow isthmus
What hormones are secreted by the thyroid?
- Thyroxine (T4)
2. Tri-iodothyronine (T3)
Describe the basic histology of the thyroid?
- 2 types of endocrine cells: thyroid hormone secreting cells and clear cells/C cells/para-follicular cells.
- follicles filled with colloid store thyroid hormones
What is required for the synthesis of Thyroid hormones?
iodine and tyrosine.
What can enhance all aspects of thyroid hormone synthesis?
TSH
How are thyroid hormones transported?
- 99% is bound with plasma proteins (released slowly to tissue cells by proteins)
What % of hormones released from the thyroid are T3 and what % are T4?
93% = T4 7% = T3
What tissues are effected by thyroid hormones?
almost all
Which thyroid hormones (T3 or T4) is more active than the other?
T3 is more active.
What happens to most of the T4 that is secreted?
converted to T3 in peripheral tissues by de-iodination
How do thyroid hormones work?
- activate nuclear transcription of a large number of genes resulting in the synthesis of many proteins
- this affects nearly all the cells in the body, which increases the functional activity of the whole body
What is the function of thyroid hormones?
- increases cellular metabolic activity
- increases O2 consumption in most tissues
- produces heat (calorigenic effect)
- maintains level of metabolism in the body
- increases number and activity of mitochondria
- increases activity of Na+-K+ATPase throughout the body
- enhances cardiac output
- increases rate of utilisation of food
What is the major factor that defines the body’s BMR?
plasma conc. of thyroid hormones
Describe thyroid levels in endotherms compared to ectotherms
generally elevated in endotherms, but in ectotherms it is selectively elevated during periods of metabolically demanding activity
What is the role of thyroid hormones in relation to body growth and the nervous system?
- increases GH secretion
- needed for development of CNS in foetus and after birth
- needed for normal CNS activity in adults
- needed for normal gonadal functions
- required for metamorphosis in frogs
How are thyroid hormones regulated?
- by the hypothalamo-pituitary-thyroid axis
- cold exposure enhances secretion
- starvation reduces seretion
What does euthyroid mean?
normal thyroid function
What is hyperthyroidism?
excess thyroid activity
What is the most common disorders in animals?
thyroid disporders
What is hypothyroidism?
deficient thyroid activity
What is one of the most common endocrine disorders in cats?
hyperthyroidism
What is the function of the adrenal medulla?
- produces catecholamines: adrenaline and noradrenaline (aka epinephrine and norepinephrine)
What stimulates the adrenal medulla?
- sympathetic nervous system, fight or flight response
- hypoglycaemia
- decreased BP
- cold exposure
What is the target of adrenaline and noradrenaline?
heart, smooth muscle, pancreas, eyes
What is Pheochromocytoma?
tumour of the adrenal medulla causing excessive secretion of catecholamines
What is the function of the adrenal cortex?
secretion of steroid hormones
What are the three layers of the adrenal cortex? (outside to inside)
- zona glomerulosa
- zona fasciculata
- zona reticularis
What is secreted by the zona glomerulosa?
mineralcorticoids
What is secreted by the zona faciculata?
glucocorticoids
What is secreted by the zona reticularis?
sex steroids (androgens)
What is the main glucocorticoid in mammals?
cortisol
What are the metabolic effects of glucocorticoids?
- increases blood sugar (needed during starvation)
- enhances protein catabolism and lipid lipolysis
- inhibits growth
- adapts body to long term stress
- anti-inflammatory and immunosuppressive effects
What is the clinical use for glucocorticoids?
- used to suppress inflammatory response (inhibits production of substances that promote inflammation)
- inhibit allergic reactions
How are glucocorticoids regulated?
- by hypothalmic-pituitary-adrenal axis
- influenced by stress and circadian rhythm
When are glucocorticoid levels highest in a normal animal?
- in diurnal animals: highest in the morning
- reverse in nocturnal animals
What is the main mineralocorticoid?
aldostrone
how are mineralocorticoids regulated/ stimulated?
- angiotensin II
- elevated ECF K+ conc.
- reduction of ECF Na+ conc.
- renin
What is the target tissue of mineralocorticoids?
renal tubules
What is the effect of mineralocorticoids?
- stimulates Na+ reabsorption and excretion of K+ and H+ at the kidneys.
- major role in regulating ECF volume and therefore BP
What happens if there is no aldosterone?
- ECF K+ rises
- sodium and chloride lost
- ECF vol falls
- cardiac output decreases
- shock
- death
What is hyper-adrenocorticism (cushing’s syndrome)?
- over secretion of CRH/ ACTH
- leads to excessive glucose (adrenal diabetes), protein shortage, fat deposition in abdomen
What is primary adrenocortical insufficiency (addison’s disease)?
- low BP, brady, weak pulse, prolonged CRT
- lerthargy
- lack of appetite, diarrhoea
- syncope
- hypoglycaemia
- hyponatremia
- hypokalemia
What is hyperaldosteronism (Conn’s disease)?
caused by eithr an adrenal tumour hypersecreting aldosterone, or bilateral adrenal hyperplasia
What are the two main causes of Cushing’s syndrome?
- pituitary gland tumour
1. adrenal gland tumour
