Pituitary Hyperadrenocorticism

Question 1

• What are the two types of HAC?

Answer 1

Pituitary dependent hyperadrenocorticism & adrenal dependent hyperadrenocorticism

Question 2

• In horses what type of HAC is most common?

Answer 2

PDH in almost 100% of cases, affecting the pars intermedia

Question 3

• In dogs what type of HAC is most common?

Answer 3

PDH in 85% of cases, ADH in 15% of cases

Question 4

• List the hormones secreted by the pars distalis

Answer 4

TSH, ACTH, FSH, LH, GH/somatotropin, prolactin (PRL)

Question 5

• List the hormones that can be produced in the pars intermedia

Answer 5

ACTH, MSH & CLIP

Question 6

• How is the pars intermedia controlled?

Answer 6

Constant inhibition by dopamine, these neurones degenerate with age leading to loss of inhibition, hyperplasia and increased hormone production – increased POMC synthesis results in increased levels of ACTH, upregulated secretion continues because there is no feedback control

Question 7

• In dogs with PDH what is seen on the post mortem?

Answer 7

Bilateral adrenal gland hyperplasia

Question 8

• How is a pars intermedia tumour different if it does occur in dogs?

Answer 8

Affects the B cells (secrete ACTH), increased ACTH but not other POMC products

Question 9

• What are the differences between the control mechanisms of the pars distalis and the pars intermedia?

Answer 9

Pars distalis: -hormonal control, vascualr (portal), no nerve supply, stimulated by CRH, inhibited by cortisol (short feedback loop), primary site or ACTH release.
Pars Intermedia: - neuronal control, relatively avascualr, neurons from hyothalamic nuclei, inhibited by dopamine, stimulated by serotonin

Question 10

• What other pituitary lesions are possible?

Answer 10

Pituitary cysts, inactive chromophobes tumours & pituitary gland carcinomas

Question 11

• What are the consequences of PPID relating to αMSH & POMC products?

Answer 11

Delayed shedding, hirsuitism, lethargy, pain tolerance, laminitis

Question 12

• What is the physiology behind these signs?

Answer 12

ACTH and MSH allow hair follicles to survive for longer in catagen & prolong anagen (growth phase), hirsuitism is caused by an increase in αMSH

Question 13

• What causes polyuria/polydipsia?

Answer 13

Glucose spillover, neurogenic diabetes insipidus & antagonism of ADH receptors within the kidneys