Pituitary pars intermedia dysfunction + thyroid disease

Question 1

What is the pathophysiology of PPID?

Answer 1

• Neurodegenerative lesion - loss of dopaminergic inhibition = excess pars intermedia hormones (B-endorphin, CLIP + a-MSH + ACTH)

Question 2

What is the epidemiology of PPID?

Answer 2

• Usually >15y/o (19-24 y/o at diagnosis)
• No sex predilection
• Ponies more likely than horses

Question 3

What are clinical signs of PPID?

Answer 3

** Hypertrichosis (abnormal coat growth)
** Laminitis (50-80%) - may be sub-clinical

• Weight loss / weight redistribution
• Wasted epaxial muscles + pot belly
• Bulging supraorbital fat
• Lethargy / exercise intolerance
• Sweating
• PUPD
• Susceptibility to infections

Question 4

How is PPID diagnosed?

Answer 4

Tier 1
* Basal ACTH - most convenient
Tier 2
* Dynamic endocrine tests:
- overnight dexamethasone suppression test - most accurate
- thyrotropin-releasing hormone (TRH) stimulation test

Question 5

What is ODST? (overnight dexamethasone suppression test)

Answer 5

1. Give cortisol 40ug/Kg IM (or IV) (usually around 5pm)
2. Retest cortisol 19 hours later
   - positive if cortisol >27nmol/l (or >10ng/ml)

Question 6

What are the problems with ODST?

Answer 6

• False positives in summer/autumn
• False negatives in early/mild cases

Question 7

What is TRH stimulation test?

Answer 7

1. Measure baseline ACTH
2. Administer TRH IV (0.5mg <250kg / 1mg >250kg)
3. Resample ACTH exactly 10 mins later

Question 8

What are side effects of TRH administration?

Answer 8

• Muscle fasciculations
• Licking
• Chewing
• Flehmen response
• Yawning
• Coughing

Question 9

What can hyperinsulinaemia indicate?

Answer 9

• Predictor of laminitis risk
• Horses with PPID + high insulin = more likely to develop laminitis + not survive 2 years

Question 10

When should you treat PPID?

Answer 10

• Medical therapy improves quality of life
• Many vets + owners will wait till clinical laminitis develops

Question 11

What can you use to treat PPID?

Answer 11

• Pergolide
• start with 2ug/kg/day

Question 12

How can you monitor response to Tx?

Answer 12

• Obtain baseline endocrine values - basal ACTH, basal insulin (+glucose)
• Document CS findings
• hair coat
• appetite, water intake / urination
• BCS / muscle loss
• laminitis / lameness
• general demeanour

Question 13

What is suggested protocol for monitoring?

Answer 13

• Monthly evaluation of ACTH + insulin for 3 months
  + if stable
• 3 monthly evaluation for 9 months
• Every 6 months in selected, well managed cases
• If poor response increase dose (up to 5x starting dose)

Question 14

What may high dosage lead to?

Answer 14

• Inappetence
• reduce / stop tx for a few days
• if continued / evidence of weight loss = dental exam, blood tests to look for alternatives

Question 15

What should be done if continued laminitis / high insulin?

Answer 15

• Careful dietary control - restricting cereals/grass
• Increase exercise if possible (laminitis depending)
• Medical therapy for EMS

Question 16

What is prognosis of PPID?

Answer 16

• Life long treatment + management

Question 17

How common is hypothyroidism in horses?

Answer 17

• Very rare
  can do TRH stimulation test (measure T3 + T4 2-4hrs later)

Question 18

What can cause hypothyroidism?

Answer 18

• Feeding excess iodine
• Feeding seaweed / kelp supplements
• Seen in foals if fed to pregnant mares