Equine Endocrinopathies

Question 1

PPID.

Answer 1

Common in aged horses.
60% over 20s at PM have signs of pituitary adenoma.
- may not be seen grossly.
Clinical signs in 20-30 over 15s.
Can occur in younger ones but not really less than 10.

Question 2

Normal functioning?

Answer 2

HT produces dopamine.
Dopamine acts on pituitary and calms it.
Pituitary makes a group of things called Pro-opiomelanocortins (POMCs).

Question 3

PPID effect on functioning?

Answer 3

Still not fully understood.
Decrease in production of dopamine from HT.
- decrease in inhibition of pituitary.
- pituitary adenoma.
- overproduction of POMCs.
- range of clinical signs.

Question 4

PPID clinical signs.

Answer 4

Variable.
Long curly coat - unknown cause.
Laminitis.
- IR.
- PPID on ddx in any horse with laminitis.
PUPD.
- ?decreased secretion of vasopressin.
Weight loss.
- ?cortisol production.
- ?associated other disease.
- Note parasite burden.
– high cortisol reduces immunity.
Docile - increased CSF B-endorphin.
Neurologic impairment
- compression from adenoma.
Hyperhidrosis - cause unknown.
Change in fat distribution, bulging fat pads.
Infertility.
Skin disease.
Periodontal disease.

Question 5

Dx of PPID.

Answer 5

Clinical signs and signalment.
Gold standard = PM only.
Individual variation in POMC production.
Pars intermedia more active in Autumn (Aug-Dec).
- used to advise not to test during this period.
- now know we can test with adjusted reference ranges.
Best first-line test (field) = resting plasma ACTH concentration.
- horse should not be stressed.
- collect blood.
– cold plastic EDTA tube.
– keep below 4C.
– centrifuge within 8hrs.
– send frozen plasma for analysis.
– look for ACTH elevation.
- ADJUST REF RANGE FOR AUTUMN!
Second line test less commonly used = overnight dexamethasone suppression test.
- if administer exogenous cortisol, should down-regulate endogenous cortisol production.
TRH stimulation test next.
- first-line in hospital.
- more sensitive than ACTH.
- blood for baseline ACTH.
- inject TRH.
- they feel funky…
- blood for ACTH at 10 and 30 mins.

Question 6

Other diagnostics for PPID.

Answer 6

Baseline cortisol.
- variable.
Urinary corticoid / creatinine ratio.
- too many false positives and negatives.
ACTH stim test,
- variable results.
Combined dex suppression and TRH stim test.
- may be best test but expensive and multiple sample collections required.
Insulin concentration.
- often IR.
- may habe EMS instead / as well.
- may help management but not firm dx.

Question 7

What if positive test result?

Answer 7

Remember risk of false positives (esp. in autumn).
Start tx.
Repeat test in 4-6 weeks to check dose.
Repeat annually as disease progresses.

Question 8

1. What if negative test result but high clinical suspicion?
2. Routine checking of aged horses?

Answer 8

1. (Rare now have TRH stim).
   Start tx anyway?
   Assess clinical response.
2. Judge case by case.
   Based on finances and owner abilities.

Question 9

Tx if PPID.

Answer 9

Combination of management…
- farriery.
- clipping.
- parasite control.
- dental care.
- feeding.
…and medication.
- Dopamine agonist (Pergolide - Prascend).
– tabs.
– 0.2-5mg/horse/day, once daily.
– start with 1mg/horse.
– monitor, adjust dose, reassess annually.
– may go off food, resolves on lower dose.
– usually v. effective.
– if refractory even to higher doses, split dose to twice daily.
–

Question 10

Thyroid in the horse.

Answer 10

2 lobes over position of proximal trachea.
Usually palpable but not visible.
Understanding thyroid hormone functions important for dx and tx.
Thyroid dz in horses quite rare.

Question 11

Thyroid hormones in the horse.

Answer 11

Triiodothyroidine T3.
Thyroxine T4.
Thyroid hormones important for cell growth, differentiation, metabolism in nearly all tissues.

Question 12

Factors that affect thyroid gland function and testing.

Answer 12

Age.
Gender.
Breed.
Hormones.
Season.
Disease.
Activity.
Feeding.
Iodine supplementation.
Medication.

Question 13

Normal functioning.

Answer 13

HT produces thyrotropin releasing hormone (TRH).
TRH travels to pituitary gland which produces thyroid stimulating hormone (TSH).
TSH travels to thyroid which brings in iodine and makes T3 and T4 which is released.
All works via negative feedback system.

Question 14

Diagnostic testing for thyroid in horses.

Answer 14

Generally measure T4 initially.
If result uncertain, TSH stim test or TRH stim test.
May do:
- scintigraphy.
- U/S.
- biopsy.

Question 15

Hypothyroidism in foals cause.

Answer 15

Major cause is nutritional during gestation.
- inadequate OR excessive iodine intake by mare.
Low TH causes lack of pituitary inhibition causes excess TSH secretion causes thyroid enlargement.

Question 16

Clinical signs of hypothyroidism in foals.

Answer 16

Thyroid enlargement (goitre), but not in all.
Multiple other signs:
- stillbirth, weakness, defective ossification.
- can be born normal, develop skeletal lesions when weeks old.

Question 17

1. Dx of hypothyroidism in foals.
2. Px of hypothyroidism in foals.

Answer 17

1. Difficult.
   - TH concentrations vary in foals.
   - TSH/TRH stim tests.
   - BUT thyroid gland function may be normal once foal is born.
   Look at mare’s diet - beware seaweed supplementation.
2. Very poor even after thyroid supplement therapy.
   - as problem happened during development so cannot be reversed.

Question 18

Hypothyroidism in adult horses.

Answer 18

Seems extremely rare.
- not really tested for so rarely seen.
Non-specific signs.
Anaemia, irregular oestrous cycle, poor performance.
TH low, TRH, TSH high.
Levothyroxine as tx.

Question 19

Hyperthyroidism in horses.

Answer 19

Few documented cases.
Tremors, excitability, tachycardia, sweating.
High T3 and T4.
Anti-thyroid therapy with potassium iodide.

Question 20

Thyroid adenoma.

Answer 20

In 30% of aged horses.
In 0% of under 18s.
Enlarged thyroid, often unilateral.
Usually no thyroid dysfunction.
Biopsy - benign.
Hemithyroidectomy if size interfering with eating or exercise.
Other types of tumour are more rare.

Question 21

Granulosa-theca cell tumour.

Answer 21

Benign.
Any age affected.
Poor fertility.
Abdominal discomfort.
Classic sign = stallion-like behaviour.
- BUT not all mares with GCT show stallion-like behaviour.
- AND not all mares with stallion-like behaviour have GCT.

Question 22

Diagnosing GCT.

Answer 22

Rectal - enlarged ovary.
Usually unilateral and other often small.
U/S multiloculated.
Anti-mullerian hormone newest and best test.
(Testosterone elevated in 50-90%).
Inhibin is a better indicator.

Question 23

Tx of GCT.

Answer 23

Surgical removal.
- standing laparoscopic assisted if possible.
- OR laparotomy if very large (longer recovery).
- usually only need to remove one ovary.
- histopathology.
- surgery and histo even if AMH negative.
Fertility should return.