Lecture 22 5/1/25

Question 1

What are the characteristics of insulin dysregulation and laminitis?

Answer 1

-major predisposing factor for laminitis in both PPID and EMS
-laminitis can be induced by giving insulin
-laminitis is most common when NSC in pasture is high

Question 2

What is the current theory behind why hyperinsulinemia can cause laminitis?

Answer 2

-hyperinsulinemia causes inappropriate stimulation of insulin-like growth factor-1 receptors on lamellar epidermal cells
-stretching and elongation of lamellae occurs without disruption of basement membrane

Question 3

How is endocrinopathic laminitis managed?

Answer 3

-control pain
-support the feet
-control hyperinsulinemia; dietary management, pergolide for PPID
-weight loss

Question 4

What are the characteristics of thyroid disease?

Answer 4

-common in other species
-controversial in horses
-accurate diagnosis is difficult
-hypothyroidism thought to be rare and often a misdiagnosis

Question 5

What are the characteristics of baseline thyroid hormone concentrations in horses?

Answer 5

-includes total and free fractions of T3 and T4
-normal values vary with lab/method
-affected by physiologic, pathologic, and pharmacologic factors
-values are 10 to 20x higher in neonates than adults due to involvement in growth
-single measurements of serum concentrations are NOT accurate in diagnosis of thyroid dz

Question 6

What are the characteristics of TSH concentration measurement?

Answer 6

no commercially available, validated test for equine TSH

Question 7

What are the characteristics of trophic response tests?

Answer 7

-TRH or TSH stimulation tests
-superior to baseline thyroid hormone conc. measurement
-likely the most accurate tests for assessing the equine thyroid
-inconvenient; expensive, need multiple samples, TRH/TSH not readily accessible

Question 8

What were the hormone findings in horses with induced hypothyroidism?

Answer 8

-decrease in response to thyroid hormone concentrations
-increase in TSH concentrations
-altered response to TRH

Question 9

What should be done when suspicious of thyroid disease in a horse?

Answer 9

-consider and rule out other factors that could affect thyroid function
-measure free T4
-TRH stim. test
-do NOT begin replacement therapy based on low thyroid hormone conc. alone

Question 10

What are additional diagnostic tests that may be helpful in assessing the equine thyroid?

Answer 10

-aspirate
-biopsy
-ultrasound
-scintigraphic imaging
-basal metabolic rate

Question 11

Which conditions are ANECDOTALLY linked to hypothyroidism?

Answer 11

-obesity
-laminitis
-infertility
-anhidrosis

Question 12

What are the clinical signs seen in horses that are experimentally made hypothyroid?

Answer 12

-weight gain or loss (or no change)
-decreased heart and resp rate
-decreased temp
-dull hair coat
-appear outwardly almost normal and active

Question 13

What are the characteristics of thyroid supplementation in horses?

Answer 13

-apparent response to treatment does not confirm hypothyroidism
-euthyroid animals may experience increased basal metabolic rate and activity
-no data on whether supplementation in euthyroid animals is harmful
-can lead to decreased endogenous TSH and thyroid gland atrophy

Question 14

What are the characteristics of euthyroid sick syndrome?

Answer 14

-serum concentrations are low in animals with non-thyroidal systemic illness and normal gland function
-adaptation to reduce energy expenditure and activate innate immune response
-degree of thyroid suppression may correlate with severity of disease/mortality
-difficult to control for drugs and anorexia when testing

Question 15

What are the mechanisms behind euthyroid sick syndrome?

Answer 15

-decreased peripheral deiodination
-altered binding to carrier proteins
-hypothalamic-pituitary dysregulation

Question 16

What are the potential thyroid diseases in foals?

Answer 16

-congenital hypothyroidism and dysmaturity (CHD)
-iodine excess or deficiency
-endophyte infested fescue
-idiopathic goiter

Question 17

What are the characteristics of CHD?

Answer 17

-thyroid hyperplasia and musculoskeletal deformities
-prolonged gestation; normal in size but appear immature
-generally thyroids are not grossly enlarged

Question 18

What are the signs of CHD?

Answer 18

-contracted tendons +/- rupture of common digital extensor
-mandibular prognathism/monkey mouth
-hypo-ossification; esp. cuboidal bones

Question 19

What can lead to CHD/thyroid dz in foals?

Answer 19

-mustard plants/thyrotoxic glucosinates
-selenium deficiency
-green feed/nitrates
-lack of mineral supplementation

Question 20

What is the prognosis for CHD?

Answer 20

-varies with severity
-become euthyroid over time
-mandibular prognathism eventually corrects
-severe cases may have long-term musculoskeletal complications

Question 21

What are the characteristics of goiter in foals?

Answer 21

-occurs with iodine excess or deficiency
-may be seen in foals born to mares grazing Acremonion coenophialum-infected fescue

Question 22

What are the characteristics of thyroid tumors?

Answer 22

-seen in 32% of horses > 10 years and 75% of horses over 20 years
-can be adenoma, adenocarcinoma, or medullary/C cell tumors
-difficult to diagnose with aspirate/biopsy
-most commonly non-functional adenomas

Question 23

What are the charactersitics of hyperthyroidism in horses?

Answer 23

-uncommon
-present with weight loss, hyperactive behavior, and polyphagia
-can lead to thyroid enlargement over months to years

Question 24

Which tests are done to assess for hyperthyroidism?

Answer 24

-plasma thyroid hormone conc. (high conc.)
-T3 suppression test (lack of T4 suppression)
-TRH stim test (inappropriate increases)

Question 25

What are the characteristics of hemithyroidectomy?

Answer 25

*indications: -thyroid dysfunction -large thyroid mass *main complication is ipsilateral laryngeal hemiplegia *can also administer propylthiouracil

Question 26

What can cause hypercalcemia in horses?

Answer 26

*hypercalcemia of malignancy -PTH-related peptide *renal failure -presents with azotemia and low USG *primary hyperparathyroidism -rare -may present with fibrous osteodystrophy/osteopenia -commonly have hypophosphatemia

Question 27

What are the characteristics of adrenal disorders in horses?

Answer 27

-typically critical illness related corticosteroid insufficiency/relative adrenal insufficiency -both high and low cortisol levels have been associated with a poor prognosis in critically ill patients

Question 28

What are the characteristics of pheochromocytoma?

Answer 28

-most common adrenal medullary tumor in horses -often non-functional -functional tumors lead to increased catecholamines

Question 29

What are the types of diabetes insipidus?

Answer 29

-neurogenic: decreased release of ADH -nephrogenic: decreased sensitivity to ADH

Question 30

Which fungal endophyte causes fescue toxicity?

Answer 30

Neotyphodium coenophialum

Question 31

What are the clinical signs of fescue toxicity in adult horses?

Answer 31

-typically unaffected -anorexia/weight loss -poor hair quality -pyrexia -hypersalivation

Question 32

What are the clinical signs of fescue toxicity in pregnant mares?

Answer 32

-agalactia*** -abnormal placentation -prolonged gestation -abortion or weak foals

Question 33

What is the pathophysiology of fescue toxicity?

Answer 33

-ergopeptine alkaloids act as D2 receptor agonists -inhibits prolactin secretion -inhibits ACTH secretion; reduces fetal cortisol and progesterone secretion by placenta

Question 34

What is the treatment/management for fescue toxicity?

Answer 34

-treat with domperidone -remove pregnant mares from fescue for last 30 to 90 days of gestation; longer is better -can grow endophyte free fescue; does not grow as well and eventually gets infested anyway

Question 35

What are the characteristics of refeeding syndrome?

Answer 35

-occurs in horses being fed following starvation -feeding results in rise in insulin -affects CHO metabolism -moves electrolytes into cells -especially causes hypophosphatemia

Question 36

What are the recommendations for refeeding starved horses to try and avoid refeeding syndrome?

Answer 36

-start gradually -use a diet with a low glycemic index -supplement electrolytes if starvation was severe