Thyroid Diseases

Question 1

Thyroid Physiology

Answer 1

Hypothalamus > TRH
Pituitary Gland > TSH
Thyroid Gland > T4 and T3
>Body

Question 2

T4

Answer 2

Thyroxine
Major secretory product of the normal thyroid gland

Question 3

T3

Answer 3

Triiodothyronine
Most active thyroid hormone
Produced by deiodination of T4 (mostly in tissues)
Responsible for negative feedback mechanism to pituitary and hypothalamus

Question 4

Free T3 and T4

Answer 4

Active form

Question 5

Thyroid hormone functions

Answer 5

Increases:
-Basal metabolic rate
-O2 consumption
-HR
-Catabolism of fat and muscle
-Protein synthesis
-Erythropoiesis
-Sensitivity to catecholamines

Critical for development and differentiation of cells

Question 6

What is the most common endocrinopathy in dogs?

Answer 6

Hypothyroidism

Question 7

Clinical Signs of Hypothyroidism

Answer 7

Dermatologic: bilateral, symmetrical alopecia, pyoderma, otitis, demodicosis, facial myxoedema
Metabolic: lethargy, inactivity, weight gain
Neuromuscular: weakness, ataxia

Question 8

Canine Hyperthyroidism Lab Abnormalities

Answer 8

Hypercholesterolemia (>75%)
Mild, non-regenerative anemia

Question 10

Pathogenesis of Acquired Hypothyroidism
(Primary, secondary, and tertiary)

Answer 10

Primary: Thyroid gland doesn’t produce hormone (95% of cases)
Secondary: Pituitary doesnt produce TSH
Tertiary: Hypothalamus doesnt produce TRH

Question 11

Causes of Acquired Hypothyroidism (Primary)

Answer 11

Lymphocytic thyroiditis
Idiopathic atrophy
Neoplasia (thyroid carcinoma, squamous cell carcinoma)
Toxins
Surgical removal
I-131

Question 12

Total T4 Test

Answer 12

Good for excluding (if normal = not hypothyroid)
Frequently low with normal thyroid function (euthyroid sick syndrome, drugs, random fluctuations)
Can be artificially increased

Question 13

Free T4 test

Answer 13

Most accurate for dogs
T4 should be low if hypothyroid

Question 14

Total T3 and free T3 tests

Answer 14

Not recommended as sole test
Often WNL in hypothyroid dogs
T3 should be low in hypothyroid dog

Question 15

If patient is hypothyroid, what should T4 and T3 levels be?

Answer 15

Low

Question 16

If patient is hypothyroid, what should TSH levels be?

Answer 16

High, no negative feedback

Question 17

What age range is most common for equine hypothyroidism?

Answer 17

Foals, rare in adults

Question 18

Equine Hypothyroidism CS

Answer 18

In foals:
Enlarged thyroid glands (goiters)
Weakness
Incoordination
Poor suckle reflex
Ossification/tendon defects

Question 19

Equine hypothyroidism pathogenesis

Answer 19

Foals:
Dietary iodine deficiency in mare
Ingestion of goitrogen by mare (brassicas, sudan grass)
Adults:
Neoplasia

Question 20

Equine Hypothyroidism Diagnosis

Answer 20

total T3, T4
free T3, T4
Not TSH

Question 21

Feline Hypothyroidism

Answer 21

Rare
Most common iatrogenic (bilateral thyroidectomy, overdose of a antithyroid drug)

Question 22

Hyperthyroid in dogs and horses

Answer 22

Rare

Question 23

Feline Hyperthyroidism

Answer 23

Most common endocrinopathy in cats

Question 24

Feline Hyperthyroidism CS

Answer 24

Weight loss
Polyphagia
PU/PD
Increased activity
Restlessness
GI: V/D
Skin changes
Respiratory signs

Question 25

Feline Hyperthyroidism lab abnormalities

Answer 25

Increased liver: ALT, ALP (>90%)
ERythrocytosis
Excitement leukogram: neutrophilia, lymphocytosis
Increased BUN (creatinine WNL)
Decreased cholesterol
Falsely decr fructosamine

Question 26

Feline hyperthyroidism pathogenesis

Answer 26

Excess production of T4 and T3 by thyroid gland: almost always primary (start w/ thyroid)

Question 27

Feline hyperthyroidism diagnosis

Answer 27

Total T4 best method
(Should be high)