Thyroid Function

Question 1

What is the hormonal control of thyroid glands?

Answer 1

Thyrotropin-releasing hormone in the hypothalamus causes the release of TSH in the pituitary
TSH binds to receptors and stimulates the release of T3 and T4

Question 2

What hormones does the thyroid produce?

Answer 2

All T4, but only 10 to 40% of T3

Question 3

What is most T3 produced by?

Answer 3

Other cells throughout the body by deinodination of T4

Question 4

What inhibits the production of TSH?

Answer 4

T3

Question 5

What is the synthesis of T3 and T4 influenced by?

Answer 5

Availability of iodide, sensitivit to TSH, and ratio T3:T4

Question 6

What is thyroglobulin produced by?

Answer 6

Luminal thyroid cells

Question 7

What is thyroglobulin stored in?

Answer 7

Follicular lumina

Question 8

What are most T4 and T3 like in blood?

Answer 8

Protein bound

Question 9

What are T4 and T3 that are not bound to proteins called?

Answer 9

Free T4 and free T3

Question 10

What is the biologically active form of T4?

Answer 10

fT4

Question 11

Which is more potent, fT3 or fT4?

Answer 11

fT3 is 3-5 times more potent

Question 12

What happens when T4 and T3 enter cells and bind to receptors?

Answer 12

Increase metabolic rate
Increased O2 consumption
Increased HR
Ehnaced response to catecholamines
Bone formation and resorption
Increased catabolism of muscle and adipose tissue
Protein synthesis
Erythropoiesis
Changes in lipoprotein metabolsim

Question 13

What are T4 and T3 excreted in?

Answer 13

Bile and urine

Question 14

What sample is needed for canine T4 and T3?

Answer 14

EDTA plasma or serum

Question 15

What species is fT4 lower in?

Answer 15

Much lower in dogs than cats, horses, and humans

Question 16

What can falsely increase T4 and T3?

Answer 16

T4AA and T3AA

Question 17

What is the best test for fT4?

Answer 17

Equilibiurm dialysis

Question 18

What causes increase production of T4?

Answer 18

Increased production of T4 by thyroid neoplasia

Administration of T4, TSH, or TRH

Question 19

What is the most common presenting problem in a cat with increased T4 due to hyperthyroidism?

Answer 19

Weight loss and polyphagia

Question 20

What causes decreased tT4?

Answer 20

Primary hypothyroidism
Ssecondary hypothyroidism caused by TSH deficiency
Tertiary hypothyroidism caused by TRH deficiency
Defective T4 production
Sick euthyroidism
Inflammatory, neoplastic, metabolic, and endocrine
Steroids
Lower in larger dogs (Greyhounds)

Question 21

What causes increased tT4 by equilibrium dialysis?

Answer 21

Same disorders that increases T4

Question 22

What causes decreased tT4 by equilibrium dialysis?

Answer 22

Causes of decreased production of T4
Hyperadrencocorticism
Inflammation
Treatments with certain drugs (steroids)

Question 23

What do basal tT3 and fT3 assess?

Answer 23

Conversion of T4 in T3

Question 24

Why is tT3 measured?

Answer 24

To monitor therapy and owner compliance

Question 25

What causes increased canine TSH?

Answer 25

Primary hypothyroidism

Question 26

What do 60 to 75% of dogs with hypothyroidism have?

Answer 26

Increased TSH

Question 27

What was once the gold standard for establishing thyroid hypoplasia?

Answer 27

Poor or inadequate response to TSH

Question 28

What does a TRH response tetst do?

Answer 28

TRH --> TSH --> T4 (indirect)

Question 29

Why do most cats with hyperthyroidism no need a T3 suppression test?

Answer 29

The have increased T4

Question 30

What should differentiate hyperthyroid from euthyroid in cats?

Answer 30

T3 Suppression Test