Chapter 17

Question 1

What is the structure of thyroid glands?

Answer 1

Lobes containing follicles, lined by thyroid epithelium, contains colloid.

Question 2

What is the function of colloid?

Answer 2

Storage substance for thyroglobulin

Question 3

What is the most important enzyme in thyroid hormone synthesis?

Answer 3

TPO

Question 4

What is the main plasma protein thyroid hormone binds to in dogs?

Answer 4

Thyroxine-binding globulin

Question 5

What is the main plasma protein thyroid hormone binds to in cats?

Answer 5

Albumin and transthyretin

Question 6

What proportion of T4 is protein bound in dogs and cats?

Answer 6

99.9%

Question 7

What proportion of T3 is protein bound in dogs and cats?

Answer 7

99%

Question 8

How is thyroid hormone production regulated?

Answer 8

Hypothalamus produces TRH
Anterior pituitary produces TSH
TSH acts on thyroid cells, promotes trapping of iodine and hormone synthesis
Excess T4/T3 inhibit hypothalamus and pituitary

Question 9

What proportion of TgAA positive, euthyroid dogs become hypothyroid within a year?

Answer 9

5%

Question 10

Which breeds have had defects in thyroid peroxidase described?

Answer 10

Fox, Rat and Tenterfield Terriers

Question 11

What are the common CBC and biochem findings with hypothyroidism?

Answer 11

Mild non-regen anaemia
^ Trigs
^ Chol
^ ALKP
^ CK

Question 12

How do sulphonamides interfere with thyroid testing? What is the mechanism?

Answer 12

v T4 ^ TSH
Inhibit TPO

Question 13

What are the limitations of measuring TT3?

Answer 13

• High prevalence of T3AAs
• Deiodinase upregulated in failing thyroid function - TT3 maintained
• More sensitive to NTI
• Suppressive effect of medications

Question 14

What is the prevalence of TgAAs in hypothyroidism?

Answer 14

50%

Question 15

What the the prevalence of thyroid hormone antibodies in hypothyroid dogs?

Answer 15

TgAA +ve - T3AA 37%, T4AA 11%
TgAA -ve - exceptionally rare

Question 16

What % hypothyroid dogs have normal TSH?

Answer 16

15%

Question 17

How is thyroid hormone production in cats with hyperthyroidism?

Answer 17

Normal architecture replaced by multiple hyperplastic nodules

Question 18

What % of hyperthyroid cats have a) unilateral b) bilateral adenomas or c) carcinomas

Answer 18

a - 30%
b - 70%
c - <2%

Question 19

What are the common CBC findings in hyperthyroid cats?

Answer 19

Erythrocytosis
Macrocytosis
Stress leukogram

Question 20

What are the common biochemical findings in hyperthyroid cats?

Answer 20

^LE
Azotaemia
Hyperphosphataemia
Hyperglycaemia
Reduced fructosamine

Question 21

What % of hyperthyroid cats are azotaemic at diagnosis?

Answer 21

20%

Question 22

How does the prognosis differ between hyperthyroid cats who are a) azotaemic at diagnosis, b) non-azotaemic at diagnosis, c) non-azotaemic at diagnosis but develop azotaemia with treatment?

Answer 22

a - MST 6 months
b - MST 20 months
c - same as b

Question 23

What % of cats with hyperthyroidism have RR T4)

Answer 23

10-20%

Question 24

What % of sick euthyroid cats have elevated fT4?

Answer 24

20%

Question 25

What clinical pathological side effects are seen with antithyroid drug treatment?

Answer 25

Mild, transient lymphocystosis, eosinophilia of leucopenia in 15%
Neutropenia, thrombocytopenia +/- haemolytic anaemia in 5%
Hepatopathy in 2%

Question 26

What proportion of treated hyperthyroid cats develop azotaemia?

Answer 26

1/3

Question 27

What % of cats with thyroid carcinoma develop hyperthyroidism?

Answer 27

10-20%