Chapter 17 Flashcards
What is the structure of thyroid glands?
Lobes containing follicles, lined by thyroid epithelium, contains colloid.
What is the function of colloid?
Storage substance for thyroglobulin
What is the most important enzyme in thyroid hormone synthesis?
TPO
What is the main plasma protein thyroid hormone binds to in dogs?
Thyroxine-binding globulin
What is the main plasma protein thyroid hormone binds to in cats?
Albumin and transthyretin
What proportion of T4 is protein bound in dogs and cats?
99.9%
What proportion of T3 is protein bound in dogs and cats?
99%
How is thyroid hormone production regulated?
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
What proportion of TgAA positive, euthyroid dogs become hypothyroid within a year?
5%
Which breeds have had defects in thyroid peroxidase described?
Fox, Rat and Tenterfield Terriers
What are the common CBC and biochem findings with hypothyroidism?
Mild non-regen anaemia
^ Trigs
^ Chol
^ ALKP
^ CK
How do sulphonamides interfere with thyroid testing? What is the mechanism?
v T4 ^ TSH
Inhibit TPO
What are the limitations of measuring TT3?
- High prevalence of T3AAs
- Deiodinase upregulated in failing thyroid function - TT3 maintained
- More sensitive to NTI
- Suppressive effect of medications
What is the prevalence of TgAAs in hypothyroidism?
50%
What the the prevalence of thyroid hormone antibodies in hypothyroid dogs?
TgAA +ve - T3AA 37%, T4AA 11%
TgAA -ve - exceptionally rare
What % hypothyroid dogs have normal TSH?
15%
How is thyroid hormone production in cats with hyperthyroidism?
Normal architecture replaced by multiple hyperplastic nodules
What % of hyperthyroid cats have a) unilateral b) bilateral adenomas or c) carcinomas
a - 30%
b - 70%
c - <2%
What are the common CBC findings in hyperthyroid cats?
Erythrocytosis
Macrocytosis
Stress leukogram
What are the common biochemical findings in hyperthyroid cats?
^LE
Azotaemia
Hyperphosphataemia
Hyperglycaemia
Reduced fructosamine
What % of hyperthyroid cats are azotaemic at diagnosis?
20%
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?
a - MST 6 months
b - MST 20 months
c - same as b
What % of cats with hyperthyroidism have RR T4)
10-20%
What % of sick euthyroid cats have elevated fT4?
20%
What clinical pathological side effects are seen with antithyroid drug treatment?
Mild, transient lymphocystosis, eosinophilia of leucopenia in 15%
Neutropenia, thrombocytopenia +/- haemolytic anaemia in 5%
Hepatopathy in 2%
What proportion of treated hyperthyroid cats develop azotaemia?
1/3
What % of cats with thyroid carcinoma develop hyperthyroidism?
10-20%
