Thyroid

Question 1

Stages of thyroid hormone synthesis (9)

Answer 1

1) Thyrogobulin is synthesised by follicular cells and secreted into the folicular lumen
2) TSH binding of TSH receptor stimulated sodium-iodide symporter to trap iodide
3) Iodide in cell transported to follicular lumen by Pendrin
4) Iodide oxidised to iodine by thyroid peroxidase
5) Iodine enters follicle lumen where it is attached to tyrosine to form MIT and DIT
6) MIT and DIT couple to form T3 and T4
7) Thyroglobulin colloid in endocytosed and combined with lysomome
8) Lysomomal enzymes cleave thyroglobulin to separate T4 and T3
9) T4 and T3 diffuse into blood streamffuse into blood stream

Question 2

Regulation of thyroid hormone secretion

Answer 2

TRH stimulates TSH
TSH bind TSH receptors on thyroid gland (cAMP messaging)
Negative feedback

Question 3

Signs of congential hypothyroidism (3)

Answer 3

Disproportionate dwarfism
Macroglossia
Delayed dental eruption

Question 4

Causes of primary hypothyroidism (5)

Answer 4

Lymphocytic thyroiditis
idiopathic atrophy
neoplastic destruction
iodine deficiency
iatrogenic (surgical removal)

Question 5

Causes of congenital hypothyroidism (2)

Answer 5

Thyroid gland dysgenesis
Dyshormogenesis
Thyroid peroxidase deficiency - Fox terrier

Question 6

Causes of secondary hypothryoidism (3)

Answer 6

Pituritary malformation, neoplastic destruction of the pituitary gland, iatrogenic (glucocorticoids, hyposphysectomy)

Question 7

Cause of tertiary hypothyroidism (2)

Answer 7

Neoplasia - destruction of the hypothalamus

Deficient TRH molecule

Question 8

Age of hypothyroidism

Answer 8

middle aged- older dogs

Question 9

Breeds associated with hypothryoidism (2)

Answer 9

Golden Retriever and Doberman

Question 10

Pathophysiology of hypothyroidism clincial signs

Answer 10

due to decreased metabolic rate

Question 11

3 most common clinical signs of hypothyroidism

Answer 11

Lethargy, alopecia, weight gain,

Question 12

What rhythm disturbance can be seen with hypothyroidism

Answer 12

Atrial fibrilation

Question 13

What neruomuscular signs can be seen with hypothroidism? (6)

Answer 13

Polyneuropathy, myxedema coma, facial nerve paralysis, possible laryngeal paralysis, possible megaoesophagus, cricopharyngeal achalasia

Question 14

What other endrocrinopathy can hypothryoidism be associated with (3)

Answer 14

hT and DM (1-10%)
hT and hAC (4%)
Can have hT, GH excess and DM

Question 15

Laboratory changes associated with hypothyroidism and pathophysiology behind them (5)

Answer 15

Increased cholesterol and triglycerides - impaired degradation
Elevated fructosamine - reduced protein turn over
Mild anaemia - decreased EPO
Increased homocysteine
Mild increase ALP and GGt- increased hepatic lipid deposition

Question 16

Most sensitive thyroid test

Answer 16

TT4 (89-100%)

Question 17

Most specific thyroid test

Answer 17

TSH (82-100%)

Question 18

Least affected by non-thyroidal illness (dog)

Answer 18

fT4

Question 19

Prevalence TGAA

Answer 19

< 2% samples

Question 20

What affects the sensitivity and specificty of T3

Answer 20

Low due to high circulating anti-T3 antbodies

Question 21

Why does TSH increase with hypothyroidism

Answer 21

loss of negative feedback

Question 22

Impact TGAA on T4

Answer 22

False increase using RIA (most common)

Other techniques would decrease

Question 23

Breeds with low T4

Answer 23

sighthounds e.g. Greyhounds (measure T3 may be helpful in them)

Question 24

Drugs that reduce Tt4 (5)

Answer 24

Prednisolone, phenobarbital, sulfonamides, aspirin, ketoprofen

Question 25

Testing thyroid function after cessation of treatment

Answer 25

Generally 6-8 weeks after but a recent study suggests can assess 1 week later

Question 26

Clinical manifestation of myxedema coma

Answer 26

Profound weakness, hypothermia, bradycardia, stupor/coma

Question 27

T4 and TSH consistent with non-thyroidal illness (dog)

Answer 27

low TT4 and normal TSH

Question 28

Confirmation of hypothyroidism

Answer 28

Low TT4, high TSH

Question 29

Possible causes of normal TT4 and high TSH

Answer 29

Subclinical hypothyroidism
Interference TGAA
Recovery from illness
Recent withdrawal of leveothyroxine

Question 30

Likleyhood normal fT4 and TgAA negative

Answer 30

unlikely

Question 31

Evidence of lyphocytic thyroiditis

Answer 31

decrease fT4, TGAA positive

Question 32

Thyroid stimulation tests and their utility (3)

Answer 32

1) TSH stimulation - minimal stim if hT (<20mmol/l)
2) TRH stimulation - less reliable
3) TRH stimulated GH concentration - GH higher in hT than ill dogs with minimal overlap. possible future test.

Question 33

Supplementation of T4 or T3 in dogs with hT

Answer 33

T4 (levothyroxine)

Safer as replicates normal physiological process (mroe T4 than T3) and reduces risk of thyrotoxicosis.

Question 34

Peak concentrations levothyroxine

Answer 34

3-5 hours

Question 35

How long to steady state with thyroid supplementation

Answer 35

within 2 weeks

Question 36

Monitoring of hT

Answer 36

serum 6 hours post pill - TSH tells about contorl (provided was increased at diagnosis) T4 just tells you about control that day

Question 37

Causes of treatment failure for hT (4)

Answer 37

1) inadequate dose
2) Poor intestinal absorption (may need T3)
3) Daily pred
4) Incorrect diagnosis (HAC, hAC, atopy)

Question 38

Presentation spontaneous hypothyroidism in cats

Answer 38

7 cats, 6/7 goitre, 6 male, age 3-11y

Question 39

Is fT4 helpful in hypothryoid cats

Answer 39

No