Hypothyroidism

Question 1

What are the main clinical signs of hypothyroidism?

Answer 1

Lethargy
Weight gain
bradycardia
Symmetrical alopcia

Question 2

What are the types of hypothyroidism?

Answer 2

90% Primary - 50% lymphocytic thyroiditis, 50% degeneration (idiopathic)
10% Secondary - Pituitary. Either congenital panhypopituitaryism (disproportionate dwarfism) or from trauma/ neoplasia

1 case of tertiary - hypothalamic patholgy

Can also get peripheral failure of T3/T4 in circulation

Question 3

What are the normal changes in biochemistry/ hematology?

Answer 3

35-50% normochromic normocytic anaemia
75% high cholesterol, can be marked
88% high triglycerides
35% mild increase CK
30% mild increase liver enzymes

Question 4

What is the use of basal TT4?

Answer 4

To exclude hypothyroidism

Can be below ref ranges at different times of the day in healthy dogs

Question 5

What are the main causes of low tT4?

Answer 5

Hypothyroidism
Euthyroid sick syndrome
Lots of drugs - sulphonamides, carprofen, phenobarb, frusemide,
Older animals/ sight hounds

Question 6

What normally happens to t4 in euthyroid sick syndrome?

Answer 6

Low TSH secretion
Low T4 synthesis and protein binding

Normally on bloods see low T4 and normal TSH

Question 7

What are some of the difficulties with using TSH T4 for testing?

Answer 7

20% hypothyroid dogs have normal TSH
12% sick euthyroid dogs have raised cTSH
if secondary hypothroidsism, cTSH may be low

Question 8

What is the use of free T4?

Answer 8

this is the active form (99% is protein bound, 1% free and active)
Often euthyroid sick animals with have a normal fT4, whereas hypothyroid will be low
Except euthyroids with cushings, on preds, phenobarb, some sighthounds

Question 9

What is the best way of diagnosing a patient when there is not a clear cut answer?

Answer 9

Combine tests
TT4, fT4, TSH
Only 1.8% non thyroid illness will have low TT4, low fT4, and high TSH

Question 10

Outline thyroglobulin auto antibodies (TgAA)

Answer 10

+ve results confirm thyroid pathology but not hypothyroidism - -ve does not rule out hypothyroidism
Present in 50% dogs with lymphocytic thyroiditis (so 25% in total)
Not affected by drug Tx
Doesn’t indicate function - can be present in euthyroid dogs before clinical hypothyroidism develos and not all dogs will develop hypothyroidism from there
Will become -ve in advanced disease

Question 11

Outline T3 and T4 autoantibodies

Answer 11

Present in approx 10% dogs with TgAA
Not important in vivo as do not affect availability of T3/T4
In vitro can cause artificially high or low t3/ t4 results
Usually leads to an increase in TT4 - masks hypothyroidism
Test for in cases that are v clinically suggestive with high t4

Question 12

What is the use of t3?

Answer 12

None
fluctuates a lot
Normal in 15-50% hypothyroid dogs

Question 13

What is the ideal way to test for hypothyroidsim

Answer 13

Ensure off all drugs for 4-6 weeks

Low T4 with high TSH