Thyroid Flashcards
Hyperthyroid signalment
Common in cats
Usually only see as a malignant thyroid tumor in dogs
Hypothyroidism
Markedly decreased tT4 and fT4 is diagnostic for primary, secondary or tertiary
Primary hypothyroidism hallmark
Decreased fT4 and increased TSH
Secondary hypothyroidism
Deficiency in secretion of TSH from pituitary
Tertiary hypothyroidism
Deficiency of TRH secretion from hypothalamus
Sick euthyroid
Hypothyroxemia due to non thyroidal diseases (Neoplasia, hyperadrenocorticism, infammatory disease, drugs)
Processes of sick euthyroid
-Decreased protein bound
-Inhibition of TSH secretion
-Inhibition of T4 production
Measuring tT4
Rule in hyperthyroidism in cats
Rule out hypothyroidism in dogs
Can be suppressed in non thyroidal illnesses
Increased tT4
Increased production by thyroid neoplasia
-Thyroid adenoma or adenocarcinoma
-Positive interference by T4AA in dogs (false increase)
Decreased tT4
-Decreased production: Primary is the most common (secondary and tertiary are rare
-Multifactorial: Sick thyroid, drugs
tT4 in healthy dogs
Much lower than cats and horses so must use a lower detection limit to measure normal and decreased
Why does T4AA cause positive interference
Binds to labelled T4 in the test tube so there is less competition for those binding sites and when you measure the radioactivity of the sample, it makes it look like there is more pt T4 than there really is
tT4 test
Competitive binding assay
Combine pt serum with a known amount of labelled T4 and they compete for binding spots on antibodies in the tube. The more pt T4 the less labelled T4 that binds and vice versa
TSH measurement
Always interpret with tT4 and or fT4
Use to differentiate cause of hypothyroidism
TSH usually increased if primary
Increased TSH with decreased TT4 and fT4
Almost 100% specific for primary hypothyroidism
Autoantibody titers
Used to diagnose autoimmune thyroiditis not thyroid function
Thyroglobulin autoantibody (TgAA)
Immune response to thyroglobulin due to damaged cells releasing and reaction as a foreign protein
Decrease tT4, TSH WRI and positive TgAA
Supports hypothyroidism
T4AA
Dogs with lymphocytic thyroiditis recognize T4 as foreign
More important because of test interference
fT4
1% of serum thyroxin but good for diagnostics
Less suppression by non thyroid diseases and drugs
fT3 WRI
Rule out hypothyroidism in dogs
Assays for fT4
Equilibrium dialysis
Serum sample filtered through membrane that only fT4 can pass through ( not protein or antibodies) and then measure
Increased fT4
Increased production by thyroid neoplasia
Often seen in concurrence with tT4
Decreased fT4
Usually in primary hypothyroidism with decreased tT4
