Total T4 (tT4) is generally used
as the diagnostic test for hyperthyroidism and is used to monitor
T4 levels post treatment.
Free T4
Free T4 (fT4) it has been suggested
that fT4 may be more sensitive in assessing thyroid function
in sick animals where the T4 may be depressed.
Free T4 by Equilibrium
Dialysis (fT4ED)
Free T4 by equilibrium dialysis
(fT4ED) is the most accurate way to measure fT4 and is more
sensitive in assessing thyroid function in sick animals where
the T4 may be depressed. fT4ED is less affected by altered protein
levels as the samples are dialysed prior to assay.
T3 Suppression Test
This is a useful test for diagnosing
borderline cases of hyperthyroidism where the tT4 is consistently
within the normal range. It is very important to measure Total
T3 (tT3) as well to ensure the cat actually absorbed the T3
dose.
1. Collect basal blood sample for tT4 and tT3
2. Administer T3 (Tertroxin) orally every 8 hours for a total
of 7 doses according to body weight: cats <5 kg 20 micrograms
of Tertroxin cats >5 kg 30 micrograms of Tertroxin
3. Collect second blood sample 2 - 6 hours after final dose
4. Label both samples clearly and request tT4 and tT3.
Interpretation
tT3 suppression normal cats usually show at least 50% reduction
in tT4 levels following suppression. Hyperthyroid cats generally
show limited suppression. tT3 should be above the normal range
indicating the T3 tablets have been absorbed.
TRH Stimulation
Test
This test is also useful
in cases of mild or borderline hyperthyroidism.
1. Collect basal blood sample.
2. Administer 0.1 mg/kg i/v slowly over one minute
3. Collect second blood sample 4 hours later
4. Label samples clearly and request tT4.
Interpretation Normal cats increase 1.5 -
2 times following stimulation.
Hyperthyroid cats generally
show little or no rise in tT4 levels.
Feline
Hypothyroidism
Feline hypothyroidism is considered
to be a rare condition. The TRH stimulation test may be
used for diagnosis.
See above for TRH Stimulation
Test method
Interpretation Normal cats - post stimulation
result is expected to be about 1.5 - 2 times greater than
the basal T4 concentration.
Hypothyroid cats - post stimulation
result is expected to be greater than 2 times than the
basal T4 concentration.
