Mooney C.T.
Conference Proceedings, (1992). British Small Animal Veterinary Association: p.168
It has been suggested that the thyrotropin (thyroid stimulating hormone (TSH)) stimulution test may be helpful in confirming a diagnosis of feline hyperthyroidism when the serum total thyroxine (T4) concentration is equivocal. To date, only small numbers of cases have been studied and the value of the test remains controversial. We have performed theTSH stimulation tcst in a series of five healthy, euthyroid and 42 hyperthyroid cats in order to assess its efficacy as an additional diagnostic test for feline thyrotoxicosis.
Blood was obtained from conscious animals by jugular venepuncture both before and six hours after the intravenous injection of 0.5 IU/kg bovine TSH (Thytropur, Armour Pharmaccuticul Company). Serum total T4 and total triiodothyronine (T3) concentrations were measured in each sample by means of double antibody radioimmunoassays specifically optimised and validated for use with cat serum.
In the healthy cats, the mean ±sd serum total T4 concentration increased from basal values of 35.1 ± 10.9 nmol/l (range, 19.7-46.8nmol/l) to 120.8 ±49.2nmol/l (range, 57.4-168.1nmol/l) aflerTSH administration, representing a mean ±sd absolute increment of 85.7 ±40.7nmol/1 (range, 27.4-121.3 nmol/l). The mean ± sd ratio of serum post-TSH to basal total T4 concentrations was 3.44 ±0.92 (range, 1.91-4.27).
In the hyperthyroid cats, the mean ±sd serum total T4 concentration was 236.2 ± 146. nmol/l (range 45.6-642.H nmol/I) prior to, and 308.1±178.9 nmol/l (range, 94.9-802.4nmol/l) after TSH administration representing a mean±sd absolute increment of 71.9±80.0nmol/1 (range, ±98.4- 260.5nmol/I). The mean±sd ratio of serum post-TSH to baal total T4 concentrations was 1.40+/-0.43 (range. 0.72-3.16) which was significantly (P <0.001) lower ill this compared with the euthyroid group.
Three hyperthyroid cats had basal serum total T4 concentrations below the thyrotoxic range. These cats were separated from the main group for further statistical analysis.There was no significant difference (p>0.05 in cach case) between the post-TSH serum total T4 concentrations, the absolute increase in serum total T4 concentrations and the ratio of serum POST-TSH 10 basal total T4 concentrations in this compared with the euthyroid group.
The ratio of serum post-TSH to basal T3 concentrations was also significantly lower (P<0.01) between all the hyperthyroidcompared with thc euthyroid cats but because ofthc more variable T3 response in the latter,no further information was gained from measurement of this analyte.
A lack of serum total T4 response to TSH stimulation may confiun a diagnosis of hyperthyroidism in thyrotoxic cats with markedlyelevated basal serum total T4 concentrations. However, in cats with equivocal concemrutions, where additional diagnostic tests are actually required. The changes in this analyte alter injection of TSH too closely resemble the situation in euthyroid animals to be or diugnostic value ill a clinical situation.