Chciuk K., Behrend E.N., Martin L., et al.
Conference Proceedings, (2013). American College of Veterinary Internal Medicine:
Iatrogenic hypothyroidism following treatment of feline hyperthyroidism can have deleterious effects on renal function. Serum total thyroxine concentration (T4) is commonly used to evaluate therapy, but no study has examined the use of both serum free thyroxine by equilibrium dialysis (FT4ed) and thyroid-stimulating hormone (TSH) concentrations. The purpose of this study was to compare the ability of T4, FT4ed, and TSH concentrations to diagnose treatment-induced hypothyroidism in hyperthyroid cats receiving methimazole. We hypothesized that FT4ed would identify more cats with iatrogenic hypothyroidism as compared to T4.
A total of 65 samples from previously diagnosed hyperthyroid cats receiving methimazole therapy and with T4 concentrations < 48 nmol/L were included. Samples had been submitted to the diagnostic laboratories at Auburn University (n = 22) and Michigan State University (n = 43). T4, FT4ed and TSH concentrations were measured via assays previously validated for use in cats. Correlation was tested via a Spearman Rank Order test. Significance was set at the p < 0.05 level.
The median (range) T4, FT4ed, and TSH concentrations were 20 (3–48) nmol/L, 20 (3–57) pmol/L, and 0.1 (.08–9.1) ng/ml, respectively. Overall, 23 cats (35%) had an elevated TSH concentration (> 0.30 ng/ml). For cats with elevated TSH concentrations, median T4 and FT4ed concentrations were 11 (3–40) nmol/L and 12 (3–35) pmol/L, respectively. Cats with normal TSH concentrations (< 0.30 ng/ml) had median T4 and FT4ed concentrations of 23.5 (5–48) nmol/L and 27.0 (6–57) pmol/L, respectively. The percentage of cats with an elevated TSH concentration in combination with a low T4 (< 10 nmol/L), FT4ed (< 10 pmol/L), or both T4 and FT4ed concentration were 17%, 12% and 11%, respectively. Eleven cats (17%) had an elevated TSH despite normal T4 and FT4ed concentrations. One cat (1.5%) had a normal TSH despite low T4 and FT4ed concentrations. Of 24 cats with T4 concentrations between 10–25 nmol/L (low end of the reference range 10 nmol/L), 7 (29%) had an elevated TSH. Of 26 cats with FT4ed concentrations between 10–25 pmol/L (low end of the reference range 10 pmol/L), 11 (42%) had an elevated TSH. A significant positive correlation was found between T4 and FT4ed concentrations (p = < 0.001). A significant negative correlation was found between both T4 and TSH concentrations (p = < 0.001) and between FT4ed and TSH concentrations (p = < 0.0001).
The data suggest that FT4ed does not identify more cats with iatrogenic hypothyroidism as compared to T4. As some cats had an elevated TSH concentration despite having a normal T4 or FT4ed, further investigation may be warranted.