Effects of Recombinant Human Thyroid Stimulating Hormone on Thyroid Uptake of Radioactive Iodine and Serum Total T4 Concentration in Hyperthyroid Cats: A Preliminary Study
Administration of recombinant human thyroid stimulating hormone (rhTSH) increases the radioactive iodine uptake (RAIU) and allows a reduction of therapeutic radioiodine (131I) dose in humans with nodular goiter (Nieuwlaat et al., JCEM 2003;88:3121-3129). A similar dose reduction of 131I for treatment of hyperthyroidism in the cat could possibly be achieved after rhTSH administration, thereby respecting the ALARA (as low as reasonably achievable) principle. The objective of this study was to investigate the effect of rhTSH on RAIU and serum total T4 (TT4) concentration in hyperthyroid cats.
Inclusion criteria were clinical signs compatible with hyperthyroidism and increased serum TT4 concentration. On day 1, a blood sample for measurement of TT4 (serum TT40) was taken by jugular venipuncture and 25 µg rhTSH was injected IV. One hour later a dose of 11.4 ± 4.1 (mean ± sd) µBq 123I was injected IV. Blood samples for TT4 measurement were taken and RAIU was measured 6, 12 and 24 hours after rhTSH injection (RAIU-rhTSH). Blood was centrifuged and stored for 3 days (-20° C) until analysed for serum TT4 concentration. A static planar ventral image of 200.000 counts using a low energy high resolution (LEHR) collimator was made with the cat in sternal recumbency under general anesthesia. A syringe with a known amount of radioactivity (2.5 ± 1.6 MBq) was located next to the animal. This was used as the standard activity necessary to calculate RAIU. On day 9, the study from day 1 was repeated with injection of 2 ml NaCl 0.9% instead of rhTSH (RAIU-blanco). Results are expressed as mean ± sd.
Five hyperthyroid cats were included (age 13 ± 2 years, 3.3 ± 0.4 kg bodyweight). Percentages of RAIU-rhTSH (and RAIU-blanco) at 6, 12 and 24 hours after administration of rhTSH were 34 ± 18 (31 ± 21), 46 ± 20 (38 ± 18) and 47 ± 15 (36 ± 14), respectively. There was an overall statistically significant effect of rhTSH administration on RAIU (P=0.043) with an overall mean difference of 7.33 % between RAIU-rhTSH and RAIU-blanco. There was no statistically significant difference in effect between the timepoints after rhTSH administration (P=0.070). There was no statistically significant effect of rhTSH on serum TT4 concentration 6 hours (P=0.250), 12 hours (P=0.313) and 24 hours P=0.313) after administration. Baseline serum TT40 concentrationinfluenced significantly RAIU-rhTSH at 6 hours (P=0.037) but not at 12 hours (P=0.074) nor at 24 hours (P=0.522) after administration of rhTSH. No statistically significant effect of baseline serum TT40 was found on RAIU-blanco at 6 hours (P=0.052), at 12 hours (P=0.079) or at 24 hours (P=0.464) after administration of NaCl 0.9%.
This preliminary study shows promising results with an increased RAIU being observed after rhTSH administration. Future studies are needed to optimize the dose and timing of rhTSH.