Recombinant Human Thyrotropin Administration Enhances Thyroid Uptake of Radioactive Iodine in Hyperthyroid Cats

Background: Hyperthyroidism is the most diagnosed endocrine disorder in cats and radioiodine (I-131) is the treatment of choice. The dose emission rate and radioactivity in urine, saliva, and on hair and paws are determined by the dose of administered I-131. A dose reduction of therapeutic I-131 could possibly be achieved after recombinant human thyrotropin (rhTSH) administration as in humans with nodular goiter. Hypothesis: rhTSH will increase radioiodine uptake in hyperthyroid cats. Animals: Five hyperthyroid cats. Methods: Twenty-five micrograms rhTSH (day 1) or 2 mL 0.9% sodium chloride (NaCl) (day 9) was injected IV. One hour later, 11.4 +/- 4.1 (mean +/- SD) MBq I-123 was injected IV. Radioactive iodine uptake (RAIU) was measured 6, 12, and 24 hours after rhTSH (RAIU-rhTSH) or NaCl (RAIU-blanco) injection. Blood samples for measurement of TT4 were taken before injection of rhTSH or NaCl (TT4(0)) and at the time of imaging. Results: 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). There was a statistically significant effect of rhTSH administration on RAIU (P = .043) but not on serum TT4 concentration. Baseline serum TT4(0) concentration influenced RAIU-rhTSH significantly at 6 hours (P = .037). Conclusion and Clinical Importance: The increased RAIU observed after rhTSH administration in hyperthyroid cats could lead to a lower therapeutic dose of I-131 after rhTSH administration in hyperthyroid cats and decreased risk of environmental and owner contamination during and after hospitalization.