Broome MR, Peterson ME.
Hyperthyroidism develops in cats secondary to 1 or more autonomously functional thyroid adenomas. The progressive thy- rotoxicosis that ensues causes the chronic suppression of endoge- nous TSH release and ultimately the atrophy of normal thyroid tissue in these cats. This thyroid atrophy can lead to a period of transient hypothyroidism following curative radioiodine therapy. Once T4 values fall, circulating TSH levels increase, leading to reactivation of the previously suppressed and atrophied thyroid tissue in the large majority of these cats.Between 30-40% of cats with hyperthyroidism have pre-exist- ing chronic kidney disease (CKD). Iatrogenic hypothyroidism has been shown to contribute to worsening of azotemia and shortened life expectancy in cats with pre-existing CKD (Wil- liams et al, J Vet Intern Med. 2010;24:1086). In hyperthyroid cats with concurrent azotemia, the transient hypothyroidism that fol- lows radioiodine therapy may contribute to additional renal func- tion decline and worsening of the cats’ CKD stage.The purpose of this study was to evaluate if prevention of this transient hypothyroidism would blunt the progression of azote- mia commonly seen following the resolution of thyrotoxicosis in these cats with preexisting CKD. In this study, 195 hyperthyroid cats with concurrent CKD (IRIS stage 2 to 3) were treated with radioiodine (range, 1-10 mCi, median, 3 mCi). Of the 195 CKD cats, 85 cats were discharged on L-T4 (0.1 mg, PO q24 h), whereas the remaining 110 cats served as controls (no L-T4 sup- plementation). In both groups, total T4, BUN, and creatinine levels were recorded before treatment and then again at 1, 3 and 12 months following radioiodine therapy.Following successful radioiodine therapy, both groups of cats with preexisting CKD demonstrated increases in serum BUN and creatinine levels that gradually progressed over the 12-month per- iod (Table 1). However, the percent rise in median creatinine con- centrations in the 85 cats treated with L-T4 was significantly less than the rise in the 110 cats not supplemented with L-T4 (12.5% vs 33.3%; P < 0.05). These results suggest that L-T4 supplementation of radioiodine-treated cats with CKD may help limit progression of azotemia, presumably by avoiding the transient hypothyroidism that commonly develops after radioiodine therapy.