The recovery potential of central conduction disorder in hypothyroid rats

In an aim to detect the dysfunction of central nervous system among rats with varied durations of hypothyroidism and to elucidate the recovery potential after thyroxine replacement, a series of BAEP were conducted and compared with age-matched controls. BAEP was performed in five groups of the hypothyroid animals 1, 3, 5, 7, and 9 months after thyroidectomy respectively. Following initial electrophysiological assessment, thyroxine replacement was administered to each group of hypothyroid rats, and BAEP was performed at two month intervals, up to two successive normal studies or six months after the initiation of therapy, whichever came first. Before thyroxine treatment, prolonged I-V interpeak latency was the most consistent abnormal finding in all groups of hypothyroid rats, and longer hypothyroid state correlated well with more severe central conduction disorder. Hearing impairment was also noted among those with long duration of hypothyroidism. After thyroxine replacement, the central conduction dysfunction usually returned to normal if the hypothyroid state was not more than 5 months in duration. However, when hypothyroid state persisted over 7 months or more, there would be an incomplete recovery for central conduction disorder. The present study brings out the concept of 'therapeutic window' in reversing the central nervous dysfunction caused by hypothyroidism in adult rats.