Corrective effects of thyroxine on cochlear abnormalities induced by congenital hypothyroidism in the rat. II. Electrophysiological study.

In order to study the corrective effects of thyroxine (T4) on functional abnormalities induced by congenital hypothyroidism, small doses of T4 were injected to propylthiouracil-treated (PTU-treated) rat pups for 2 consecutive days on selected periods of development (days 3 and 4, 6 and 7, 9 and 10, 12 and 13, 18 and 19). Some animals also received thyroid replacement therapy from days 12 to 17. The animals were tested electrophysiologically on day 30, by recording the compound action potential and the cochlear microphonic from the round window after click and tone burst stimulation. PTU-treated animals given T4 for 2 consecutive days demonstrated both AP and CM threshold shifts. On the contrary, PTU-treated animals given T4 from days 12 to 17 demonstrated a normal CM output of the cochlea, but still showed elevated AP thresholds. These results are discussed with previous data concerning the corrective effects of T4 on cochlear structures in PTU-treated rats previously described.