Permanent defects in rat peripheral auditory function following perinatal hypothyroidism: determination of a critical period.

Rats were treated with propylthiouracil (PTU) for 10-day periods beginning at different ages. Daily injections of L-thyroxine (T4) were administered concurrently with PTU to a group of rats which served as one control group. Peripheral auditory function was evaluated by the brainstem response audiometry (BSRA) technique performed at 12, 16, 25 and 120 days of age. PTU treatment significantly increased wave I latency (cochlear nerve compound action potential) in adult rats when administered from 3 days before delivery through 6 days of age, but was without permanent effect (wave I latencies and thresholds) when administered for 10 days starting at 10 days after birth. T4 replacement during the first 10 postnatal days prevented permanent abnormalities. These data suggest that the period of greatest vulnerability to thyroid hormone depletion in the peripheral auditory system extends from at least 3 days before delivery through between 5 and 10 days of age.