Evidence of hypothyroidism in the genetically epilepsy-prone rat

A number of neurochemical and behavioral similarities exist between the genetically epilepsy-prone (GEPR) rat and rats made hypothyroid at birth. These similarities include lower brain monoamine levels, audiogenic seizure susceptibility and lowered electroconvulsive shock seizure threshold. Given these similarities, thyroid hormone status was examined in GEPR rats. Serum samples were collected from GEPR-9 and non-epileptic control rats at 5, 9, 13, 16, 22, 31, 45, 60, 90, 150 and 350 days of age. Serum thyroxine (T4) levels were significantly lower in GEPR-9 rats compared to control until day 22 of age. GEPR-9 thyrotropin (TSH) levels were significantly elevated during the period of diminished serum T4. GEPR-9 triiodothyronine (T3) levels were lower than control throughout the first year of life. The data indicate that the GEPR-9 rat is hypothyroid from at least the second week of life up to 1 year of age. The critical impact of neonatal hypothyroidism on brain function coupled with the development of the audiogenic seizure susceptible trait by the GEPR-9 rat during the third week after birth suggests that neonatal hypothyroidism could be one etiological factor in the development of the seizure-prone state of GEPR-9 rats.