Metabolism of L-Thyroxine and L-3:5:3′-Triiodothyronine by Brain Tissue Preparations1

THYROID gland preparations have been known for many years to have a marked effect on the development and function of the central nervous system (1). More recently, the thyroid hormones, l-thyroxine and l-3:5:3′-triiodothyronine, have been shown to increase the excitability of the brain, as measured by the threshold for electric shock seizures (2, 3). Both substances also influence the electrolyte distribution and in particular, the distribution of sodium in the brain tissue (3). Absence of the thyroid gland has marked effects on the electroencephalogram and causes profound changes in cerebral vascular resistance and blood flow (4). Moreover, changes in mental activity, mood and even psychosis maybe associated with either over- or under-activity of the thyroid (5, 6, 7, 8). There is, however, no change in oxygen consumption in brain tissue removed from either thyroidectomized or hyperthyroid rats (9) although hyperthyroidism may effect a slight change in succinoxidase activity (10).