Accumulation of [131I]triiodothyronine in neurons and other tissues following intravenous injection of the labeled hormone

The accumulation of 131 I in motor neurons, liver, muscle and spinal cord grey matter after intravenous injection with 131 I-labeled triiodothyronine was determined by liquid scintilattion counting procedures. It was observed that the thyroid hormone which accumulates in the brain (spinal cord) was largely present in the neurons, which had significantly higher accumulations than did masses of spinal grey matter which would contain comparable motor neurons. The nerve cell 131 I activity was also significantly higher than that seen in striated muscle. Liver, which takes up thyroid hormones very rapidly, had levels of radioactivity which were significantly higher than muscle, spinal grey matter, plasma or ventral horn motor neurons at most time intervals investigated. A chromatographic analysis of unfixed, ethanol (pH 8.5) extracted neurons revealed that most of the 131 I detected in the motor neuron samples was associated with triiodothyronine, which is comparable to what has been previously reported for whole brain after intravenous injection of [ 131I ]triiodothyronine.