Thyroid hormone deiodination in the brain of young chickens acutely adapts to changes in thyroid status.

The brain represents a special organ in respect of thyroid hormone handling. This was analyzed in one-week-old chickens. The effect of sham-operation (SH), thyroidectomy (TX) and thyroidectomy plus thyroxine supplementation (TX+T4) on the intracerebral triiodothyronine (T3) content and on the activity of different types of deiodinases was investigated. it was found that in spite of very low T3 levels in the serum of TX animals, the brain displayed close to normal tissue T3 levels. Kinetic studies of the deiodinase system showed an increase in type II activity (increased T4 to T3 conversion) and decreased type III activity (decreased degradation of intracellular T3) in the brain of TX animals vs. SH or TX+T4. It is concluded that a considerable part, if not the total of the T3 preserved in hypothyroidism may be ascribed to adaptive changes of the deiodinase system in the brain of young chickens.