The effects of neonatal treatment with methylthiouracil and triiodothyronine on physical and behavioural development

The important role of hormones in regulating physiological processes in man and animals has been well established by many workers in endocrinology. The site and mode of action of hormones at the cellular level, however, is not yet clear. This is also true for the thyroid hormones L-thyroxine and L-triiodothyronine. The thyroid hormones accelerate many biological processes; examples are their calorigenic and protein anabolic action. It is not yet certain whether the thyroid hormones have different distinct actions or that their different effects are manifestations of one single action at the cellular level. During the last decade evidence has been put forward for the view that thyroid hormones act on the mitochondria and on transcriptional processes involved in gene expression.