This chapter discusses the regulation of pineal thyroxine degradation and this is related to the circadian metabolic activity of the gland as a whole. The pineal gland has been described as a veritable factory for the metabolism of a wide variety of diverse compounds. The best-known metabolic pathway in the pineal undoubtedly is that for serotonin. Besides serotonin synthesis and metabolism, polypeptide products are identified in the pineal gland and these compounds are espoused as hormonal candidates although the data are not as compelling as for melatonin. Additionally, varieties of products that are generally considered retinal in nature have been isolated from the mammalian pineal gland even though this organ in mammals is generally considered to be devoid of photoreceptive capabilities. Recently, 3,3',5,5'-tetraiodothyronine (thyroxine), the main secretory product of the thyroid gland, was found to undergo metabolism in the pineal. The ability of the mammalian pineal gland to convert T4 to T3 in a rhythmic manner was only recently discovered. The expression of the 5'D-II cycle is remarkably similar in terms of its phasing and regulation to another important rhythm in the pineal gland—that is, that of melatonin synthesis and secretion.
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