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Enterohepatic circulation of triiodothyronine

By W.W. (Wouter) de Herder


The major factor regulating thyroid function is thyroid stimulating hormone\ud (TSH), a glycoprotein released by the thyrotropic cells of the pituitary gland.\ud Determinants of the TSH secretion rate are inhibition by thyroxine (T 4),\ud 3, 3', 5-triiodothyronine (T3), dopamine, glucocorticoids and somatostatin, and\ud stimulation by TSH-releasing hormone (TRH), (nor)adrenaline and perhaps\ud neurotensin [156]. In the control of thyroid hormone bioavailability, an\ud important role is also played by iodothyronine transport into cells, enzymatic\ud deiodination and conjugation.\ud Several groups have studied the enterohepatic metabolic pathways of\ud iodothyronines. It has been generally accepted that these pathways have no\ud more than a passive function in the elimination of the hormone. However, if\ud enterohepatic circulation (EHC) of iodothyronines occurs, the intestinal tract\ud may constitute an important pool of exchangeable hormone. Until recently,\ud only few and inconclusive data existed concerning this EHC.\ud In our studies we have attempted to document the possible existence of\ud an EHC of thyroid hormone and the role it may play in regulating overall\ud hormone metabolism and excretion in the rat. Especially, we wanted to assess\ud the importance of the intestinal microflora for this process.\ud We have studied the biliary clearance of T3 and its conjugates, the hydrolysis\ud of iodothyronine conjugates by intestinal bacteria and intestinal contents,\ud and the metabolism of T3 and its conjugates in conventional (CV) and\ud intestine-decontaminated (ID) rats.\ud It is the purpose of this thesis to discuss the role of the EHC of iodothyronines\ud in thyroid hormone metabolism, with special emphasis on the\ud results of my own studies of this subject, described in detail in the appendix\ud papers

Topics: thyroid stimulating hormone, TSH, circulation, iodothyronines, metabolism
Year: 1990
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