§ 4.221 ENDODERMAL ENDOCRINE GLANDS OF VERTEBRATA 141 



time, trapping of iodides and the formation of the intrafolUcular 

 colloid is stimulated; although the amount trapped depends upon 

 the level already in store (Barker, 1955). In the absence of TSH, 

 both trapping of iodine and secretion of thyroxine may be 

 reduced to 10 per cent of normal in 5 days; the binding of iodine 

 to the colloidal proteins may fail or proceed only slowly, and in 

 some cases diiodotyrosine (or even monoiodotyrosine) is formed 

 instead of thyroxine. It has been shown that TSH is inactivated 

 in vitro by exposure to thyroid, but not to other tissues ; and that 

 it can be re-activated by treatment with reducing agents such as 

 thiouracil. This may be related to the fact that the latter destroys 

 the thyroxine in the thyroid. 



The increase in height of the follicular epithelium in the thyroid 

 gland, which is often used as a means of assay for TSH activity, 

 accompanies the normal release of thyroxine from the gland (Fig. 

 4-7 d-e) ; but increase in weight of the gland is not necessarily 

 related to its rate of secretion, though it may also follow prolonged 

 treatmentwithTSH (or with the growth-promotingfraction thereof). 



The rate of secretion of TSH, rather than the quantity of it that 

 is stored in the hypophysis, seems to be increased by some form of 

 control from the hypothalamus, since it can be disturbed and 

 reduced by hypothalamic lesions in the brain (Bogdanove, 1957). 

 On the other hand, its secretion can be decreased by high con- 

 centrations of thyroxine, circulating in the blood ; this feed-back 

 reaction tends to maintain a steady level of production of the 

 metabolic hormone. 



Evidence for there being tw^o distinct, though closely similar, 

 substances both included under the name of thyrotrophin is 

 provided by experiments on mice, in which the growth-promoting 

 action of TSH responds differently from its secretion-promoting 

 action. Hypophysectomy results in reduction in size of the thyroid 

 gland, as well as in its secretion. If the hypophysis is transplanted 

 elsewhere in the body, the mice do not show any increase in 

 thyroid weight, when compared with hypophysectomized controls ; 

 but such reimplantation does restore the iodine content and thy- 

 roxine formation and secretion by the thyroid glands to 66 per cent 

 of normal, as compared with only 10 per cent in hypophysecto- 

 mized controls without implants (Greer, Scow and Grobstein, 



