Two hormones known in the female as follicle stimulating hormone (FSH) and corpus luteum stimulating hormone (LSH) are secreted by the anterior lobe of the pituitary gland. These hormones are real gonadotrophins because they directly govern the function of the ovaries. The anterior pituitary is in turn governed by the diencephalon, and so when there is an ovarian deficiency the diencephalic center concerned is hard put to correct matters by increasing the secretion from the anterior pituitary of FSH or LSH, as the case may be. When sexual deficiency is clinically present, this is a sign that the diencephalic center concerned is unable, in spite of maximal exertion, to cope with the demand for anterior pituitary stimulation.6 When then the administration of HCG increases the functional capacity of the diencephalon, all demands can be fully satisfied and the sex deficiency is corrected.
That this is the true mechanism underlying the presumed gonadotrophic action of HCG is confirmed by the fact that when the pituitary gland of infantile rats is removed before they are given HCG, the latter has no effect on their sex-glands. HCG cannot therefore have a direct sex gland stimulating action like that of the anterior pituitary gonadotrophins, as FSH and LSH are justly called. The latter are entirely different substances from that which can be extracted from pregnancy urine and which, unfortunately, is called chorionic gonadotrophin. It would be no more clumsy, and certainly far more appropriate, if HCG were henceforth called chorionic diencephalotrophin.
Excerpt from Pounds and Inches Hcg Diet manuscript