The maternal thyroid gland is also stimulated by large quantities of hCG. In some women with gestational trophoblastic disease, biochemical and clinical evidence of hyperthyroidism sometimes develops (Chap. 20, p. 391). This once was attributed to formation of chorionic thyrotropins by neoplastic trophoblasts. It was subsequently shown, however, that some forms ofhCG bind to TSH receptors on thyrocytes (Hershman, 1999). And, treatment of men with exogenous hCG increases thyroid activity. The thyroid-stimulatory activity in plasma of irst-trimester pregnant women varies appreciably from sample to sample. Modiications of hCG oligosaccharides likely are important in the capacity of hCG to stimulate thyroid function. For example, acidic isoforms stimulate thyroid activity, and some more basic isoforms stimulate iodine uptake (Kraiem, 1994; Tsuruta, 1995; Yoshimura, 1994). Finally, the LH-hCG receptor is expressed by thyrocytes, which suggests that hCG stimulates thyroid activity via the LH-hCG receptor as well as by the TSH receptor (Tomer, 1992).
Other hCG functions include promotion of relaxin secretion by the corpus luteum (Duy, 1996). LH-hCG receptors are found in myometrium and in uterine vascular tissue. It has been hypothesized that hCG may promote uterine vascular vasodilatation and myometrial smooth muscle relaxation (Kurtzman, 2001). Chorionic gonadotropin also regulates expansion of dNK cell numbers during early stages of placentation, thus ensuring appropriate establishment of pregnancy (Kane, 2009).
here are several clinical circumstances in which substantively higher maternal plasma hCG levels are found. Some examples are multifetal pregnancy, erythroblastosis fetalis associated with fetal hemolytic anemia, and gestational trophoblastic disease. Relatively higher hCG levels may be found in women carrying a fetus with Down syndrome. This observation is used in biochemical screening tests (Chap. 14, p. 281). The reason for the elevation is not clear, but reduced placental maturity has been speculated. Various malignant tumors also produce hCG, sometimes in large amounts-especially trophoblastic neoplasms (Chaps. 9, p. 159 and 20, p. 391).
Relatively lower hCG plasma levels are found in women with early pregnancy wastage, including ectopic pregnancy (Chap. 19, p. 373). hCG is produced in very small amounts in normal tissues of men and nonpregnant women, perhaps primarily in the anterior pituitary gland. Nonetheless, the detection of hCG in blood or urine almost always indicates pregnancy (Chap. 9, p. 158).
This is a single, nonglycosylated polypeptide chain with a molecular weight of 22,279 Da. The sequences of hPL and of human growth hormone (hGH) are strikingly similar, with 96-percent homology. Also, hPL is structurally similar to human prolactin (hPRL), with a 67-percent amino acid sequence similarity. Because of these similarities, it was called human placental lactogen or chorionic growth hormone. Currently, human placental lactogen is used by most.