B-HCG structure, source and function

HCG: Human chorionic gonadotropinhcg-hpl

  • Detected day 8 after LH surge
  • Rise in curvilinear fashion for 40 day, then more slowly until about 10 wks
  • Doubling time 1 ½ -2d
  • 1500-2000 IU/L seen on TV Scan
  • Secreted by the trophoblast of the placenta approximately 8 weeks after ovulation.
  • It’s composed of alpha and beta subunits
  • It prevents the corpus luteum from regression and directs it to synthesize estradiol and progesterone during the 1st trimester to maintain the endometrium for implantation.
  • During the 2nd and 3rd trimester, the placenta synthesizes its own estradiol and progesterone and the corpus luteum degenerates.
  • HCG positive pregnancy test indicates an implanted blastocyt.
  • HCG urine level is always lower than the HCG serum level , therefore,
  • HCG is detectable in the blood 8 days after ovulation, but detectable in the urine 14 days after ovulation.
  • HCG levels are maximal at gestational week 9 and then decline.
  • About 85% of normal pregnancies will have the hCG level double every 48 – 72 hours .
  • For home urine pregnancy test, false negatives can occur but false positives are rare

Pregnant – Only effective as a test for the first 10 weeks

  • hCG is produced by a fertilized egg (trophoblast)
  • hCG then tells the corpus luteum….DON’T DIE!
  • Progesterone is continually produced

Progression of hCG in Pregnancy

  • First marker of trophoblast differentiation
  • hCG plasma half life=24 -36 hours
  • First 6 weeks, [hCG] doubles every 1.7-2 days
  • Peak again briefly when maternal blood flow is established in the intervillous space

Alpha and Beta Subunit Continued

  • Held together by electrostatic and hydrophobic forces
  • Both start out as large precursor molecules and are then cleaved by endopeptidase
  • Alpha: synthesized from a single gene
    • Along with LH, FSH, and TSH
  • Beta: synthesized from 6 genes (SPECIFIC FOR ONLY HCG)

Biochemical Composition

  • Polypeptide Hormone
  • Glycoprotein with 237 AA
  • Has highest carbohydrate content of any hormone
    • This along with a terminal sialic acid, protects molecule from catabolism

Function of hCG

  • Pregnancy Test
  • Can be produced in gestational trophoblastic neoplasia (GTN) by hydatidform mole and choriocarcinoma
    • If hCG is high due to GTN, then they can become hyperthyroid because of hCG acting on TSH receptors