- 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)
- 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