What is human chorionic gonadotrophin (hCG)?

Human chorionic gonadotrophin (hCG) is one of the many hormones produced by the placenta.

The doctor asked me to do a hCG test to check if I am pregnant

Yes , detection of hCG is often used as a confirmation of early pregnancy.

What types of hCG tests are available ?

hCG can be tested in the urine as well as in the blood.

The common type of tests available (also known as commercial testing kits) is the urine testing kit.

The commercial testing kits are for common use and can diagnose pregnancy 4 weeks after ovulation . These slide tests depends on agglutination-inhibition assay. The presence of hCG in the urine sample inhibits an agglutination reaction between hCG coated latex particles and an antiserum. This test is usually done in an early morning sample of urine.

What is the difference between urine hCG test and the blood hCG tests?

a)The blood( serum) hCG test is more sensitive and specific than the urine hCG.

b) The serum test is an enzyme linked immunoassay using a monoclonal antibody ,therefore it is more sensitive. The urine hCG test uses the agglutination-inhibition assay.

c) The serum hCG test can diagnose pregnancy as early as 8-9 days after ovulation.

d) The blood tests for hCG are more specific immuno-assays. They detect the beta hCG component and therefore cross reactivity with hormones such as LH, FSH or TSH does not occur.

What are the plasma (Blood) levels of  hcg?

The baseline plasma hCG (at the time of a missed menses) is about 100iu/l.

a peak level of about 10000iu/l is reached at 8 to 10 weeks of gestation. Thereafter it decreases to about 10000 to 20000iu/l by 18 to 20 weeks of gestation and remains at this level till delivery.

During the first six weeks of pregnancy the beta hCG levels doubles every 48 hours.

The secretion of hCG is detectable in the blood plasma of a pregnant woman around the 8th day after ovulation,ie one day after implantation.

However biomolecular techniques used for research purposes has demonstrated hCG in the 8th cell stage of the embryo.

Some Advanced Questions and Answers on HCG

Is it possible for a non-pregnant woman to have elevated hCG?

Yes. This is also sometimes referred to as phantom hCG or false positive hCG values;

Usually false positive hCG  are low level elevations.

Some of the common causes of a false positive hCG elevation:

i) Molar Pregnancy -Gestational Trophoblastic Diseases(Hydatidiform Mole and Choriocarcinoma).- an extensive topic which will be discussed on its own.

ii) Heterophile antibodies in the patients serum can interfere with hCG immunometric assays. These heterophile antibodies cannot pass into the urine ,therefore patients who are suspected to have heterophile antibodies should have their urinary hCG levels asseyed.

iii) Ectopic pregnancy. In ectopic pregnancy the rise in the hCG levels is much less than in normal pregnancy.

Failure to visualize a sac by transvaginal ultrasound when the hCG titers are greater than 1000 to 1500 iu/l should suggest ectopic pregnancy.

iv) High levels of hCG are found in some diabetics.

v) Erythroblastic fetus.

vi) Multiple pregnancy.

vii) A Down’s syndrome fetus may also result in an elevated hCG, especially noticeable in the 2nd trimester. It is also used as part of a triple screen for Down’s syndrome.

What are the physiological functions of hCG? (Benefits of elevated hCG)

i) hCG is immunosuppressive in nature and may be involved in the immunological tolerance of the fetus by the mother.

ii) hCG rescues and supports the corpus luteum. It takes over from Luteinizing Hormone (LH) from about the 8th day after ovulation.

iii)hCG probably promotes the secretion of relaxin by the corpus luteum.

iv)There are LH/hCG receptors in the myometrium and in myometrial blood vessels. It is thought that hCG promotes myometrial smooth muscle relaxation or myometrial vasodilatation.

v) hCG stimulates steroidogenesis in the early fetal testis. It enters the fetal plasma from the syncytiotrophoblast. hCG may also have a role in sexual differentiation.

vi) hCG has thyroid stimulating activity.

Can hCG be detected in men?

Yes, hCG can be detected in normal men and women using very sensitive tests. It is thought that it comes from the pituitary glands.

Where is hCG synthesized?

hCG is synthesized primarily in the syncytiotrophoblast.

Immunoreactive hCG has been localized in the cytotrophoblast before 6 weeks of gestation. There is a cellular shift in the formation of hCG from the cytotrophoblast to the syncytiotrophoblast around the 6th week of gestation.

What is the structure of hCG?

hCG is a glycoprotein with a very high carbohydate content (30%, ie the highest of any known human hormone).

It is structurally similar to follicle stimulating hormone (FSH), thyroid stimulating hormone (TSH) and luteinizing hormone.

Just like all other glycoproteins hCG is made up of alpha and beta subunits. The alpha subunit is common to all of the above glycoprotein hormones.The beta subunit differs slightly from the other glycoprotein hormones. This slight alteration of the beta subunit is responsible for the uinque biological activity of the glycoproteins.

Physiology of the hCG hormone

hCG has a very long half life (24 hours) compared to LH (2 hours). This long half life is attributed to the sialic acid content.

hCG can mimic all the functions of LH and even shares the same cell membrane receptors, known as the LH/hCG receptor.

The maternal circulation contains about 30 different isoforms of hCG . These isoforms depends on the stage of the pregnancy and presence of gestational trophoblastic disease. The regular hCG has a molecular weight of 36000

What is hyperglycosylated hCG

This is a recently discovered isoform of hCG.

It is an invaluable marker for the monitoring of GTD’s (Gestational Trophoblastic Disease).

Since it is only produced by invasive cytotrophoblast, it can distinguish between invasive(malignant) and non-invasive(Non-malignant).

References

i)Medical Disorders in Pregnancy-An Update by Hiralal Konar and Pralhad Kushtagi-1st edition-Jaypee Brothers Publishers, New Delhi 2006.

ii)Current Medical Diagnosis and Treatment 2006. Edited by Lawrence M Tierney,Jr., Stephen J. McPhee and Maxine A. Papadakis Mc Graw Hill 45th Edition

iii)Essential Antenatal Care-A guide for the trainee by Deepal S. Weerasekera.

iv)Dewhurst’s Textbook of Obstetrics and Gynaecology for Postgraduates. Sixth edition edited by- D. Keith Edmonds

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