Research Article
No access
Published Online: 4 February 2009

Thyrotropic Action of Human Chorionic Gonadotropin

Publication: Thyroid
Volume 5, Issue Number 5

Abstract

Hyperthyroidism or increased thyroid function has been reported in many patients with trophoblastic tumors. In these cases, greatly increased human chorionic gonadotropin (hCG) levels and suppressed TSH levels suggest that hCG has thyrotropic activity. Recent investigations have clarified the structural homology not only in the hCG and TSH molecules but also in their receptors, and this homology suggests the basis for the reactivity of hCG with the TSH receptor. The clinical significance of the thyrotropic action of hCG is now also recognized in normal pregnancy and hyperemesis gravidarum. Highly purified hLH binds to recombinant hTSH receptor and is about 10 times as potent as purified hCG in increasing cAMP. The βsubunits of hCG and hLH share 85% sequence identity in their first 114 amino acids but differ in the carboxy-terminal peptide because hCGβ contains a 31-amino acid extension (β-CTP). A recombinant mutant hCG that lacks β-CTP showed almost identical potency to LH on stimulation of recombinant hTSH receptor. If intact hCG were as potent as hLH in regard to its thyrotropic activity, most pregnant women would become thyrotoxic. One of the roles of the β-CTP may be to prevent overt hyperthyroidism in the first trimester of pregnancy when a large amount of hCG is produced by the placenta. Nicked hCG preparations, obtained from patients with trophoblastic disease or by enzymatic digestion of intact hCG, showed approximately 1.5- to 2-fold stimulation of recombinant hTSH receptor compared with intact hCG. This suggests that the thyrotropic activity of hCG may be influenced by the metabolism of the hCG molecule itself. Deglycosylation and/or desialylation of hCG enhances its thyrotropic potency. Basic hCG isoforms with lower sialic acid content extracted from hydatidiform moles were more potent in activating adenylate cyclase, and showed high bioactivity/immunoactivity (B/I) ratio in CHO cells expressing human TSH receptors. This is consistent with the finding that the β-CTP truncated hCG with higher thyrotropic potency is substantially deglycosylated and desialylated in the β-subunit relative to intact hCG because all four O-linked glycosylation sites occur within the missing C-terminal extension. The desialylated hCG variant also interacts directly with recombinant hTSH receptors transfected into human thyroid cancer cells.
There is thyroid-stimulating activity in sera of normal pregnant women, and this correlates with serum hCG levels. The thyroid gland of normal pregnant women may be stimulated by hCG to secrete slightly excessive quantities of T4 and induce a slight suppression of TSH, perhaps being about 1 mU/L less than nongravid levels, but not high enough to induce overt hyperthyroidism. Maternal thyroid glands may secrete more thyroid hormone during early pregnancy in response to the thyrotropic activity of hCG that overrides the normal operation of the hypothalamic—pituitary—thyroid feedback system. Biochemical hyperthyroidism associated with hyperemesis gravidarum has been attributed to hCG. In patients with hyperemesis gravidarum, thyrotropic activity in serum correlated with hCG immunoreactivity, and the severity of vomiting as indicated by clinical and biochemical parameters correlated with the degree of thyroid stimulation. To understand the thyrotropic action of hCG, it is necessary to know whether hCG activates the same domain of the TSH receptor as does TSH. The identification of the molecular structure of the hCG isoform with the highest thyrotropic potency will resolve the enigma of gestational thyrotoxicosis and the hyperthyroidism associated with trophoblastic disease and hCG-producing tumors.

Get full access to this article

View all available purchase options and get full access to this article.

Information & Authors

Information

Published In

cover image Thyroid®
Thyroid
Volume 5Issue Number 5October 1995
Pages: 425 - 434
PubMed: 8563483

History

Published online: 4 February 2009
Published in print: October 1995

Permissions

Request permissions for this article.

Topics

    Authors

    Affiliations

    Metrics & Citations

    Metrics

    Citations

    Export citation

    Select the format you want to export the citations of this publication.

    View Options

    Get Access

    Access content

    To read the fulltext, please use one of the options below to sign in or purchase access.

    Society Access

    If you are a member of a society that has access to this content please log in via your society website and then return to this publication.

    Restore your content access

    Enter your email address to restore your content access:

    Note: This functionality works only for purchases done as a guest. If you already have an account, log in to access the content to which you are entitled.

    View options

    PDF/EPUB

    View PDF/ePub

    Media

    Figures

    Other

    Tables

    Share

    Share

    Copy the content Link

    Share on social media

    Back to Top