Chorionic Gonadotropin hCG, HCG

Overview

Chorionic gonadotropin (hCG; Chorulon®) is a human glycoprotein reproductive hormone used in veterinary medicine for a variety of theriogenologic conditions. The hormone mimics the physiologic effects of luteinizing hormone (LH) and is primarily administered to influence reproductive function in both male and female animals.

In females, hCG is most commonly used to induce ovulation and support reproductive management protocols. In queens it may be used to induce ovulation during the later stages of estrus, while in bitches it has been investigated for delayed ovulation and for diagnostic purposes such as identifying ovarian remnant syndrome through stimulation of estrogen secretion.

In males, hCG stimulates the testicular interstitial (Leydig) cells, leading to increased androgen production and transient increases in testosterone secretion. Although this may temporarily stimulate reproductive hormone activity, improvements in libido are not always observed and behavioral changes such as increased aggression may occur.

Mechanism of Action (MOA): Chorionic gonadotropin acts as a luteinizing hormone analog. By binding to LH receptors in the gonads, it stimulates steroidogenesis and reproductive hormone production. In females this promotes ovulation and ovarian hormone secretion, while in males it stimulates androgen production by Leydig cells.

Human chorionic gonadotropin is not orally active because it is destroyed in the gastrointestinal tract; therefore, it must be administered parenterally. After intramuscular administration, peak plasma concentrations occur approximately 6 hours after injection, and the hormone is distributed primarily to the ovaries in females and testes in males.

Indications

Chorionic gonadotropin (hCG) is used in veterinary reproductive medicine to influence gonadal hormone production and ovulation. Because it mimics luteinizing hormone (LH), it is used primarily for ovulation induction, reproductive diagnostics, and short-term stimulation of androgen secretion in male animals.

• Induction of ovulation in queens: Commonly administered during the later stages of estrus to trigger ovulation in breeding management protocols.
• Delayed ovulation in bitches: May be used in attempts to manage delayed ovulation; however, clinical results are inconsistent and ovulation may not reliably occur.
• Diagnostic testing for ovarian remnants: In bitches, hCG administration may stimulate estrogen secretion during anestrus and can be used to help identify ovarian remnant syndrome.
• Short-term stimulation of testosterone in males: Used to stimulate Leydig cells and temporarily increase testosterone secretion, although behavioral effects such as increased aggression may occur without consistent improvement in libido.

Dosage (Reference)

Dog

In dogs, chorionic gonadotropin (hCG) is administered by intramuscular injection for reproductive indications such as delayed ovulation or stimulation of testosterone production in males. Dosing protocols depend on the clinical objective.

Cat

Chorionic gonadotropin is not specifically authorized for use in cats; however, when used in feline reproductive management, dosing recommendations are generally the same as those used in dogs.

Warnings & Precautions

Chorionic gonadotropin (hCG) should be used cautiously in veterinary patients because of its potent hormonal effects and the potential for hypersensitivity reactions. Proper handling precautions are also recommended due to its classification as a hazardous drug.

• Hypersensitivity: Contraindicated in animals with known hypersensitivity to chorionic gonadotropin. Although uncommon, allergic reactions including anaphylaxis are possible.
• Androgen-dependent neoplasia: Based on human medical guidelines, hCG should be avoided in patients with androgen-responsive tumors (e.g., prostatic carcinoma).
• Precocious puberty: In humans, the drug is contraindicated in patients with precocious puberty; these precautions may also be considered when evaluating veterinary patients.
• Antibody formation with repeated use: Repeated administration may lead to antibody formation against hCG; however, ovulation-inducing activity generally remains clinically effective.
• Occupational exposure risk: The National Institute for Occupational Safety and Health (NIOSH) classifies hCG as a hazardous drug. Appropriate personal protective equipment (PPE) should be used during preparation and administration to reduce exposure risk.
• Pregnancy considerations: Safety in animals has not been fully established. The drug should be used during pregnancy only when potential benefits outweigh potential risks to the fetus.

Drug Interactions

No clinically significant drug interactions have been reported with chorionic gonadotropin (hCG) in veterinary or human medicine. However, as with any hormonal therapy, clinicians should evaluate concurrent medications when designing reproductive management protocols.

• No documented interactions: Available literature has not identified specific drug interactions associated with chorionic gonadotropin administration.
• Concurrent reproductive hormones: When used as part of reproductive management protocols, hCG may be administered alongside other hormonal agents under veterinary supervision.

Side Effects & Overdose

Side Effects

Adverse effects associated with chorionic gonadotropin (hCG) are uncommon in veterinary patients. Most animals tolerate the drug well when administered at recommended doses, although hypersensitivity reactions may occasionally occur.

• Hypersensitivity reactions: Allergic responses are possible, including rare anaphylactic reactions following administration.
• Injection site reactions: Mild discomfort or local irritation may occur after intramuscular injection.
• Reproductive effects in mares: Repeated doses administered prior to approximately day 35 of pregnancy may result in abortion, possibly due to increased estrogen concentrations.
• Transient allergic signs: Some animals may develop temporary hives or respiratory distress shortly after administration.
• Human-reported effects: In people, reported effects include headache, irritability, depression, edema, and gynecomastia, although these reactions are not well documented in animals.

Overdose

No overdose cases have been reported with chorionic gonadotropin (hCG). However, excessive administration of hormonal agents may theoretically increase the risk of adverse hormonal or hypersensitivity reactions.

• No documented overdose cases: Published literature has not described specific overdose events associated with hCG administration.
• Monitoring: Animals receiving high or repeated doses should be monitored for allergic reactions or abnormal reproductive responses.
• Management: Treatment of suspected toxicity is supportive and based on the clinical signs present.

Key Notes

Practical clinical considerations that may help optimize the use of chorionic gonadotropin (hCG) in veterinary reproductive management:

• Parenteral administration only: hCG is a peptide hormone that is destroyed in the gastrointestinal tract and therefore must be administered by injection.
• Target tissue distribution: After administration, the hormone primarily distributes to the ovaries in females and testes in males where it stimulates gonadal hormone production.
• Biphasic elimination: The drug is cleared from circulation in two phases, with an initial shorter elimination phase followed by a longer terminal phase.
• Veterinary regulatory status: Some commercial veterinary formulations are specifically approved for use in food-producing animals such as cattle and fish.
• Product handling after reconstitution: Reconstituted injectable preparations may have limited stability depending on the product and should be stored according to manufacturer instructions.
• Large vial preparations: Veterinary formulations often contain large unit quantities (e.g., 10,000 units), and aliquoting or careful storage may be required to minimize drug waste.