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Isolated hypogonadotropic hypogonadism (GNRHR)
This condition occurs when the body does not produce enough of certain hormones from the pituitary gland, which are needed to stimulate the sex organs. It affects both males and females, leading to absent or incomplete puberty and infertility.
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Overview
Isolated Hypogonadotropic Hypogonadism (IHH) refers to a group of rare genetic conditions where the body does not produce enough of the hormones crucial for sexual development and reproduction. Specifically, it involves a deficiency in gonadotropin-releasing hormone (GnRH) or a problem with how the body responds to it. This leads to reduced levels of luteinising hormone (LH) and follicle-stimulating hormone (FSH) from the pituitary gland, which are essential for stimulating the gonads (testes in males, ovaries in females) [PMID:33675073].
Individuals with IHH typically do not enter puberty spontaneously or experience an incomplete pubertal development. The condition affects both sexes and results in infertility if left untreated. The specific genetic cause, such as changes in the GNRHR gene, helps to classify different forms of IHH.
Symptoms & clinical features
The primary symptoms of Isolated Hypogonadotropic Hypogonadism relate to a lack of pubertal development. In males, this may include small testes and penis, absence of facial and body hair growth, a high-pitched voice, and reduced muscle mass. Females may experience an absence of breast development, primary amenorrhoea (no menstrual periods by age 16), and underdeveloped reproductive organs [PMID:24075110].
Additionally, individuals with IHH might have reduced bone mineral density due to the lack of sex hormones, which can increase the risk of osteoporosis later in life. Some forms of IHH, such as Kallmann syndrome, also involve an impaired sense of smell (anosmia or hyposmia), but this is generally not a feature of IHH caused solely by GNRHR variants.
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Affected organs
Isolated Hypogonadotropic Hypogonadism primarily affects the reproductive system and the endocrine glands that regulate it. The hypothalamus and pituitary gland, located in the brain, are central to this. The hypothalamus produces GnRH, which signals the pituitary gland to release LH and FSH. These hormones then act on the gonads - the testes in males and ovaries in females.
When there is an issue with GnRH signalling, such as in GNRHR-related IHH, the pituitary does not receive the correct signals, leading to insufficient production of LH and FSH. Consequently, the testes and ovaries do not develop or function properly, impacting sexual maturation and fertility. Other organs, such as bones, can also be indirectly affected due to the long-term absence of sex hormones.
Risks & severity
The severity of Isolated Hypogonadotropic Hypogonadism can vary, ranging from complete absence of puberty to partial development. Without treatment, individuals with IHH are infertile and will not experience normal pubertal changes. The long-term absence of sex hormones can also increase the risk of bone density problems (osteopenia or osteoporosis) later in life [PMID:24075110].
While IHH itself is not life-threatening, its impact on quality of life, including sexual development, fertility, and bone health, can be significant. Early diagnosis and appropriate hormone replacement therapy can help manage many of these risks and facilitate pubertal induction and, in some cases, fertility.
Genetic causes
Isolated Hypogonadotropic Hypogonadism can be caused by changes in several different genes. One such gene is GNRHR, which provides instructions for making the gonadotropin-releasing hormone receptor. This receptor is found on the surface of specific cells in the pituitary gland.
The GnRH receptor acts like a lock that fits with the GnRH hormone, which acts as a key. When GnRH binds to its receptor, it triggers the pituitary cells to produce and release LH and FSH. Pathogenic variants in the GNRHR gene can lead to a faulty or absent GnRH receptor, meaning the pituitary cells cannot properly respond to GnRH. This prevents the release of LH and FSH, leading to the hormonal deficiencies seen in IHH [PMID:33675073].
- GNRHR gonadotropin releasing hormone receptor
Inheritance pattern
Isolated Hypogonadotropic Hypogonadism caused by variants in the GNRHR gene is inherited in an autosomal recessive pattern. This means that a person must inherit two altered copies of the gene - one from each parent - to develop the condition.
Parents who each carry one altered copy of the GNRHR gene are known as carriers. Carriers typically do not show symptoms of the condition. When two carriers have children, there is a 25% chance with each pregnancy that their child will inherit two altered gene copies and develop IHH, a 50% chance the child will be a carrier, and a 25% chance the child will inherit two normal gene copies and not be affected or a carrier.
When both parents are carriers, each child has a 25% chance of being affected, 50% of being a carrier, and 25% of being unaffected.
Diagnosis & testing
Diagnosis of Isolated Hypogonadotropic Hypogonadism typically involves a thorough clinical assessment, including physical examination, hormone blood tests, and imaging. Hormone tests measure levels of LH, FSH, oestrogen (in females) or testosterone (in males), and GnRH. Low levels of LH and FSH alongside low sex hormones are indicative of IHH.
Genetic testing is crucial for confirming the diagnosis and identifying the specific genetic cause, such as variants in the GNRHR gene. Genetic testing for IHH is available within the NHS Genomic Medicine Service, typically following a referral from a specialist paediatrician or endocrinologist. The relevant NHS National Genomic Test Directory R-code for hypogonadotropic hypogonadism (including IHH) is R72. Genetic counsellors can provide support and information throughout the diagnostic process.
Management & lifestyle
Management of Isolated Hypogonadotropic Hypogonadism focuses on inducing puberty and, if desired, achieving fertility. This typically involves hormone replacement therapy. For pubertal induction, oestrogen and progesterone are given to females, and testosterone is given to males. This treatment helps to develop secondary sexual characteristics and maintain bone health.
For fertility, treatment usually involves pulsatile GnRH therapy or gonadotropin injections (LH and FSH) to directly stimulate the gonads. This can induce sperm production in males and ovulation in females. Management is typically guided by an endocrinologist or a reproductive specialist, with ongoing monitoring and support from the NHS Genomic Medicine Service and genetic counselling services. Lifestyle considerations, such as a balanced diet and weight-bearing exercise, can also support bone health.
UK care pathway
In the UK, individuals suspected of having Isolated Hypogonadotropic Hypogonadism are typically referred to a specialist, such as a paediatric endocrinologist or an adult endocrinologist. Following clinical assessment and initial hormone investigations, referral for genetic testing can be made through the NHS Genomic Medicine Service. The specific test for this condition falls under R-code R72 (Hypogonadotropic Hypogonadism) in the National Genomic Test Directory.
Clinical genetics teams and genetic counsellors play a vital role in explaining the genetic diagnosis, inheritance patterns, and implications for family members. They also help navigate the ongoing care pathway and support access to appropriate treatments.
Frequently asked questions
What is the main difference between IHH and Kallmann syndrome?
The primary difference is that Kallmann syndrome is a specific type of Isolated Hypogonadotropic Hypogonadism that also involves an impaired sense of smell (anosmia or hyposmia). IHH, including the form caused by GNRHR variants, does not typically affect the sense of smell.
Can individuals with GNRHR-related IHH have children?
With appropriate medical intervention, such as hormone therapy specifically designed to induce fertility, many individuals with GNRHR-related IHH can achieve pregnancy or father children. This often involves treatment with gonadotropin hormones or pulsatile GnRH.
What are the long-term health concerns for someone with IHH?
Beyond the lack of pubertal development and infertility, long-term health concerns primarily involve bone health. Without adequate sex hormones, there is an increased risk of developing osteopenia or osteoporosis, making regular monitoring and treatment important.
How is GNRHR-related IHH diagnosed in the UK?
Diagnosis involves clinical assessment, hormone blood tests, and genetic testing. Your GP would typically refer you to an endocrinologist who can arrange hormone tests and, if IHH is suspected, refer for genetic testing through the NHS Genomic Medicine Service using the R72 R-code.
If I am a carrier of a GNRHR variant, will I have symptoms?
No, individuals who carry only one altered copy of the GNRHR gene (carriers) typically do not experience symptoms of Isolated Hypogonadotropic Hypogonadism. The condition only manifests when two altered copies are inherited, one from each parent, due to its autosomal recessive inheritance pattern.