Resources and Glossary
This section defines key terms for XLH and puts all the resources found throughout the XLH Community Guidebook in one place.
Glossary
Explore this section to learn about key words and definitions related to XLH.XLH A to Z: Words to Know for XLH
A
Abscess
A pocket of pus that can develop when the body fights an infection.1 Pus can contain white blood cells, bacteria, and dead cells.1
Alkaline phosphatase (ALP)
ALP is a protein that helps build strong bones (called bone mineralization).2,3
An ALP test measures the amount of ALP in the blood.2 Healthcare providers may order this test to monitor bone health in people with disorders like XLH.2
Appeal
A request for a health insurance company to reconsider a decision that denies a benefit or payment.4
B
Bowed legs
A condition, also known as genu varum, where the knees curve further outward than the rest of the legs when feet and ankles are together.5
C
Calcification
The process of calcium buildup in body tissues.6 Calcification normally occurs in bones and teeth, but it can be abnormal when calcium builds up where it shouldn’t, like in arteries or as bone spurs.6
See also enthesopathy
Clinical trials
Studies that test how well new medicine works in people with a certain condition.7
There are several types of clinical trials.7 Some study the risks and benefits of only one medicine, while others may compare one medicine against another, or against a placebo (a substance designed to look like the medicine, but does not contain the active ingredient of the medicine).7,8 All trials collect and study information about safety and side effects.7
All clinical trials follow a strict set of rules known as the study protocol.7
Coinsurance
The percentage of costs for a medical service paid by a person after the insurance deductible is reached.9 If a person has met their deductible and has a 20% coinsurance for a $100 office visit, they will pay $20 out-of-pocket.9
See also copay
Copay
Also known as a copayment, this is a fixed amount a person pays for medical services, like doctor visits or prescription drugs.10 The amount is set by an individual’s insurance plan and can be different depending on the service received.10
See also coinsurance
Craniosynostosis
A condition, sometimes associated with XLH, in which a child’s skull bones fuse (merge) together earlier than expected.11 Usually, fusing takes 2 to 3 years.11 If fusing happens early, the skull may have a different shape, and in some cases may reduce the amount of space for the brain to grow.11
Creatinine
A waste substance made when muscles use energy.12 Healthy kidneys remove creatinine from the blood and pass it out of the body in urine.12
Measuring creatinine levels in the blood or urine helps doctors assess how well the kidneys are working.12
D
Deductible
The amount a person pays for covered medical services before the insurance plan begins to cover costs.13
Delayed tooth eruption
Tooth eruption is the process of a tooth growing through the gums and moving to the correct position in the mouth.14 Delayed tooth eruption means it takes longer than expected for the tooth to break through the gums.15
Denial
The decision by an insurance company to not cover a medicine, service, or other type of care.4
E
Endocrine system
The body system that produces hormones.16 These hormones act as chemical messengers.16 They send instructions throughout the body about things like growth and development, mood, and metabolism (including body temperature, digestion, and breathing).16
Enthesopathy
A group of conditions that affect the areas where tendons (connections between muscle and bone) and ligaments (connections between bones) attach to bone.17 In XLH, enthesopathies begin with calcification in the tendons and ligaments.18
See also calcification
Estimated glomerular filtration rate
Also known as eGFR, this is a calculation of how well the kidneys are functioning, specifically how quickly they clean toxins from the blood.19
F
FGF23
A protein called fibroblast growth factor 23, or FGF23.20 The protein is made by the bones and it tells the kidneys how much phosphorus to release into the urine.20
In XLH, a change in the PHEX gene causes the body to make too much FGF23.21 This prevents the kidneys from reabsorbing enough phosphorus back into the blood.21 As a result, there could be too little phosphorus in the blood.21 This may affect the bones and teeth.21
See also PHEX gene
Flexible Spending Account (FSA)
A program through a person’s employer that allows them to pay for medical expenses with pre-tax dollars.22 Unlike HSAs, these accounts are not tied to having a high-deductible health plan.22 Also, these funds may not roll over to the next year, depending on the employer’s plan.22
See also Health Savings Account (HSA)
G
Gene
A small piece of DNA that tells the body how to make a protein.23
Genes are passed down (inherited) from a person’s parents and help decide traits such as hair and eye color and height.24 Some genetic changes occur randomly, rather than being inherited.25
See also spontaneous genetic change
Genetics
The study of how genes or variants in genes affect traits, such as hair and eye color, height, and health conditions like XLH.24 It is also the study of how these traits and conditions can be passed from parents to children.24 Some genetic changes occur randomly, rather than being inherited.25
See also hereditary and spontaneous genetic change
Growth plates (epiphyseal plate)
The softer area near the ends of most long bones in children where growth happens.26 As kids finish growing, these areas harden into solid bone.26 This is called mineralization.3
H
Health Savings Account (HSA)
A type of savings account that allows a person to set aside money for qualified medical expenses on a pre-tax basis, to be used at any time.27 A person can only have an HSA if they have a HSA-eligible plan.27 This is usually a high-deductible health plan that only covers preventative services before the deductible.27
See also Flexible Spending Account (FSA)
Hyperparathyroidism
A condition in which parathyroid glands (small glands in the neck) make too much parathyroid hormone, or PTH.29 PTH helps control the amount of calcium in the blood.29
Hypophosphatemia
A condition in which there is low level of phosphorus in the blood.30
See also phosphorus
I
In-network
Refers to hospitals, doctors, institutions or other healthcare providers who are contracted to work with a person’s insurance.31 These doctors and facilities are referred to as in-network.31
Insurance Accumulators
A program used by some health insurance plans that generally does not allow third-party co-pay assistance (for example, from a drug manufacturer) to count toward annual deductibles and out-of-pocket maximums.32
Insurance Maximizers
A health insurance program that sets a person’s monthly payment for a medicine based on the maximum co-pay assistance offered by the drug maker.32 The maximizer may be applied either in even intervals throughout the year or front-loaded in the early months to obtain the maximum amount of co-pay assistance as early as possible.32 Some maximizers don’t go into effect until the person satisfies their deductible through other health care costs.32 Like accumulators, maximizers do not count toward a person’s deductible and out-of-pocket maximum.32
Insurance Premium
The amount a person pays for their health insurance coverage each month.31
K
Knock knees
A condition, also known as genu valgum, in which a person’s knees touch each other, but the ankles do not.33 As a result, the legs turn inward.33
M
Mental health
The emotional and social well-being of a person.34 This can impact the coping strategies and approaches people use to deal with life challenges, relationships, and stress.34
N
Nephrocalcinosis
A condition where too much calcium builds up in the kidneys.35
O
Osteoarthritis
A type of arthritis caused by a loss of cartilage at the ends of the bones.36 This most often impacts the joints in the hands, knees, spine, and hips.36 Osteoarthritis can result in pain, stiffness, and swelling, among other symptoms.36
Osteomalacia
Softening of the bones, which can result from a lack of calcium, phosphate, or vitamin D.37
Out-of-network
Institutions and healthcare providers who are not covered by a person’s insurance are called out-of-network.31 A higher percentage of the cost is usually covered by insurance for services provided in-network than out-of-network.31
Out-of-pocket maximum
The most a person will pay for medical services over the course of the year.38 After a person reaches the limit from paying co-pays, co-insurance, and deductibles, the insurance plan pays the full amount of additional services that year.38
P
Pain relief
Steps taken to reduce pain resulting from injury or a medical condition. Approaches to relieving pain can include over-the-counter medications like aspirin or acetaminophen (Tylenol), prescription medications, physical therapy, massage, or heat pads and ice packs.39
Peer-to-Peer Review
Process where a doctor discusses a patient’s medical needs with the insurance company if a service, procedure, or medication has been denied.40
PHEX gene
The phosphate-regulating endopeptidase X-linked, or PHEX, gene tells the body how to make a protein related to bone and teeth health.41 The PHEX protein helps regulate another protein called fibroblast growth factor 23, or FGF23.41
In XLH, a change in the PHEX gene increases the amount of FGF23 in the body.21 This causes the kidneys to lose too much phosphorus in the urine instead of returning phosphorus to the blood.21 As a result, there could be too little phosphorus in the blood.21 This may affect the bones and teeth.21
See also phosphorus, FGF23, and phosphate wasting
Phosphate wasting
The loss of too much phosphorus from the body through the urine.42 This leads to low phosphorus in the blood.42
See also phosphorus, FGF23
Phosphorus
A mineral found in bones and teeth, also known as phosphate.43 A main function of phosphorus is building and maintaining bones and teeth.43
Prescription drug coverage
Health insurance that helps pay for medicines prescribed by a person’s doctors.44
Pseudofracture
An area where the bone is weakened because of poor mineralization but not fully broken.45 Also called an incomplete fracture, pseudofractures can be confused with fractures because they appear similar in X-rays.45
Prior authorization
Approval from a health insurance plan for coverage of a medicine or a medical procedure before a person can receive it.46
R
Referral
An order from a primary care doctor for a person to see a specialist.47 Health insurance plans may require a referral to cover the service.47
Rickets
A condition in children resulting in weakening of the part of the bones called growth plates while children are still growing.48 Rickets can be caused by a lack of calcium, vitamin D, and/or phosphorus.48 If there is too little of these, the bones cannot harden as they should, which causes the weakness.48 Rickets can lead to bones that bend or curve (such as bowed legs).48
For more information about how rickets relates to XLH and misdiagnosis, see Understanding XLH
See also growth plates and bowed legs
S
Serum phosphorus
The amount of phosphorus found in the blood of a person, measured using a blood test. Individuals may be advised by their healthcare providers to fast (not have food or beverages for a certain amount of time) before blood is drawn for the test.49 Eating or drinking a few hours before the blood is drawn may change the results of the blood test.49
See also phosphorus
Spinal stenosis
A narrowing of the space in the spine that protects the spinal cord.50 This can put pressure on nerves and spinal cord, often causing pain.50
Spontaneous genetic change
Changes in a gene that occur randomly rather than being inherited from one’s parents.25 These changes can be passed to a person’s children.25
Step therapy
A process in which an insurance company requires a patient to try one or more management options first before the approach the doctor prescribed.51 This is also known as “fail first.”51
T
Tinnitus
A condition in which a person hears ringing, clicking, hissing, or buzzing in one or both ears.52 The sound can be loud, soft, high- or low-pitched.52
Transition of care
The process of moving from one healthcare provider to another.53 For example, from pediatric healthcare providers to adult healthcare providers because a child ages out of pediatric care.53
V
Variant (mutation)
A change in a gene.54 Depending on the gene and type of change, a variant can have no impact or can cause a genetic disease like XLH.54
Vitamin D
An important nutrient that helps the body absorb calcium and phosphorus to build strong bones.55 There are three forms of vitamin D, including55:
- Vitamin D3 and D2: the forms you get from food, supplements, and sunlight
- Storage vitamin D, or 25(OH)D: the form made in the liver
- Active vitamin D, or Calcitriol, or 1,25(OH)2D: the form made by the kidneys after processing storage vitamin D and is ready to be used by the body
W
Waddling gait
A way of walking that looks like the waddle of a duck.56 Some conditions, like XLH, cause bones to develop differently, which can lead to a waddling gait.56
X
X-linked
A condition that is passed to a child through a genetic change on the X chromosome.57 If the X-linked condition is dominant like XLH, a child with one X chromosome containing the variant will have the condition.57 Some genetic changes occur randomly, rather than being inherited.25
See also gene, genetics, hereditary, PHEX gene, and spontaneous genetic change
Resources
The XLH Community Guidebook helps connect XLHers with useful resources at every stage of their journey. Explore all of the resources featured in the guidebook here.
Important:
The information on this website is provided by the XLH Network and Kyowa Kirin for educational purposes only and should not take the place of talking with your doctor or healthcare professional.
It should not be used as medical advice or for diagnosing or treating a health problem or disease. If you have any questions about your medical condition, contact your doctor.
This is a curated list and is not meant to be an exhaustive list of all organizations, services, or online resources. Many of these resources are created by and hosted on third-party websites that are not reviewed, controlled, or endorsed by the XLH Network or Kyowa Kirin.
Diagnosing and Managing XLH
Find a Doctor or Care Provider
XLH Link
National Society of Genetic Counselors
- Finding a Genetic Counselor
- This tool is offered to assist in locating genetic counselors. Healthcare providers listed in this directory have not received nor have they been provided any fees or remuneration for their inclusion. Kyowa Kirin and the XLH Network do not endorse or recommend any healthcare providers included on the list. This list is not comprehensive and is updated as healthcare providers opt in or out of the directory.
Children’s Hospital Association
- Children’s Hospital Association Directory
- This tool is offered to assist in locating children’s hospitals. Hospitals listed in this directory have not received nor have they been provided any fees or remuneration for their inclusion. Kyowa Kirin and the XLH Network do not endorse or recommend any hospitals included on the list. This list is not comprehensive and may not be updated as hospitals opt in or out of the directory.
National Organization for Rare Disorders (NORD)
- Rare Disease Centers of Excellence (NORD)
- This tool is offered to assist in locating rare disease centers of excellence identified by NORD. Care providers listed in this directory have not received nor have they been provided any fees or remuneration for their inclusion. Kyowa Kirin and the XLH Network do not endorse or recommend any care providers included on the list. This list is not comprehensive and may not be updated as care providers opt in or out of the directory.
Make the Most of Doctor Appointments
XLHLink
XLH Community Impact Survey
Advocate for Dental Care
XLH Network
XLH Link
The XLH Community Impact Survey
Preparing for Key Transitions
Find a Doctor or Care Provider
The XLH Network
XLH Link
Plan for Pediatric to Adult Care Transition
XLH Link
Child Neurology Foundation
Global Genes
Navigate Insurance
Health Insurance Marketplace
Dating, Relationships & Family Planning
National Organization for Rare Disorders (NORD)
Work
Rehabilitation Services Administration
Aging
National Organization for Rare Disorders (NORD)
Eosinophilic & Rare Disease Cooperative
State Health Insurance Assistance Program (SHIP)
Daily Living with XLH
Connect with Others
XLH Link
XLH Patient Organizations
Rare Disease Patient Advocacy Organizations
Special Interest Advocacy Organizations
Navigate Work, Employment, or Jobs
Workplace Accommodations
- Americans with Disabilities Act
- Job Accommodation Network
- Understood.org
Employment Benefits
Employment Opportunities
- Rehabilitation Services Administration
Advocate for School Support
Center for Parent & Information Resources
PROGRESS Center
Understood.org
Navigate Health Insurance
National Organization for Rare Disorders (NORD)
Healthcare.gov
Social Security Administration
Medicaid.gov
Medicaid.gov
Plan for Travel
U.S. Department of Transportation
Johns Hopkins Healthcare
Global Genes Rare Parenting
Learn About Aids and Adaptations
United Disability Services
National Organization for Rare Disorders (NORD)
ALS Association
- Adaptive Equipment (PDF)
- Not all information may be relevant to people with XLH, as the material was written for the ALS community.
Prepare for Emergencies
U.S. Department of Homeland Security
Fostering Health and Wellness
Advocacy Organizations Providing Mental Health Resources for Chronic and Rare Conditions
Find Mental Health Support and Resources
Support for Mental Health in XLH
- 988 Suicide & Crisis Lifeline
- Call or text 988
- 988 Suicide Prevention Lifeline
- 211
- XLH Community Impact Survey
- Global Genes
- Stomp Out Bullying
- NORD
- Center for Chronic Illness
Social Media Support for Teens
- National Alliance on Mental Illness (NAMI)
Support for Kids
- National Alliance on Mental Illness (NAMI)
- The Kids Mental Health Foundation
- Stomp Out Bullying
Using Health Insurance
- Caregiver Action Network
- Give an Hour
References
- Medline Plus. Abscess. August 29, 2024. medlineplus.gov/ency/article/001353.htm.
- MedlinePlus. Alkaline phosphatase: MedlinePlus lab test information. November 5, 2024. medlineplus.gov/lab-tests/alkaline-phosphatase.
- Science Direct. Bone mineralization. 2016. sciencedirect.com/topics/medicine-and-dentistry/bone-mineralization.
- Patient Advocate Foundation. Engaging with Insurers: Appealing a Denial. https://education.patientadvocate.org/wp-content/uploads/2022/07/Engaging-with-Insurers.-Appealing-a-Denial.pdf.
- MedlinePlus. Bowlegs. October 20, 2024. medlineplus.gov/ency/article/001585.htm.
- MedlinePlus. Calcification. July 23, 2024. medlineplus.gov/ency/article/002321.htm.
- MedlinePlus. Clinical trials. May 18, 2018. medlineplus.gov/clinicaltrials.html.
- NIH National Human Genome Research Institute. Clinical research FAQ. August 11, 2017. www.genome.gov/FAQ/Clinical-Research.
- HealthCare.gov. Coinsurance. www.healthcare.gov/glossary/co-insurance.
- HealthCare.gov. Copayment. www.healthcare.gov/glossary/co-payment.
- Medline Plus. Craniosynostosis; December 31, 2023. medlineplus.gov/ency/article/001590.htm.
- Mayo Clinic. Creatinine test. June 12, 2025. www.mayoclinic.org/tests-procedures/creatinine-test/about/pac-20384646.
- HealthCare.gov. Deductible. www.healthcare.gov/glossary/deductible.
- Suri L, Gagari E, Vastardis H. Delayed tooth eruption: pathogenesis, diagnosis, and treatment. A literature review. Am J Orthod Dentofacial Orthop. 2004;126(4):432-445. doi:10.1016/j.ajodo.2003.10.031.
- Prachi J, Manu R. Anatomy, head and neck, tooth eruption. StatPearls. January 2025. www.ncbi.nlm.nih.gov/books/NBK549878/.
- MedlinePlus. Endocrine diseases. November 18, 2025. medlineplus.gov/endocrinediseases.html.
- Alvarez A, Tiu T. Enthesopathies. StatPearls [Internet]. June 5, 2023. www.ncbi.nlm.nih.gov/books/NBK559030.
- Imel E. Enthesopathy, osteoarthritis, and mobility in X-linked hypophosphatemia. J Clin Endocrinol Metab. July 2020, Pages e2649–e2651. https://doi.org/10.1210/clinem/dgaa242.
- Estimated glomerular filtration rate (eGFR). May 28, 2025. https://my.clevelandclinic.org/health/diagnostics/21593-estimated-glomerular-filtration-rate-egfr.
- MedlinePlus. FGF23 gene. August 1, 2012. medlineplus.gov/genetics/gene/fgf23.
- Guo, Y., Yuan,Q. Fibroblast growth factor 23 and bone mineralisation. Int J Oral Sci. February 2015. doi: 10.1038/ijos.2015.1.
- HealthCare.gov. Flexible Spending Account (FSA). www.healthcare.gov/glossary/flexible-spending-account-fsa.
- MedlinePlus. Genes. April 1, 2025. medlineplus.gov/ency/article/002371.htm.
- MedlinePlus. Genetics. March 31, 2024. medlineplus.gov/ency/article/002048.htm.
- ScienceDirect. Spontaneous Mutation. www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/spontaneous-mutation.
- NIH National Institute of Arthritis and Musculoskeletal and Skin Diseases. Growth plate injuries. December 2023. www.niams.nih.gov/health-topics/growth-plate-injuries.
- HealthCare.gov. Health Savings Account (HSA). 2019. www.healthcare.gov/glossary/health-savings-account-HSA.
- Merriam-Webster. Hereditary. www.merriam-webster.com/dictionary/hereditary.
- Merriam-Webster. Hyperparathyroidism. www.merriam-webster.com/dictionary/hyperparathyroidism.
- MedlinePlus. Hypophosphatemia. December 31, 2023. medlineplus.gov/ency/article/000307.htm.
- U.S. Department of Health & Human Services. Health insurance basics. September 2023. www.cms.gov/files/document/nsa-health-insurance-basics.pdf.
- Autoimmune Association. Understanding health Insurance accumulators and maximizers. November 19, 2025. autoimmune.org/advocacy/accumulators-and-maximizers.
- Cleveland Clinic. Treating knock knees. November 7, 2024. consultqd.clevelandclinic.org/treating-idiopathic-genu-valgum-knock-knees.
- MedlinePlus. Mental health. April 15, 2024. medlineplus.gov/mentalhealth.html.
- MedlinePlus. Nephrocalcinosis. July 1, 2023. medlineplus.gov/ency/article/000492.htm.
- Mayo Clinic. Osteoarthritis. Apr 08, 2025. www.mayoclinic.org/diseases-conditions/osteoarthritis/symptoms-causes/syc-20351925.
- MedlinePlus. Osteomalacia. May 20, 2024. medlineplus.gov/ency/article/000376.htm.
- Healthcare.gov. Out-of-pocket maximum/limit. www.healthcare.gov/glossary/out-of-pocket-maximum-limit.
- MedlinePlus. Pain Relievers. July 30, 2025. medlineplus.gov/painrelievers.html.
- HealthCare.gov. External Review. www.healthcare.gov/appeal-insurance-company-decision/external-review.
- MedlinePlus. PHEX gene. February 13, 2026. medlineplus.gov/genetics/gene/phex.
- ScienceDirect. Hypophosphatemia. www.sciencedirect.com/topics/medicine-and-dentistry/hypophosphatemia.
- MedlinePlus. Phosphorus in diet. January 21, 2025. medlineplus.gov/ency/article/002424.htm.
- HealthCare.gov. Prescription drug coverage. www.healthcare.gov/glossary/prescription-drug-coverage.
- NIH National Center for Biotechnology Information. Pseudofractures. www.ncbi.nlm.nih.gov/medgen/866681.
- HealthCare.gov. Prior authorization. www.healthcare.gov/glossary/prior-authorization.
- HealthCare.gov. Referral. www.healthcare.gov/glossary/referral.
- Mayo Clinic. Rickets. April 25, 2025, www.mayoclinic.org/diseases-conditions/rickets/symptoms-causes/syc-20351943.
- MedlinePlus. Phosphate in the blood. October 25, 2023. medlineplus.gov/lab-tests/phosphate-in-blood.
- MedlinePlus. Spinal Stenosis. September 24, 2025. medlineplus.gov/spinalstenosis.html.
- Autoimmune Association. Understanding Step Therapy. autoimmune.org/advocacy/understanding-step-therapy.
- MedlinePlus. Tinnitus. May 2, 2024. medlineplus.gov/ency/article/003043.htm.
- Dahir K., et al. Health care transition from pediatric- to adult-focused care in X-linked hypophosphatemia: expert consensus. J Clin Endocrinol Metab. November 6, 2021. doi:10.1210/clinem/dgab796.
- NIH National Cancer Institute. Mutation. www.cancer.gov/publications/dictionaries/cancer-terms/def/mutation.
- NIH Office of Dietary Supplements. Vitamin D fact sheet for health professionals. June 27, 2025. ods.od.nih.gov/factsheets/VitaminD-HealthProfessional.
- Cleveland Clinic. Gait disorders and abnormalities. February 22, 2023. my.clevelandclinic.org/health/diseases/21092-gait-disorders.
- NIH National Cancer Institute. X-linked dominant inheritance. www.cancer.gov/publications/dictionaries/genetics-dictionary/def/x-linked-dominant-inheritance.
