CTX most commonly refers to one of two things in medicine: a blood test that measures bone breakdown, or a rare genetic disorder called cerebrotendinous xanthomatosis. Which one matters to you depends on context. If your doctor ordered a “CTX test” or you saw it on lab results, you’re almost certainly looking at the bone marker. If you or someone you know received a CTX diagnosis, that refers to the genetic condition. Here’s what you need to know about both.
CTX as a Bone Resorption Marker
C-terminal telopeptide, abbreviated CTX (sometimes called beta-CrossLaps), is a fragment of type I collagen, the most abundant protein in bone. When your body breaks down old bone tissue through a normal recycling process, these collagen fragments get released into your bloodstream. The more bone breakdown happening, the more CTX shows up in a blood draw. That makes it a reliable indicator of how fast you’re losing bone.
Doctors order CTX tests primarily to monitor osteoporosis and to check whether bone-protecting medications are working. Because the marker responds quickly to changes in bone activity, it can show treatment effects well before a bone density scan would pick up any difference. Studies have confirmed that higher levels of bone resorption markers like CTX predict a greater risk of fractures, and that bigger drops in CTX during treatment correspond to bigger reductions in spine and hip fractures.
What Normal CTX Levels Look Like
CTX is measured in nanograms per milliliter (ng/mL). In premenopausal women aged 30 to 54, the typical range is 0.05 to 0.67 ng/mL. Postmenopausal women tend to run higher, with a range of 0.09 to 1.05 ng/mL, reflecting the increased bone turnover that comes with declining estrogen. For men, CTX decreases with age: young men (25 to 29) range from 0.12 to 0.83 ng/mL, while men in their late 70s range from 0.05 to 0.58 ng/mL.
If your results fall above these ranges, it suggests your body is breaking down bone faster than expected. That doesn’t automatically mean you have osteoporosis, but it’s a signal your doctor will factor in alongside bone density scans and your overall risk profile.
How the CTX Blood Test Works
CTX levels fluctuate significantly throughout the day, peaking between midnight and 8 AM and dropping to their lowest point in the afternoon. Of all bone markers, CTX shows the most dramatic swing from morning to evening. To get a consistent reading, the test requires a 12-hour fast and a morning blood draw before 10 AM. If you take biotin supplements or multivitamins containing biotin (vitamin B7), you should skip them for at least 8 hours before the draw, as biotin can interfere with the assay.
CTX as a Genetic Disorder
Cerebrotendinous xanthomatosis is a rare lipid storage disease, estimated to affect roughly 1 in every million people worldwide. It’s caused by mutations in a gene called CYP27A1, which provides instructions for an enzyme your body needs to make bile acids. Without enough of this enzyme, your liver can’t produce adequate bile acids (particularly one called chenodeoxycholic acid), and cholesterol byproducts, especially a substance called cholestanol, accumulate in tissues throughout the body. The brain, eyes, and tendons take the biggest hit.
CTX is inherited in an autosomal recessive pattern, meaning a child must receive a faulty copy of the gene from both parents to develop the condition. It’s notably more common in the Moroccan Jewish population, where the incidence rises to about 1 in 108 individuals.
How CTX Symptoms Progress Over Time
One of the tricky aspects of cerebrotendinous xanthomatosis is that it unfolds gradually across decades, and the earliest signs are easy to mistake for other conditions.
In infancy, the first clue is often chronic diarrhea or neonatal jaundice. These are nonspecific enough that many cases go unrecognized at this stage. By the first decade of life, around 75% of affected children develop cataracts, typically in both eyes. Vision problems in a young child should raise suspicion, particularly if combined with a history of persistent digestive issues.
During the teens and twenties, fatty deposits called xanthomas begin to appear on tendons, most commonly the Achilles tendon but also tendons in the elbows, hands, knees, and neck. These are the visible hallmark of the disease and often the finding that finally leads to a diagnosis.
The neurological decline typically becomes apparent in the third decade. More than half of affected individuals experience progressive dementia with a slow deterioration in thinking ability. Muscle stiffness and coordination problems (from damage to the brain’s motor and balance centers) almost invariably show up between ages 20 and 30. Other neurological features include seizures, movement abnormalities resembling Parkinson’s disease, and peripheral nerve damage causing numbness or weakness in the hands and feet. Psychiatric symptoms can be prominent as well: behavioral changes, hallucinations, agitation, depression, and in some cases suicide attempts.
How CTX Is Diagnosed
Doctors look for the combination of childhood cataracts, tendon xanthomas, chronic diarrhea, and progressive neurological problems. Blood tests reveal the biochemical signature: elevated cholestanol is the primary marker, often running more than seven times the normal upper limit. Levels of other sterols, including a cholesterol precursor called 7-dehydrocholesterol, also tend to be elevated. Genetic testing of the CYP27A1 gene confirms the diagnosis.
CTX can be confused with other rare lipid disorders. Sitosterolemia, for instance, also causes xanthomas, but the blood sterol profile is different: sitosterolemia patients accumulate plant sterols like sitosterol, while CTX patients accumulate cholestanol. The distinction matters because the treatments are completely different.
Treatment for Cerebrotendinous Xanthomatosis
The core treatment replaces what the body can’t make on its own. An FDA-approved bile acid called chenodiol (brand name Ctexli) is taken as a 250 mg tablet three times daily, with or without food. By supplying the missing bile acid, this treatment restores the feedback loop that normally keeps cholesterol metabolism in check, which slows or stops the buildup of cholestanol in tissues.
When treatment starts early, before significant neurological damage has occurred, outcomes are substantially better. Cataracts and diarrhea often improve, cholestanol levels drop, and neurological progression can be slowed or halted. Once brain damage is established, however, it generally cannot be reversed. That’s why early recognition of the disease, ideally in childhood, makes such a critical difference in long-term quality of life.