Most liver cysts are present from birth, formed by a minor glitch during fetal development when tiny bile ducts fail to connect properly and instead fill with fluid. These simple cysts are by far the most common type, showing up on about 22% of ultrasound scans in the general population. They’re almost always harmless. But liver cysts can also develop from inherited genetic conditions, parasitic infections, or, rarely, abnormal cell growth that carries a risk of becoming cancerous.
Simple Cysts: A Developmental Leftover
Simple liver cysts form when small bile duct glands become obstructed during fetal development. These glands normally start branching from the liver’s ductal system around the seventh week of pregnancy and continue developing through adolescence. When a duct doesn’t connect the way it should, fluid accumulates in a sealed pocket. The resulting cyst doesn’t contain bile and has no connection to the working bile duct system.
Because they originate so early in development, simple cysts are technically congenital, even if they aren’t detected until decades later. Detection rises sharply with age: fewer than 1% of people under 30 have a visible cyst on ultrasound, compared with about 29% of people in their 60s and over 34% of those 80 and older. This doesn’t mean new cysts are constantly forming. It largely reflects the fact that small cysts grow slowly over a lifetime and eventually become big enough for imaging to pick up.
The vast majority of simple cysts cause no symptoms at all. Roughly 5% eventually grow large enough to press on surrounding structures, producing upper abdominal pain, bloating, nausea, or a feeling of fullness. Treatment is rarely needed unless a cyst exceeds about 10 to 15 centimeters or grows more than 3 centimeters per year.
Polycystic Liver Disease: A Genetic Cause
When dozens or even hundreds of cysts develop throughout the liver, the cause is usually genetic. Polycystic liver disease (PLD) follows an autosomal dominant inheritance pattern, meaning a single copy of a mutated gene from one parent is enough to cause the condition. The most well-characterized mutations occur in the PRKCSH gene, which plays a role in how proteins are processed inside liver cells. When this gene is disrupted, the cells lining small bile ducts proliferate abnormally and form fluid-filled sacs.
PLD can also occur alongside polycystic kidney disease, a related but distinct genetic condition. In both cases, the cysts themselves are benign, but the sheer number and size of them can enlarge the liver dramatically, causing pain, difficulty eating, and reduced quality of life over time.
Estrogen and Cyst Growth
Estrogen plays a significant role in how fast liver cysts grow, particularly in people with polycystic liver disease. In the cells lining the inside of hepatic cysts, estrogen stimulates proliferation. It also suppresses expression of the PRKCSH gene, compounding the genetic imbalance already present in PLD.
A large cross-sectional study found that every year of estrogen-containing oral contraceptive use correlated with a 1.45% increase in total liver volume among premenopausal women with PLD. Over a decade, that adds up to a roughly 15.5% larger liver compared with women who never used hormonal birth control. Postmenopausal hormone replacement therapy with estrogen also increased liver volume in a separate prospective study of women with polycystic kidney disease. This hormonal sensitivity helps explain why PLD tends to be more severe in women than men, and why symptoms often worsen during pregnancy or with long-term contraceptive use. For people with PLD, nonhormonal contraceptives are generally recommended.
Parasitic Cysts From Tapeworm Infection
In parts of the world where livestock farming is common, liver cysts can be caused by a tapeworm called Echinococcus granulosus. This infection, known as hydatid disease, occurs when a person accidentally swallows tapeworm eggs shed in the feces of infected dogs or other canines. The eggs hatch in the small intestine, and the larvae burrow through the intestinal wall into the bloodstream, eventually settling in the liver (or sometimes the lungs).
Once lodged in the liver, a larva builds a thick-walled cyst around itself that grows slowly over months to years, producing internal daughter cysts and larval structures. Unlike simple cysts, hydatid cysts are biologically active, housing a living parasite. They can grow quite large and cause pain, allergic reactions, or serious complications if they rupture. Hydatid disease is rare in the United States and Western Europe but remains a significant health concern in South America, the Mediterranean, Central Asia, and parts of Africa.
Neoplastic Cysts: When Abnormal Cells Are Involved
A small fraction of liver cysts arise from abnormal cell growth rather than developmental or infectious causes. The most notable of these is a mucinous cystic neoplasm (previously called biliary cystadenoma), a slow-growing, fluid-filled tumor that typically appears as a single large, multi-chambered cyst. Its exact origin is debated. Some researchers point to congenital tissue remnants from the embryonic foregut that become trapped in the liver. Others suggest it develops as a reactive process following focal injury to the bile ducts. Evidence of endocrine cells within these tumors also suggests they may originate from glands surrounding the bile ducts.
The key concern with neoplastic cysts is malignant transformation. Studies have reported that up to 23 to 30% of these tumors show cancerous changes when removed surgically, though no large controlled study has pinned down the exact risk for cysts left in place. Because of this uncertainty, surgical removal is the standard approach whenever a neoplastic cyst is identified.
How Different Cysts Look on Imaging
The cause of a liver cyst often determines what it looks like on ultrasound, CT, or MRI, which is how most cysts are discovered in the first place (usually by accident during imaging for something else).
A simple cyst appears as a well-defined, round pocket of clear fluid with a thin wall so fine it’s barely visible. It shows no internal structures, no solid components, and no enhancement when contrast dye is injected. On CT, its density matches water. On MRI, it lights up brightly on certain sequences, consistent with pure fluid.
Features that raise concern include a thick or irregular wall, internal dividers (septations), solid nodules within the cyst, enhancement after contrast injection, or fluid that doesn’t behave like water on imaging. Mucinous cystic neoplasms, for example, typically show multiple internal chambers with enhancing walls and septations. Cystic metastases from cancers elsewhere in the body show thick walls, nodules, and contrast enhancement. An infected cyst may contain gas bubbles or layered fluid of different densities. A hemorrhagic cyst (one that has bled internally) shows higher density than water on CT and may have mobile internal strands, but those strands won’t enhance with contrast, helping distinguish it from a tumor.
Rare Complications of Liver Cysts
For the overwhelming majority of people, a liver cyst will never cause a problem. The rare complications tend to occur with larger cysts. Hemorrhagic rupture, where a cyst bleeds and bursts, is extremely uncommon but potentially life-threatening. It can present as sudden, severe abdominal pain with signs of internal bleeding such as a rapid heart rate and dropping blood pressure. Infection of a cyst can also occur, causing fever, localized pain, and characteristic changes on imaging. In people already known to have liver cysts, sudden or worsening abdominal pain warrants prompt evaluation to rule out these complications.