Ovarian cysts are almost always caused by the normal hormonal process of ovulation. Each month, a small fluid-filled sac (follicle) grows on the ovary to release an egg, and when that process doesn’t complete as expected, the follicle can fill with fluid and become a cyst. These functional cysts are by far the most common type, but cysts can also form from misplaced tissue, abnormal cell growth, infection, or hormonal imbalances that disrupt ovulation entirely.
Functional Cysts: The Most Common Cause
To understand functional cysts, it helps to know what happens during a normal menstrual cycle. Each month, several follicles begin developing on the ovaries under the influence of hormones, particularly FSH (follicle-stimulating hormone). One follicle is selected as the dominant one, growing larger while the others shrink away. When a surge of LH (luteinizing hormone) hits, the dominant follicle ruptures and releases its egg. That’s ovulation.
A follicular cyst forms when this process stalls. The dominant follicle grows but never ruptures. Instead of releasing its egg, it continues to accumulate fluid and can swell to several centimeters. These cysts are typically painless and resolve on their own within one to three menstrual cycles.
A corpus luteum cyst forms after ovulation does occur. Normally, the emptied follicle collapses, transforms into a structure called the corpus luteum, and begins producing estrogen and progesterone to prepare for a potential pregnancy. Sometimes, though, the opening where the egg escaped seals shut. Fluid builds up inside, and the corpus luteum balloons into a cyst. These can occasionally grow large enough to cause a dull ache or, rarely, twist the ovary or rupture and bleed.
Both types are considered “functional” because they arise from the ovary doing its job. They’re so common that most people with ovaries will develop one at some point without ever knowing it.
Endometriomas: Cysts From Misplaced Tissue
Endometriomas are cysts that develop as part of endometriosis, a condition where tissue similar to the uterine lining grows outside the uterus. When this tissue attaches to an ovary, it responds to the same hormonal shifts that drive your menstrual cycle. It thickens, breaks down, and bleeds each month, just as it would inside the uterus. But because the blood has nowhere to go, it collects inside a cyst on the ovary.
Over many cycles, the trapped blood darkens and thickens, which is why these are sometimes called “chocolate cysts.” Unlike a simple blood collection, endometriomas are lined with sticky endometrial tissue and contain more fibrous material, which makes them less likely to resolve on their own. The most widely accepted explanation for how the endometrial tissue gets there in the first place is retrograde menstruation, where menstrual blood flows backward through the fallopian tubes and into the pelvic cavity, seeding tissue in places it doesn’t belong.
Dermoid Cysts and Cystadenomas
Not all ovarian cysts come from the monthly cycle. Dermoid cysts are benign growths that develop from embryonic cells, the same type of cells that form skin, hair, and teeth during fetal development. Because of their origin, dermoid cysts can contain surprisingly varied tissue: hair follicles, oil-producing glands, and even fragments of bone or cartilage. They grow slowly and can be present for years before being discovered incidentally on an ultrasound. They’re most common in younger women of reproductive age.
Cystadenomas are another type of non-functional cyst. These develop from the cells on the outer surface of the ovary. Serous cystadenomas are filled with thin, watery fluid, while mucinous cystadenomas contain a thicker, gel-like substance. Some mucinous types share a developmental origin with dermoid cysts, suggesting they come from the same embryonic cell lines. Cystadenomas can grow quite large, sometimes reaching 10 centimeters or more, which increases the risk of ovarian torsion (the ovary twisting on itself).
How PCOS Causes Multiple Cysts
Polycystic ovary syndrome produces cysts through a different mechanism than the occasional functional cyst. In PCOS, the ovaries produce unusually high levels of androgens (often called “male hormones,” though all bodies make them). These elevated androgens prevent follicles from maturing fully and releasing their eggs. Instead of one dominant follicle completing the cycle, multiple small follicles stall at an early stage and accumulate fluid, lining the ovaries in a characteristic “string of pearls” pattern visible on ultrasound.
Insulin resistance plays a central role. When cells don’t respond well to insulin, the body compensates by producing more, and elevated insulin signals the ovaries to make even more androgens. This creates a self-reinforcing loop: more androgens mean less ovulation, which means more immature follicles accumulating as small cysts. People with PCOS also tend to have chronic low-grade inflammation, which further disrupts normal ovarian function. The key distinction is that PCOS follicles are not the same as a single large functional cyst. They represent a systemic hormonal imbalance rather than a one-time glitch in the ovulation cycle.
Thyroid Problems and Ovarian Cysts
An underactive thyroid can trigger ovarian cysts through an unexpected quirk of hormone biology. The hormones TSH, FSH, and LH all share a similar chemical structure, specifically a common protein building block. When the thyroid is severely underactive, TSH levels climb very high to try to stimulate the sluggish gland. At high enough concentrations, TSH can activate FSH receptors on the ovary, essentially tricking the ovary into thinking it’s being told to grow more follicles.
This cross-reactivity can cause ovarian hyperstimulation, where multiple follicles develop and fill with fluid. The relationship between hypothyroidism and ovarian cysts was first described in the 1960s, and case reports continue to document women whose ovarian cysts resolve completely once their thyroid hormone levels are corrected. This is one reason doctors sometimes check thyroid function when evaluating recurrent or unexplained ovarian cysts.
Pelvic Infections
Severe pelvic infections, particularly pelvic inflammatory disease (PID), can spread to the ovaries and cause cysts. PID usually starts as a bacterial infection in the uterus or fallopian tubes, often from sexually transmitted bacteria. When the infection reaches the ovary, it can lead to a tubo-ovarian abscess, a walled-off collection of infected fluid involving both the fallopian tube and the ovary. These are not the same as functional cysts and require treatment for the underlying infection.
Why Some People Get Cysts More Often
Certain factors make cyst formation more likely. Irregular ovulation is the biggest driver. Anything that disrupts the precise hormonal sequence of follicle development, egg release, and corpus luteum breakdown creates more opportunities for fluid to accumulate where it shouldn’t. This is why cysts are far more common before menopause, when the ovaries are actively cycling. In premenopausal women, cysts are found on ultrasound roughly 70% of the time, compared to about 15% in postmenopausal women.
Fertility medications that stimulate ovulation can increase the risk of both follicular and corpus luteum cysts by pushing more follicles to develop at once. Previous ovarian cysts also raise the likelihood of recurrence, since the same underlying hormonal patterns tend to repeat. Endometriosis and PCOS both create ongoing conditions where cysts form repeatedly rather than as isolated events. And as noted above, untreated hypothyroidism can quietly drive cyst formation until the thyroid issue itself is addressed.