Frozen shoulder is caused by inflammation and progressive scarring of the joint capsule, the flexible tissue that surrounds and encloses the shoulder joint. This scarring thickens and tightens the capsule so much that the shoulder loses its ability to move freely. While the exact trigger remains unclear in many cases, several well-established risk factors, particularly diabetes, thyroid disease, and prolonged immobilization, dramatically increase the likelihood of developing it.
What Happens Inside the Joint
The shoulder joint is wrapped in a flexible capsule of connective tissue that normally allows a wide range of motion. In frozen shoulder (known clinically as adhesive capsulitis), this capsule becomes inflamed, and that inflammation eventually gives way to fibrosis, a buildup of dense scar tissue. Immune signaling molecules flood the joint lining, triggering an overgrowth of the cells that produce collagen. The result is excessive collagen deposits that thicken the capsule, shrink the space inside the joint, and progressively lock the shoulder in place.
The scarring concentrates in a specific area called the rotator interval, where a key ligament connecting the shoulder blade to the upper arm bone becomes abnormally thick and rigid. On a microscopic level, the joint lining shows new blood vessel growth and an overgrowth of its cellular layer. These changes restrict both active movement (what you can do on your own) and passive movement (what someone else can move your arm through), which is a hallmark that distinguishes frozen shoulder from muscle injuries or tendon problems.
Diabetes Is the Strongest Risk Factor
Diabetes is the single most studied and most significant risk factor. A meta-analysis published in BMJ Open estimated that about 13.4% of people with diabetes develop frozen shoulder, far higher than the 2 to 5% rate seen in the general population. The connection likely comes down to how chronically elevated blood sugar changes connective tissue. When blood sugar stays high, simple sugars bond to proteins throughout the body in a process called glycation. This produces compounds that cross-link collagen fibers, making them stiffer and more resistant to normal stretching. In the shoulder capsule, this accelerated collagen cross-linking may prime the tissue for the kind of fibrotic scarring that defines frozen shoulder.
People with diabetes also tend to have more severe cases that last longer and respond less readily to treatment. Both type 1 and type 2 diabetes carry increased risk.
Thyroid Disease and Hormonal Links
Thyroid disorders are another well-documented contributor. Research comparing people with frozen shoulder to healthy controls found that someone with a thyroid condition has roughly 2.7 times the risk of developing frozen shoulder. Hypothyroidism shows an especially strong connection. One study found hypothyroidism in 17% of frozen shoulder patients compared to just 4% of people with rotator cuff injuries and 2% of healthy controls. Higher levels of thyroid-stimulating hormone (TSH) have also been linked to bilateral frozen shoulder, meaning both shoulders are affected, and to more severe stiffness overall.
Hyperthyroidism carries a smaller but still meaningful increase in risk, estimated at about 1.2 times that of the general population. The exact mechanism linking thyroid dysfunction to shoulder fibrosis isn’t fully understood, but thyroid hormones influence connective tissue metabolism throughout the body, and their disruption appears to make the shoulder capsule more vulnerable to the inflammatory-fibrotic cascade.
The Role of Inflammation and the Immune System
Growing evidence points to frozen shoulder as partly an immune-driven condition. People with adhesive capsulitis consistently show elevated levels of pro-inflammatory signaling molecules in and around the affected joint. A 2024 genetic study published in Frontiers in Immunology identified two specific immune proteins that appear to directly increase risk: one involved in recruiting inflammatory cells to tissue (IP-10) and another that attracts white blood cells to sites of inflammation (RANTES). Higher circulating levels of both were associated with a greater chance of developing the condition.
This immune connection helps explain why frozen shoulder clusters with other conditions tied to immune system disruption, including diabetes, thyroid disease, and Dupuytren’s contracture (a thickening of tissue in the palm). It also explains why the early “freezing” phase feels so intensely painful before the joint simply stiffens. The pain reflects active inflammation; the stiffness reflects the fibrotic aftermath.
Immobilization and Secondary Causes
Any situation that keeps the shoulder still for an extended period can trigger what’s called secondary frozen shoulder. Common culprits include recovery from a shoulder fracture, rotator cuff surgery, mastectomy or breast cancer treatment, and even cardiac surgery. A stroke or other neurological event that limits arm use can have the same effect. In these cases, the lack of movement allows the joint capsule to stiffen and adhere, and inflammation from the underlying injury or surgery accelerates the scarring process.
This is one reason physical therapists emphasize early, gentle movement after shoulder injuries and surgeries. Once the fibrotic cycle begins, it can become self-reinforcing: the stiffness causes pain, which discourages movement, which worsens the stiffness.
Who Gets Frozen Shoulder
Women are affected roughly twice as often as men. In one large study of over 1,300 patients treated for adhesive capsulitis, 69% were female. The peak age range is between the 40s and 60s, with an average age at diagnosis around 54 to 55. The condition is rare before age 40 and uncommon after 70.
Neurological conditions also appear in frozen shoulder populations at higher rates. Parkinson’s disease is a notable example. The rigidity, postural changes, and reduced arm swing associated with Parkinson’s may contribute to shoulder stiffness, and some researchers believe that early, undiagnosed Parkinson’s may actually be misidentified as frozen shoulder in its initial stages. Cerebrovascular disease, including stroke, shows up more frequently in people with frozen shoulder as well, likely because of the immobilization and altered movement patterns that follow a stroke.
The Three Stages and Their Timeline
Frozen shoulder follows a predictable pattern, though the total duration varies widely from person to person.
- Freezing stage (2 to 9 months): Pain is the dominant symptom, often worse at night. Range of motion gradually decreases. This is when active inflammation is at its peak.
- Frozen stage (4 to 12 months): Pain often eases somewhat, but the shoulder is at its stiffest. Daily tasks like reaching overhead, fastening a seatbelt, or dressing become difficult or impossible.
- Thawing stage (5 to 24 months): Motion slowly returns as the fibrotic tissue remodels. Recovery can be gradual, and some people retain a small degree of permanent stiffness.
From start to finish, the full cycle typically takes one to three years. People with diabetes tend to land toward the longer end of that range and are more likely to have the condition recur in the opposite shoulder.
Why the Exact Trigger Is Still Unclear
Despite decades of research, the precise mechanism that initiates frozen shoulder in an otherwise healthy joint remains elusive. The inflammatory and fibrotic processes are well characterized, and the associated risk factors are clearly established, but many cases appear without an obvious trigger. No prior injury, no surgery, no clear metabolic condition. These “primary” or “idiopathic” cases suggest there may be a genetic susceptibility or an immune system predisposition that hasn’t yet been pinpointed. What is clear is that once the inflammatory cascade begins in the joint capsule, it follows a remarkably consistent pattern of inflammation, fibrosis, and eventual (though slow) resolution.