Thyroid eye disease (TED) is caused by an autoimmune attack on the tissues behind and around the eyes. The immune system produces antibodies that mistakenly target cells in the eye socket, triggering inflammation, swelling, and the characteristic bulging eyes associated with the condition. About 50% of people with Graves’ disease develop some degree of TED, though roughly 10% of cases occur in people whose thyroid hormone levels are completely normal.
The Autoimmune Trigger
The root cause of TED is a case of mistaken identity by the immune system. In Graves’ disease, the body produces antibodies that stimulate the thyroid gland by latching onto receptors on thyroid cells called TSH receptors. These same receptors also exist on cells in the eye socket, specifically on a type of connective tissue cell called an orbital fibroblast. When the antibodies reach the eye socket and bind to those receptors, they set off an inflammatory chain reaction in tissue that was never meant to be a target.
What makes the process especially damaging is a second receptor on those same orbital fibroblast cells: the IGF-1 receptor, which normally responds to a growth factor involved in cell metabolism. The TSH receptor and the IGF-1 receptor sit extremely close together on the cell surface, held in place by a scaffolding protein that keeps them within about 40 nanometers of each other. When the autoimmune antibodies activate the TSH receptor, that signal crosses over and activates the neighboring IGF-1 receptor as well. This “crosstalk” between the two receptors amplifies the immune response far beyond what either receptor would produce alone, driving the inflammation and tissue remodeling that define TED.
What Happens Inside the Eye Socket
The eye socket is a bony, rigid space with very little room for expansion. When orbital fibroblasts become activated by the autoimmune process, two major changes occur that crowd this tight compartment.
First, the activated cells begin producing large amounts of hyaluronan, a sugar-based molecule with an enormous capacity to absorb and retain water. As hyaluronan accumulates, it draws fluid into the orbital tissues like a sponge, causing significant swelling. At the same time, the enzymes that normally break down hyaluronan become less active, so the molecule builds up faster than it can be cleared. The result is progressive, waterlogged expansion of the soft tissue behind the eyes.
Second, some orbital fibroblasts transform into fat cells, a process called adipogenesis. This new fat tissue adds volume to an already overcrowded space. Together, the fluid retention from hyaluronan and the expansion of fat tissue push the eyeball forward (proptosis), compress the muscles that move the eye, and in severe cases can squeeze the optic nerve. That compression, which occurs in fewer than 5% of TED patients, is considered an ocular emergency because it threatens permanent vision loss.
Smoking Is the Strongest Modifiable Risk
Smoking is by far the most significant environmental risk factor for developing TED. A study published in JAMA found that smokers had 7.7 times the odds of developing Graves’ eye disease compared to non-smokers. That’s not a modest increase. It places smoking in a category of risk that dwarfs nearly every other controllable factor. Smoking appears to worsen the autoimmune response in the orbit, intensify inflammation, and make the disease less responsive to treatment. Quitting smoking is one of the most impactful things a person with Graves’ disease can do to lower their chances of developing eye complications.
How Thyroid Treatment Affects Risk
The way hyperthyroidism is treated can influence whether TED develops or worsens. Radioactive iodine therapy, one of the most common treatments for an overactive thyroid, carries a notably higher risk of triggering or worsening eye disease compared to surgical removal of the thyroid. Research shows that patients who underwent thyroidectomy had a 74% reduced risk of progressing to TED compared to those treated with radioactive iodine. The leading theory is that radioactive iodine destroys thyroid cells, releasing a burst of thyroid antigens that can ramp up the immune response and redirect it toward the eye socket.
Antithyroid medications, whether used alone or combined with radioactive iodine, did not significantly reduce the risk of TED in the same study. This means the choice of hyperthyroidism treatment is a meaningful conversation for anyone with Graves’ disease, particularly those who already show early signs of eye involvement.
Genetics and Other Risk Factors
There is a genetic component to TED susceptibility. Certain immune system genes, particularly a variant called HLA-BW35, have been linked to a significantly higher risk of severe eye disease. In one study, people carrying this genetic marker had 13 times the risk of developing severe orbital inflammation compared to the general population. This helps explain why some people with Graves’ disease develop devastating eye involvement while others never experience eye symptoms at all.
Beyond genetics, several other factors tilt the odds. Women develop Graves’ disease far more often than men, but men who do develop it tend to have more severe eye disease. Age also plays a role: TED is more common and tends to be more aggressive in people over 50. Poorly controlled thyroid hormone levels, whether too high or too low, can fuel the orbital inflammation, making stable thyroid management a priority for prevention.
TED Without Hyperthyroidism
About 10% of TED cases occur in people who are euthyroid, meaning their thyroid hormone levels are normal. Some of these individuals have Hashimoto’s thyroiditis (an underactive thyroid condition) or no detectable thyroid disease at all. This happens because the autoimmune antibodies responsible for TED can target the eye socket independently of their effect on the thyroid gland. The orbital fibroblasts respond to the antibodies regardless of what the thyroid itself is doing. This is one reason TED can be tricky to diagnose: a person with normal thyroid labs and progressive eye swelling may not immediately be connected to an autoimmune thyroid process.
How New Treatments Target the Cause
Understanding the crosstalk between the TSH receptor and the IGF-1 receptor led directly to a targeted therapy: a medication that blocks the IGF-1 receptor on orbital fibroblasts. By preventing the IGF-1 receptor from being activated, the drug interrupts the amplified signaling that drives inflammation and tissue expansion. This approach works because the medication only blocks the autoimmune signal when the two receptors are physically close together on the cell surface. If researchers artificially separated the receptors in lab experiments, the drug lost its effectiveness, confirming that the receptor pairing is central to how TED develops. This treatment represents a shift from managing symptoms with steroids or surgery to directly interfering with the molecular cause of the disease.