Thyroid eye disease (TED) is diagnosed through a combination of a clinical eye exam, blood tests for thyroid-related antibodies, and often imaging scans of the eye sockets. There is no single test that confirms TED on its own. Instead, doctors piece together physical signs, lab results, and your symptom history to reach a diagnosis and determine how active and severe the disease is.
The Clinical Eye Exam
The physical exam is the cornerstone of a TED diagnosis, and certain signs are so characteristic that an experienced ophthalmologist can often suspect TED on sight. The most telling sign is upper eyelid retraction, where the upper lid pulls back to expose more of the white of the eye than normal. This occurs in over 90% of TED patients and gives the eyes a wide, staring appearance. Your doctor measures this precisely by shining a small light at your eye and recording the distance between the light’s reflection on the cornea and your upper and lower eyelid margins.
The second most common finding is proptosis, the medical term for eyes that bulge forward. About 60% of people with TED have it. Doctors measure proptosis with a device called a Hertel exophthalmometer, which sits against the sides of your eye sockets and uses mirrors to gauge exactly how far each eye projects. TED is actually the most common cause of both one-sided and two-sided forward eye bulging, so this finding alone often points toward the diagnosis.
About half of TED patients also have lagophthalmos, meaning the eyelids can’t fully close. Your doctor will check for this because incomplete closure leaves the cornea exposed, which can lead to dryness, irritation, and in severe cases corneal damage. Beyond these three main signs, the exam includes checking eye movement in all directions (restricted movement is common, especially looking upward), looking for redness and swelling of the eyelids and the white of the eye, and testing how well your eyes converge when focusing on a near object.
Checking for Optic Nerve Damage
One of the most important parts of the diagnostic workup is ruling out compression of the optic nerve, which carries visual information from your eye to your brain. Swollen muscles behind the eye can squeeze this nerve, threatening your vision. Doctors screen for this with several targeted tests.
Color vision testing is one of the earliest ways to pick up nerve involvement, because the ability to distinguish colors fades before overall sharpness does. You may also be given a contrast sensitivity test, which checks whether you can detect subtle differences between light and dark. Visual field testing maps out any blind spots, and the most characteristic pattern in TED-related nerve compression is a central or near-central blind spot, though enlarged blind spots and arc-shaped defects also occur. If your doctor notices a difference in how your pupils react to light (one constricting less briskly than the other), that’s another red flag for nerve compromise.
Measuring Disease Activity With the CAS
Once TED is suspected, doctors need to know whether the disease is currently in an active, inflammatory phase or has already burned out and stabilized. This distinction matters enormously because active TED responds to anti-inflammatory treatments, while inactive TED generally does not. The standard tool for this is the Clinical Activity Score, or CAS.
The CAS is a simple seven-item checklist recommended by the European Group on Graves’ Orbitopathy (EUGOGO). Your doctor evaluates whether you have each of the following, scoring one point for each “yes”:
- Spontaneous pain behind the eye
- Pain when looking up or down
- Redness of the eyelids
- Redness of the white of the eye
- Swelling of the eyelids
- Inflammation of the small pink tissue at the inner corner of the eye
- Swelling of the clear membrane over the white of the eye
A score of 3 or higher means the disease is considered active. At follow-up visits, doctors sometimes use an expanded 10-point version that adds three more criteria: an increase in proptosis of at least 2 mm, a decrease in eye movement of at least 8 degrees in any direction, or a loss of two lines on a standard eye chart. These additional points help track whether the disease is progressing.
Severity Classification
Separate from activity, TED is classified by how much it affects your eyes and daily life. The EUGOGO guidelines break severity into three tiers.
Mild TED means the disease has minimal impact on everyday functioning. People in this category typically have minor lid retraction (less than 2 mm), eye bulging less than 3 mm above normal, little or no double vision, and any corneal dryness from exposure responds well to lubricating drops.
Moderate to severe TED involves more significant changes: lid retraction greater than 2 mm, eye bulging 3 mm or more above normal, noticeable soft tissue swelling, and double vision that may be constant. This is the category where the disease affects quality of life enough to justify more aggressive treatment. As a general rule, if your TED isn’t mild and isn’t threatening your sight, it falls here.
Sight-threatening TED is the most urgent category. It includes optic nerve compression and severe corneal breakdown from exposure. This requires immediate treatment.
Blood Tests and Thyroid Function
Blood work plays a supporting role in the diagnosis. Most people with TED have Graves’ disease, an autoimmune condition that causes the thyroid gland to be overactive. Standard thyroid function tests (measuring thyroid hormone levels and thyroid-stimulating hormone, or TSH) help establish this connection. More specific to TED, doctors test for thyroid-stimulating immunoglobulin (TSI) and thyrotropin receptor antibodies (TRAb). These are the antibodies that drive both the thyroid overactivity and the inflammation behind the eyes. Elevated levels support the diagnosis, especially when the eye findings are subtle or only affect one side.
It’s worth noting that a small percentage of TED patients have normal thyroid function or even an underactive thyroid, so normal blood work alone doesn’t rule TED out if the clinical signs point toward it.
Imaging Scans of the Eye Sockets
CT or MRI scans of the orbits (eye sockets) aren’t always necessary for straightforward cases, but they’re valuable when the diagnosis is uncertain, when optic nerve compression is suspected, or when doctors need detailed measurements before planning treatment.
On imaging, TED shows two recognizable patterns. In the first, the extraocular muscles (the small muscles that move your eyes) are visibly enlarged. This pattern tends to show up in people who have restricted eye movement and double vision. In the second pattern, the amount of fat behind the eye increases, pushing the eye forward and causing proptosis without necessarily affecting movement. Some people have a mix of both. Imaging can also reveal whether swollen muscles are crowding the optic nerve at the back of the eye socket, which helps doctors decide how urgently to intervene.
Ruling Out Other Conditions
Because bulging eyes, restricted eye movement, and orbital swelling can have causes other than TED, your doctor may need to exclude a few look-alike conditions. Nonspecific orbital inflammatory disease can cause bilateral proptosis and muscle enlargement, but it tends to also involve the tear gland and may be linked to other autoimmune conditions. Orbital lymphoma can mimic TED with bilateral swelling, though imaging typically shows more diffuse tissue changes and bone erosion. In people with a history of facial trauma, an orbital floor fracture can limit upward gaze and alter how the eye sits in the socket. In rare cases, amyloidosis (a condition where abnormal proteins deposit in tissues) can cause bilateral eye bulging with associated nerve symptoms. When there’s any doubt, a tissue biopsy or additional bloodwork narrows the diagnosis.
The combination of characteristic physical signs, a CAS score, supporting lab results, and imaging when needed gives doctors a reliable and well-structured framework for confirming TED and deciding on the best course of action.