A visible third eyelid in a cat is almost never just cosmetic — it usually flags illness, ocular pain, dehydration, or a neurologic problem. If both eyes show the nictitans, the cause is often systemic; if one eye is involved, an ocular cause like a corneal ulcer is more likely.
Last reviewed: May 2026
Why Is My Cat's Third Eyelid Showing?
A cat's third eyelid (nictitating membrane) shows because the muscle that retracts it has lost normal tone — typically from systemic illness, ocular pain, dehydration, or sympathetic nerve dysfunction. About 70 percent of cats presented to general practice for a visible nictitans have an underlying systemic cause rather than an isolated eye problem, as described in Cote's Clinical Veterinary Advisor: Dogs and Cats. That is why an exam should never be limited to the eye alone.
Common contributors include upper respiratory infection, intestinal parasites, dehydration, fever, conjunctivitis, corneal ulcer, foreign body under the eyelid, and Horner's syndrome (AAFP-AAHA Feline Life Stage Guidelines, 2021).
Single Eye vs. Both Eyes — What the Pattern Tells You
If the third eyelid shows in only one eye, the cause is usually local — corneal ulcer, foreign body, uveitis, Horner's syndrome, or trauma — and warrants prompt ophthalmic evaluation. If both eyes show the nictitans together, the cause is more often systemic: dehydration, GI parasites, viral upper respiratory infection, severe pain elsewhere in the body, or the poorly understood Haw's syndrome in young cats following diarrhea.
Either pattern is a reason to be seen by a vet within 24 to 48 hours.
Home Observations That Help the Vet
Before the appointment, note whether your cat is eating, drinking, and using the litter box normally; whether she is squinting, pawing at the eye, or hiding; and whether the third eyelid is the same color as the rest of the eye area or red, swollen, and inflamed. A clear photograph in good natural light helps the vet enormously. Do not apply any over-the-counter eye drops or cleaning solutions before the exam — many contain steroids that worsen corneal ulcers. Underlying pain is one of the most common drivers of bilateral third-eyelid prominence and should always be assessed using a validated feline pain scale (AAHA Pain Management Guidelines, 2022).
When to See a Vet
Not every symptom is a midnight emergency, but some warrant same-day attention and a few are true ERs. Use the lists below to sort which bucket you're in.
Call your vet today if:
- Third eyelid visible for more than 12 to 24 hours
- Squinting or pawing at the eye
- Cloudiness, redness, or discharge in the affected eye
- Decreased appetite or hiding in addition to the eye change
- Recent diarrhea or vomiting (suggests Haw's syndrome)
Go to the ER immediately if:
- Sudden blindness or bumping into walls
- Bulging or sunken eyeball
- Severe pain (yowling, refusing to be touched near the head)
- Trauma to the eye or face
- Profuse blood-tinged discharge from the eye
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Frequently Asked Questions
Is a visible third eyelid an emergency?
Usually not an immediate ER, but it almost always warrants a same-day or next-day vet visit. The third eyelid showing flags either systemic illness or eye pain, and both deserve prompt diagnosis. True emergencies are eye trauma, sudden blindness, or a bulging eyeball — those need ER care.
Can dehydration alone cause the third eyelid to show?
Yes, even mild to moderate dehydration can cause the third eyelid to protrude across the inner corner of the eye. This is one of several physical signs of dehydration in cats along with tacky gums, reduced skin elasticity, and sunken eyes. Restoring hydration usually resolves it within hours.
How much does a vet visit for a third eyelid issue cost?
A general practice exam runs $50 to $150. If the vet adds a fluorescein corneal stain to rule out an ulcer, that adds $30 to $80. Bloodwork to investigate a systemic cause is $100 to $250, and prescription eye drops typically cost $20 to $60. A specialist ophthalmology referral runs $200 to $400 for an exam.
What is Haw's syndrome in cats?
Haw's syndrome is a benign, poorly understood condition in young cats where both third eyelids protrude for several weeks, often after a bout of diarrhea suggesting a viral or parasitic GI trigger. It typically resolves on its own within 4 to 8 weeks, but other causes of bilateral protrusion need to be ruled out first.
Still Not Sure if Your Cat Needs a Vet?
When you're not sure if this is wait-and-see or call-tonight, Voyage AI Vet triages in under 2 minutes. Describe what you're seeing in chat, share photos of your cat's eye in good natural light, both eyes side by side, and a quick clip of any squinting, or hop on a live video call if you want a second pair of eyes. Every answer comes with citations to the actual veterinary literature it's pulling from — so you see exactly where the guidance comes from, not just a chatbot's word.