Thyroid eye disease (TED) is an autoimmune condition that is classically divided into active (inflammatory) and inactive (“burnt-out”) disease. Hydraulic or meta-active TED is driven by specific “mechanical” factors at the posterior orbit resulting in a characteristic increased orbital tension, congestion and venous obstruction. It is associated with posterior orbital muscle expansion and crowding. It may be present with or without a significant concurrent inflammatory component. Hydraulic apex TED manifests during or following the active (inflammatory) phase. This can be misinterpreted as active inflammation only, leading to over-treatment with, or poor response to immunosuppression. Patients with hydraulic phenotype TED have severe disease. They typically have limited moderate proptosis, firm orbits to retropulsion and deep dilated conjunctival vessels throughout. Optic neuropathy or visual obscurations when changing posture or eye movements may be present. They may suffer from double vision with limitation in their eye movements. Posterior and apical extraocular muscle enlargement is characteristic. They respond poorly to immunosuppression but well to posterior apical orbital decompression surgery. Therefore, it is essential to identify these patients and distinguish from the other phenotypes of TED in order to tailor treatment choices and avoid unnecessary immunosuppression.