The iris is a unique structure, with exquisite variations in colour and form. Pathological changes, specifically including iris cysts and tumours are relatively uncommon, difficult to diagnose, and yet potentially blinding or life-threatening. Based on a comprehensive literature review, with highly illustrated key case examples, this report aims to guide the clinician in filtering the differential diagnoses of iris cysts and tumours. Evaluation is in the context of key diagnostic clinical tools and management considerations. Diagnostic imaging techniques include serial anterior segment photography, ultrasound, anterior segment optical coherence tomography, and iris fluorescein angiography, however, the roles of computerised topography and magnetic resonance imaging are also considered in this review. Management includes categorisation in terms of solid iris tumours (melanocytic vs. non-melanocytic), or iris cysts (primary vs. secondary) that may be usefully differentiated by clinical assessment, avoiding more invasive interventions. Cystic lesions are generally benign, although implantation cysts in particular cause significant complications and surgical challenges. Most solid tumours are melanocytic and also typically benign. However, in larger lesions, rapid growth, symptoms and complications more likely indicate malignancy, requiring further investigation.

BACKGROUND: Ectopic thyroid tissue in the iris, also known as a thyroid glandular epithelial choristoma of the iris, has only been described twice in the literature. In both cases it remained asymptomatic.
METHODS: A 67-year-old female patient presented for the first time in mid-2017 with corneal endothelial decompensation, with a history of complicated cataract surgery and IStent® implantation. Slit lamp microscopy showed endothelial decompensation, pseudophakia, anterior synechiae and a whitish iris tumour adhering to the endothelium. The latter had existed since childhood. Given these findings, reduced visual acuity of hand movement perception and an intraocular pressure of 23 mmHg, we performed a keratoplasty combined with an en bloc resection of the iris tumour at 9 o\'clock and sector iridectomy at the end of 2019. Histological and immunohistological examination of the iris tumour unexpectedly revealed thyroid tissue. After the procedure described above, the patient had an increase in visual acuity while the graft stayed clear and the eye showed no evidence of tumour recurrence or other complications.
CONCLUSIONS: We report a third case of ectopic thyroid tissue in the iris. Both previous cases remained asymptomatic, whereas in our case, size and location of the ectopic thyroid tissue contributed to a more complex cataract surgery resulting in endothelial decompensation. Therefore, in such cases appropriate patient information should be provided prior to cataract surgery. Furthermore, careful histological examination and examination of the thyroid is important to exclude malignant diagnoses such as a metastasis of a follicular thyroid carcinoma.