Eye · 6 authors, 2 centres
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This cross-sectional study found that keratoconus patients have significantly higher rates of thyroid dysfunction and lower serum vitamin D levels compared to healthy controls. The Taq1 tt genotype of the vitamin D receptor was associated with a 3.5-fold increased risk of keratoconus. These findings suggest that both thyroid disorders and vitamin D deficiency are potential risk factors for keratoconus development.
**Background:** Keratoconus (KC) is a progressive ectatic corneal disease characterized by conical protrusion and thinning, leading to visual impairment. Its exact pathophysiology remains unclear, but immunological, genetic, and environmental factors are implicated. Previous studies have suggested a link between thyroid gland dysfunction and KC, as thyroxin can cause biochemical changes in the corneal stroma. Additionally, vitamin D receptors (VDR) are present in the cornea, and vitamin D deficiency has been associated with KC. This study aimed to detect serum levels of thyroid hormones, vitamin D, and VDR polymorphisms in KC patients and to identify any association between vitamin D deficiency and thyroid disorders in these patients.
**Methods:** This prospective, observational, cross-sectional study was conducted at Mansoura Ophthalmic Center, Egypt, from March to September 2021. It included 177 KC patients without previously diagnosed thyroid disorders and 85 age- and sex-matched healthy controls with normal corneal topography. KC diagnosis was confirmed by slit-lamp biomicroscopy and Pentacam imaging. Serum levels of TSH, FT3, FT4, and 25-OH vitamin D were measured using ELISA. Vitamin D status was classified according to 2012 American Endocrine Society guidelines: sufficiency (>30 ng/ml), insufficiency (21–30 ng/ml), and deficiency (<20 ng/ml). VDR polymorphisms (TaqI rs731236, ApaI rs7975232, BsmI rs1544410) were analyzed using PCR-RFLP. Statistical analysis included Mann-Whitney test, Chi-square test, and logistic regression.
**Key Results:** The KC group had significantly higher TSH (median 2.3 vs. 1.5 µIU/L, P=0.001) and FT4 (median 1.43 vs. 1.0 ng/dl, P<0.001) compared to controls. Thyroid dysfunction was more frequent in KC patients (22.0% vs. 4.7%, P=0.004), with hypothyroidism (subclinical + overt) in 10.2% of KC vs. 1.2% of controls. Serum 25(OH) vitamin D was significantly lower in KC patients (median 10.6 vs. 31.0 ng/ml, P<0.001). Vitamin D insufficiency was present in 53.7% of KC vs. 36.5% of controls, and deficiency in 35.6% vs. 12.9% (P<0.001). Among VDR polymorphisms, only TaqI showed a significant association with KC: the tt genotype was more common in KC patients (33.9% vs. 17.6%, OR=3.459, 95% CI 1.654–7.233, P=0.001). The t allele was more prevalent in KC (56.5% vs. 40.0%, P<0.001). In KC patients, vitamin D levels varied by TaqI genotype: TT had median 19.3 ng/ml, Tt 10.2 ng/ml, and tt 9.5 ng/ml (P<0.001). Deficiency was most common in tt (60.0%), while insufficiency was most common in Tt (60.0%). Logistic regression identified TSH (OR=2.173, 95% CI 1.603–2.946), FT4 (OR=61.95, 95% CI 24.145–77.06), hypothyroidism (OR=8.474, 95% CI 1.062–67.63), insufficient vitamin D (OR=5.847, 95% CI 2.936–11.643), and deficient vitamin D (OR=12.314, 95% CI 5.283–28.704) as significant risk factors for KC. Sufficient vitamin D was protective (OR=0.943, P<0.001).
**Clinical Implications:** This study demonstrates that both thyroid dysfunction and vitamin D deficiency are significantly associated with keratoconus. The TaqI tt genotype of the VDR gene may increase susceptibility to KC, possibly by influencing vitamin D metabolism. These findings suggest that screening for thyroid disorders and vitamin D deficiency in KC patients could be beneficial. Future research should explore whether vitamin D supplementation or thyroid hormone management can influence KC progression. The study is limited by its cross-sectional design and lack of correlation with KC severity or progression.