High serum thyrotropin levels are associated with retinal arteriolar narrowing in the general population

Recent studies have shown associations of hypothyroidism with arterial blood pressure, atherosclerosis, and inflammation. Based on these pathways, there might also be an association between hypothyroidism and retinal arteriolar narrowing (RAN), a marker of microvascular damage from hypertension, atherosclerosis, and inflammation. Against this background, the aim of this study was to investigate the putative association between serum thyrotropin (TSH) levels and RAN defined by arterio-venous ratio (AVR) from static vessel analysis.

We used data from 3189 individuals from the second population-based cohort of the Study of Health in Pomerania (SHIP-TREND-0). Thyroid function was defined according to serum TSH and serum diiodothyronine (3,5-T2) levels. Low and high serum TSH levels were defined by the cutoffs 0.3 mIU/L and 3.0 mIU/L. Fundus photography of the central retina was recorded with a nonmydriatic camera, and images were evaluated by one experienced reader. An AVR <0.8 was defined as decreased. Serum TSH levels, low and high TSH, and serum 3,5-T2 levels were associated with AVR by linear regression and with AVR <0.8 by Poisson regression, both adjusted for age, sex, cigarette smoking, alcohol consumption, and intake of beta-blockers.

Serum TSH levels were significantly associated with AVR (β=-0.028 [CI -0.049 to -0.007]; p=0.009) and with a decreased AVR <0.8 (relative risk=2.05 [CI 1.13-3.73]; p=0.019). Individuals with high TSH had a 1.43 higher risk for a decreased AVR ([CI 1.04-1.96]; p=0.027) than individuals with serum TSH levels within the reference range. Serum 3,5-T2 levels were also associated with a decreased AVR (relative risk for an increase of 1 nM=0.45 [CI 0.23-0.87]; p=0.017).

Our results substantiate evidence for an association between hypothyroidism and RAN. Potential mechanisms explaining this association are long-term hypertension, atherosclerotic processes, and inflammation.