# Incidence and risk factors of tocilizumab-induced hypofibrinogenemia in patients with thyroid eye disease: a single-center retrospective study

## TL;DR

Tocilizumab treatment for thyroid eye disease causes low fibrinogen in nearly half of patients, with risk factors including low baseline fibrinogen, higher weight, and faster blood clotting.

## Contribution

Identifies hypofibrinogenemia risk factors in TED patients treated with tocilizumab through a single-center retrospective study.

## Key findings

- 45.88% of patients developed hypofibrinogenemia after tocilizumab treatment.
- The most significant fibrinogen decline occurred after the first dose.
- Baseline fibrinogen, body weight, and prothrombin time were independent risk factors.

## Abstract

Tocilizumab, an interleukin-6 receptor antagonist, is increasingly used in moderate-to-severe thyroid eye disease. However, its association with hypofibrinogenemia remains underexplored in this population.

This single-center retrospective study included 194 TED patients treated with Tocilizumab at Beijing Tongren Hospital between March 2023 and May 2025. Patients were stratified into a positive group (fibrinogen < 1.5 g/L) and a negative group (fibrinogen ≥ 1.5 g/L) based on post-treatment fibrinogen levels. Demographic, clinical, and laboratory data were analyzed to identify risk factors using univariate and multivariate logistic regression.

Among 194 patients, 89 (45.88%) developed hypofibrinogenemia (fibrinogen < 1.5 g/L). The most significant fibrinogen decline occurred after the first Tocilizumab administration (median reduction: 0.88 g/L). Nadir levels were most common before the third (26.4%) or fifth administration (29.6%). Multivariate analysis identified lower baseline fibrinogen (OR = 0.37, P = 0.001), higher body weight (OR = 1.05, P = 0.001), and lower prothrombin time (OR = 0.45, P = 0.008) as independent risk factors.

Hypofibrinogenemia was observed at a higher incidence (45.88%) in thyroid eye disease patients treated with tocilizumab. It typically did not occur immediately after administration but emerged early in the treatment course and persisted with repeated dosing. Baseline fibrinogen level, body weight, and prothrombin time (PT) activity were identified as significant predictors.