Background: Premature rupture of membranes (PROM) and preterm PROM (PPROM) are the rupture of the fetal chorioamniotic membranes before the onset of labor contractions. Objectives: To evaluate whether detection of thyroid hormones in vaginal washouts can be used to diagnose premature rupture of membranes (PROM). Patients and Methods: Total T4 (thyroxin), total T3 (triiodothyronine), and free T4 concentrations in vaginal washouts were analyzed in 45 women with the diagnosis of PROM and compared with 45 normal pregnancies (controls). A Student's t-test was used for comparison of thyroid hormone values and a ROC curve was used for analyzing the diagnostic accuracy. Results: There were no differences between the patients with PROM and the control women regarding maternal age, gestational age, and parity. The mean concentration of total T4 in the PROM group and the control subjects were 2.1 ± 1.3 µg/mL and 1.55 ± 0.58 µg/, respectively (P = 0.01). Total T3 concentration in the PROM group was significantly higher compared to the controls (1.28 ± 0.42 ng/ vs. 0.8 ± 0.26 ng/, P < 0.0001). Also, free T4 concentration was significantly higher in the PROM group as compared with the controls (0.026 ± 0.034 ng/ vs.0.007 ± 0.004 ng/, P < 0.0001). The ROC curve analysis showed that total T4 = 1.685 µg/ had a sensitivity of 62%, specificity of 76%, positive predictive value (PPV) of 71.8%, and negative predictive value (NPV) of 66.7%. Total T3 = 0.82 ng/ gave a sensitivity of 91%, specificity of 64%, PPV of 72%, and NPV of 87.8%. Free T4 = 0.01 ng/ gave a sensitivity of 51%, specificity of 82%, PPV of 74.2%, and NPV of 62.7%. The best cutoff values were total T4 = 1.685 ng/, total T3 = 0.82 ng/, and free T4 = 0.01 ng/. Conclusions: Detecting total T4, total T3, and free T4 in vaginal washouts of patients suspected of PROM suspected is a simple, available, rapid, and inexpensive method for the diagnosis of PROM. However, a combination of three tests is recommended.