Hi, As such, in early pregnancy, transient hypotension can cause syncopal attacks. Since you have hypothyroidism, the basal metabolic rate of your system would be low; you have associated bradycardia also. Though your body would be functioning without any trouble in the normal state, in the pregnant condition there is increased metabolism and demands rise; as such your body is probably not able to cope with the raised demand and you pass out. You need proper evaluation from a cardiologist and an endocrinologist for future management. Wish you good health.