I've had hyperthyroidism for five years now. my latest ecg sinus tachycardia and slight left heart enlargement...what can i do or need to be aware of?

Echo, treat thyroid: Hyperthyroidism for extended time periods can produce both sinus tachycardia and hypertension. Hypothyroid states typically cause slo heart rates but hypertension can also be seen. High BP can result in cardiac hypertrophy (lvh), that can show up as heart enlargement. Over time this process can cause increased risk for arrhythmia/heart failure, and needs to be monitored closely.