What Causes Increased Heart Rate?

Hi I am a 35 yr old woman, non smoker.Cholesterol of 165, triglyceride level of 68.I am 5 9 and 200 lbs. I attend a gym 3-4 days per week, walk around 10 miles and get 3 days of cardio.I eat a diet of lean meats, veggies, low on fat and almost no fried foods.Recent blood work shows normal hormone levels but an elevated tsh level of 3.77.Recently I ve noticed when getting up in the morning my heart rate will race to between 85-105 and sometimes higher.Walking up a split level staircase of 12 steps can throw my heart rate over the 120 mark with just normal walking speed.What could cause this?