Is it important to monitor vitamin d levels annually if my calcium levels are normal? Only my vitamin d was low (>15). My doc says it's not import

It is important. It is important to try to mantain a vit d level of greater than 50ng/ml many docs do not see this as important but this is gradually changing/improving check vit d levels about every 3-4 months until a steady state is achieved and then about once per year dose is about 1000iu vitd per each 25# of body wt therefore a 100# person would use 4000iu/day.
Take 2000 units. Of vitamin d3/day and you won't need to recheck level after it's stabilized. Make sure you're taking brand name T4 replacement and you're getting adequate calcium intake or supplement.