I registered a 5 ng/ml with a blood test. I wasn't feeling tired or seeing any negative effects at that time from it and most likely it had been low for a decade. However for obvious reasons I began immediately taking actions to move that number persistently into a healthy range.

I've been using a D3 supplement of 5000 IU from NatureWise, for several years. I take one per day depending on the time of year. It's inexpensive, ~$11-12 for a year supply, and it has worked very well. I take it daily throughout Fall & Winter, and then lighten up on it during Summer.