On that note, as someone who used to be 6'4 - 150lbs, this is a rather personal issue for me:
Unless you have a serious medical issue, or are making a very specific personal choice to remain underweight, I'd encourage you to put on some healthy weight, and bump yourself up into the normal BMI range. The difference between 150 and 165 was a huge one for me. I had more energy, I had to rely on caffeine less to get through my day, and I got rid of some chronic pain that I had been dealing with. I didn't know that small change could make me feel so much better.
After four years at 165, I made a huge effort to get serious, and started a proper lifting and eating routine. In eight months, I brought myself up to a muscular 205. It changed everything about my life, physically and psychologically.
Again, if you have a medical issue, or are just plain happy at 145, disregard what I said. On the off chance that you are like I used to be, unhappy, but thinking that there was nothing you could do about it, I thought I'd let you know that it's something completely in your control, and that you can do something about it if you want to. The possibility for change exists, and there are plenty of resources to help you if you want it.