I have been told optimal = 80 and I have been at the lower side ranging from super low at 15 to 70 and various levels in between over the last 15 years of testing. NOTE: I am surgical menopause, 55 yr woman, on HRT. When I take Vitamin D3 at higher than 2k per day, I notice impact such as sleep disturbance, hair loss, estrogen fluctuation symptoms and headache. I've been told that taking 1-2K IU per day is not enough to increase my current level = 25. I've been prescribed 50k IU caps. what is the science behind the optimal =80? perhaps the level required might be different for men and women? or women on HRT? it just seems odd, hoping to hear your take on this.