Need some schooling on vitamins and supplements please?

I’ve looked at many sources as a resource to determine what works and what doesn’t. It appears as if supplementation only is beneficial if you are deficient in the vitamin or mineral. I for one am deficient in Vitamin D and this has been verified by my blood tests. This is very common malady today because many of us don’t work outside and get enough sun exposure to sustain adequate levels. When I do take a vit/supplement I will choose a high grade formulation with less fillers and front the best sources. You often get what you pay for. After taking 3,000-5,000 I.u.’s of vit D a day my levels are now high normal and I feel much better. Vit D is actually classified as a hormone and is involved in many body functions. Very often the amount found in the common multi mineral vitamin supplements don’t have high enough levels to actually boost your D at all. OTC testosterone replacements work marginally at best. Usually you have to take many pills throughout the day, and become quite costly, and the testosterone blood levels aren’t sustained for long enough periods of time. In short they don’t work that well. I’m myself am on testosterone replacement and take a shot once a week. That by far is the best bar none. It will give you adequate levels and sustained blood levels. Proven by science. Believe me the studies done on OTC testosterone replacement is on small sample sizes and boost very low t levels to maybe low normal. I also have worked for a large pharmaceutical company for many years. If you believe that there isn’t money in finding cure’s for ailments you’re wrong. There are now selective cancer treatments and ant-virals that go a long way to curing things like Hepatitis C and sending cancer in remission with minimal side effects. The companies are making billions on these. The reason why they do so well with pills to battle ailments is because our society is lazy and wants a quick fix. They don’t want to adapt a healthy lifestyle and exercise. That’s not the companies fault. Any questions feel free to reach out.
Carl