Nutritional deficiency can cause hair loss but is not a common cause in the United States. Unless you are on a starvation diet, suffer from malnutrition, or have an intestinal issue, dietary supplements will unlikely offer you help. Vitamin deficiencies are also rare causes but you could check this out with basic blood tests ordered by your primary physician. Low iron or low red blood cell counts (anemia) may be a factor. Hormonal deficiencies are more common with thyroid abnormalities being at the top of the list.
Another common cause of hair loss is called telogen effuvium which can be due to a major illness or life stress. This type of hair loss usually corrects itself.
The most common cause of gradual hair loss would be androgenetic alopecia which is due to genetics and the interaction of your hair follicles with the metabolites of testosterone. The most widely used treatment is Rogaine 5% Foam which is rubbed into the scalp once a day and has helped many people. Other oral pill treatments include prescription spironolactone for women and Propecia for men. There is now some evidence that women may also benefit from this hormone-blocking drug.
It is best for you to see a dermatologist to properly diagnose the cause of your hair loss. Blood tests, microscopic hair examination and a scalp biopsy may be needed if the cause is not obvious on a physical examination. Hair loss or alopecia is not always a simple problem, and success rates of treatment vary widely.