What is the best treatment for thinning hair and hair loss?

Depends on cause...: One's primary care doctor can check for medical or psychiatric causes. If none are found, then one can see a dermatologist with expertise in hair restoration (some treatments: minoxidil, etc.). Medical causes may include: hormonal problems (such as low thyroid), seborrhea, alopecia areata, stressful illness. Other causes: genetic male-female-pattern baldness, telogen effluvium, etc...