What Causes Sudden Thinning Of Hair Without Visual Hair Loss?

Hi doctor I think that i am going toward baldness since 3 years and a half,i am 21 year old,medical student,my scalp can be seen often especially under focused light and when my hair is wet and when my hair is short.i don t see much hair fall!the scalp is most obvious over posterior parietal area!my family told me that my hair is not changing at all but i don t believe them!i saw a dermatologist 6 months ago and told me that my hair density is normal,and she prescribe anaphase shampoo and neoptide lotion just to calm me down,i used the drug for approximately three months and still using the shampoo.but i didn t see any change!i want to knowif i am losing hair or not and if so i want to retain my losed hair! Please doctor answer me,because this problem is annoying me so much,i can t concentrate on anything! Thanks in advance