Treatment to Thicken Hair?

Yes, there are many different types of treatments to thicken the hair on the scalp. However, the real answer to this question depends on the cause of hair loss. Hair loss due to pattern baldness can by treated by minoxidil, finasteride, or hair transplant. Hair loss due to malnourishment or stress is best reversed through improved diet, vitamins, and sufficient sleep. And there are many different causes and respective treatments in between. Thyroid dysfunction, hormonal imbalance, scarring alopecia, etc, are all varying causes of hair loss with specific treatment.

There is no specific treatment to thicken the diameter of hair but there may be treatments to grow more hair such as Propecia and Rogaine.  You wound need to see a doctor to see which treatment may be right for you.

Several medications can improve the thickness of your hair, including oral finasteride (Propecia, for men) and topical minoxidil (Rogaine). They are both safe and very effective in increasing the caliber of your hair and increasing the time it spends in the actively growing phase. Just remember both require 6-8 months to start to appear full. Don't give up after a few weeks!
Some other products can be applied directly to the scalp to help increase the appearance of density. Toppik, X-fusion, and Joan Rivers' "Great Hair Day" can all be applied as a camouflage to the scalp to increase the look of fullness. Just remember that they won't GROW hair, just change how it looks.

And, if you are still not happy you may want to consider hair transplantation. If you are not on blood thinners, don't smoke, and have sufficient density in the back of the scalp, you may benefit from moving those hairs to the front where the density is thinner. Contemporary hair transplantation can offer natural results which are impossible to detect!