Hair loss was a medical condition that led to baldness.
One of the symptoms of hyper-evolution was the loss of hair. (VOY: "Threshold")
Hair loss was an unintended side-effect suffered by Morn as a result of holding liquid latinum in his second stomach for a decade. (DS9: "Who Mourns for Morn?")
In 2374, Commander Chakotay began to lose his hair while the Srivani experimented on him and the crew of the USS Voyager. (VOY: "Scientific Method")
When, in 2376, The Doctor asked Lewis Zimmerman what the initial symptoms of his terminal illness were, Zimmerman sarcastically told him "radical hair loss", before finally giving The Doctor his true symptoms of fatigue, nausea, and joint inflammation. (VOY: "Life Line")