Alopecia

Alopecia, also known as baldness, was a condition where an individual's hair on the top of their head began to recede with age or was absent due to other reasons. It could be acute, such as a small loss of hair, or advance to the point where someone did not have any hair at all. Baldness could also be a hairstyle by choice, afflicted by physical scarring, (TV: The Caves of Androzani) radiation, (TV: The Mutants) or due to biological experimentation that removed hair. (WC: The Making of the Gunslinger, TV: A Town Called Mercy)

Baldness in individuals
When he had grown quite older, the First Doctor appeared to have some degree of baldness, with most of his hair growing around the back of his head. (TV: An Unearthly Child) When Peri Brown was selected for brain surgery, she was forcibly shaved bald in preparation. (TV: Mindwarp)

The Fifth Doctor gained a temporary bald spot when the time differential in a future version of his TARDIS caused him to age very quickly. (TV: Time Crash)

When the Tenth Doctor was forced to age by the Master, he went partially bald. The second time he was aged, he went completely bald. (TV: The Sound of Drums, Last of the Time Lords)

The Eleventh Doctor once got bored and shaved his head with a razor. However, it allowed him to wear a wig and hide a TARDIS key in his coif that Tasha Lem could not discover when he set foot on a planet with a technology ban. He also exhibited true baldness at the echelon of old age. This condition reversed when his new regeneration cycle reset him back to a youthful body. (TV: The Time of the Doctor)

Though Nardole appeared to be bald, (TV: The Husbands of River Song et al.) his glass avatar would claim that he actually had invisible hair. (TV: Twice Upon a Time)

Hobo Kostinen had alopecia, (PROSE: Avatars of the Intelligence) which he had been bullied for. (PROSE: 8.46)

After the Time Lords resurrected him, the Master's new incarnation was totally bald. The Eighth Doctor ridiculed him because of it. (AUDIO: Eyes of the Master)

Baldness in species
Sontarans, at least in the case of their clone batch-borne warriors, were naturally bald. (TV: The Time Monster, The Sontaran Experiment, et al.) However, some could have vestigial hair such as beards that turned white with age like those of humans. (PROSE: Shakedown) Strax, a Sontaran, didn't understand the growth of hair on humans and other species; he once mistakenly thought River Song's curly hair meant she had a big head. Clara Oswald subsequently referred to River as "the lady with the funny name and the space hair". (TV: The Name of the Doctor)

Other species which appeared to be naturally bald included the Rostrow, (COMIC: Mission for Duh) the Technix, (TV: The Daleks' Master Plan) the Yend (PROSE: The Sons of the Crab) and the natives of Jevo. (COMIC: The Emissaries of Jevo)

Behind the scenes
Matt Lucas, who played Nardole, has alopecia.