From the experts:
Though rarely used now that many parents would rather appropriate men's names than sweeten them with feminine endings, Thomasina does have some vintage appeal.
Thomasina Coverly plays a large part in the Tom Stoppard play Arcadia. And Thomasina also has some critter cred--Thomasina Tittlemouse is a Beatirx Potter character, there's a Disney movie called The Three lives of Thomasina, about an enchanted cat, and a Paul Gallico novel titled Thomasina, the Cat Who Thought She was God.
Another version is Thomasin, newly visible via the 2016 horror movie The Witch and also used for a major character in Thomas Hardy's novel The Return of the Native. Hardy's heroine is commonly called Tamsin, the more popular short form.