Thyra is the older and/or Danish spelling of Tyra. Queen Thyra Dannebod is a famous Danish queen. The name Tyra/Thyra comes from the old Nordic name Thyrvi.
Princess of Thyra of Denmark (1853-1933), daughter of King Christian IX of Denmark, sister of King George I of Greece and Maria Feodorovna, Tsesarevna of Russia.
Dr Samuel Gee (1839-1911) was a well-known English medical specialist - his daughter was called Edith Thyra Gee. I think it's weird how he chose that "thyroid" spelling!