She is presented in medieval sources as a wise and powerful woman who ordered the building or fortification of the Danevirke, consistent with her commemoration on multiple Viking Age runestones.

[1] Much of her story is pieced together through 12th and 13th century sources that broadly disagree with one another, such as Icelandic sagas and writings of the medieval historians Saxo Grammaticus and Sven Aggesen.

[2][3][4] The Jelling 1 stone, commissioned in honour of Thyra by her husband after her death refers to her as Danmarkar bót, translating as "Denmark's adornment", "strength", "salvation" or "remedy".

[5][6] It has been further argued that the words Danmarkar bót instead refer to Gorm, with Thyra's importance being a later innovation by medieval writers, though this is not the scholarly consensus.

[5][11] An alternative proposal given by accounts such as Heimskringla and Jómsvíkinga saga is that her father was Klak Harald, an earl who ruled over a region in the area around Jutland.

However, these sources are subject to great scrutiny and do not provide a consensus on her life.The Gesta Danorum claims that Thyra stood out as a serious and cunning woman and said that she would not marry Gorm until she had all of Denmark as her dowry.

Neither accepting nor rejecting her offer, she told him that to work out if they would have a happy marriage, he had to build a house in a wood that could fit only one bed and to sleep there alone for three nights, telling her what dreams he had.

The loud noise as the waves of the sea rolled back on the Danish shores signified that mighty men would come to the land and Gorm's relations would be drawn into the wars that would follow.

She said that had he dreamed of the black boars and the rushing waves the first night, she would not have married him, but since she would be available to provide advice as queen, there would be little injury from the wars foretold.

[17] Aggesen and Saxo Grammaticus record that Thyra was a wise and steadfast queen who ordered the building of the Danevirke, a system of fortifications built to protect Denmark from invaders from the south.

[5] According to Aggesen, Thyra ordered the construction of the Danevirke as part of her plan to end Denmark's role as a tributary to the Kingdom of Germany.

She is reported to have deceived the German emperor into believing that the fortifications she was constructing on the border were to protect Germany from a invasion from her husband Gorm.

[5] It has been proposed however, based on analysis of factors such as groove depth, rune shape and spelling, that a single runecarver named Ravnunge-Tue (who is one of the earliest known Western European artisans to have carved his name on his work as the maker) wrote the inscriptions both on the Læborg and Jelling 2 stones, while another individual carved the Horne, Bække and possibly the Randbøl stones.