In June 1862, King Louis I of Portugal asked Maria Theresa to marry him in a letter sent to her father. It was urgent for him to get married as his older brother, King Peter V, had died in November 1861, without issue and two of his younger brothers, John and Ferdinand, followed him shortly after, which left the Braganza dynasty almost without heirs. He had already selected a number of brides including Princess Marie of Hohenzollern-Sigmaringen (1845-1912), sister of his late sister-in-law Stephanie, Duchess Sophie Charlotte in Bavaria (1847-1897), Princess Maria Pia of Savoy (1847-1910) and also considered some Austrian archduchesses, Maria Theresa being one of them, but didn't know which one to choose. So he sent letters to his cousin, Queen Victoria, and his great-uncle, King Leopold I of Belgium, to ask for their advice. Both agreed that the best choice was Maria Theresa. Thus, King Luís sent his letter. However, his wish was not fulfilled as her father, Archduke Albert, thought she was too young at the time (she was one month away from turning 17) and needed to finish her education. Two weeks after, Luís asked for the hand of Princess Maria Pia of Savoy and, this time, was accepted, even though Maria Pia, born in 1847, was even younger than Maria Theresa.

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