Ælfthryth, also known as Alfreda, Alfritha, Aelfnryth, or Etheldritha, is a saint, virgin, and recluse, venerated in both the Roman Catholic Church and Antiochian Orthodox Church. She was a daughter of King Offa of Mercia and his consort, Cynethryth.
Ælfthryth (c. 945 – 1000 or 1001, also Alfrida, Elfrida or Elfthryth) was Queen of the English from her marriage to King Edgar in 964 or 965 until Edgar's death in 975. She was a leading figure in the regency during the minority of her son King Æthelred the Unready between 978 and 984.
Althrith (shortened to 'Alth') could be a modern English form. Source: https://anglisc.miraheze.org/wiki/Anglish_Given_Names (See Anglo-Saxon Names category)
Elfthritha is another form. Source:
https://thehistorianshut.com/2020/08/18/the-assassination-of-edward-the-martyr-by-joseph-martin-kronheim-c-1810-1896/
Aelfthryth, Elfthryth and Elfthrith are other forms. Source: https://m.wikidata.org/wiki/Q1470395