Sudden onset dry itchy skin on face/chest and isolated patches on arms. Recently tested for hypothyroidism, one test normal, other high. Poss cause?

Xerosis. Xerosis or dryness of the skin can have multiple causes including vitamin deficiencies or hypothyroidism. Eat a healthy, balanced diet and take a multivitamin supplement with vitamin a, biotin and zinc. Also, follow up with an endocrinologist to make sure you don't need thyroid hormone supplement. Moisturizer with aveeno (oatmeal) or other sensitive skin moisturizer.