Two weeks ago, I suddenly started having these symptoms: 1) consistently LOWER than normal body temperature (low- to mid 97 degrees, reaching normal 98.6 only briefly in mid-afternoon); 2) low-level headache; 3) constant tinnitus; 4) low energy/fatigue; and 5) lack of mental focus/"brain fog". My research shows that these are common symptoms of abnormally low thryoid hormone levels. But what might cause hypothyroidism ot show up suddenly and without warning? Thank you!