hypo versus hyperthyroidism

I have been hypothyroid for 21 years and have been using synthroid. What was surprise to me is that my last test results have shown that I am suddenly hyperthyroid. I am having severe hyperthyroid symptoms like sensitivity to heat, dizziness, insomnia etc. My question is simple. How can I be hypothyroid for so many years and now hyperthyroid?

The causes of hypothyroidism are insufficient thyroid tissue, iodine deficiency, inborn errors of metabolism, and some drugs. Causes of hyperthyroidism are Grave’s Disease, toxic multinodular goiter, toxic thyroid adenoma etc. When it comes to your case, you most likely still have hypothyroidism but have decreased synthroid requirements. The usual synthroid dose that had worked in the past is now rendering your over-treated. Reasons that lead to this are: change in estrogen status-pregnancy, birth control pills, change in supplements-stopping calcium, iron or high fiber. Also, change in medicines effecting absorption/metabolism of thyroid hormone-certain anti-depressants or cholesterol medications for example.
It is less likely there is a true gain in the function of the thyroid. What ever question you have I suggest you to ask your doctor. He is the one who knows the best your condition and will be able to give you information that you are looking for.