Levothyroxine stops working after 2 months, until dose is increased. had 4 dose increases over a year. why does it work for a while then stop?

Thyroid failure: The most common cause of hypothyroidism in an adult is hashimoto's thyroiditis. This is a progressive destruction of the thyroid over months to years. So a dose of thyroid hormone that works now, may not be enough a year from now if the thyroid has less function. Eventually, you will be on a stable dose of thyroid hormone when the output of thyroid hormone from the thyroid ceases.