Can an under-active thyroid gland become over-active?

Yes, it is possible for an underactive thyroid gland (hypothyroidism) to become overactive (hyperthyroidism) in a condition known as "transient thyrotoxicosis" or "subacute thyroiditis". This usually occurs during the recovery phase of thyroiditis, an inflammation of the thyroid gland.

During thyroiditis, the damaged thyroid gland may release an excessive amount of thyroid hormones into the bloodstream, leading to temporary hyperthyroidism. This overproduction of thyroid hormones can cause symptoms of thyrotoxicosis such as increased heart rate, anxiety, weight loss, heat intolerance, and increased appetite. However, this phase is usually transient and eventually, the thyroid function returns to normal or becomes underactive again.

It's essential for individuals who have been diagnosed with hypothyroidism to continue regular monitoring of their thyroid hormone levels and discuss any changes in their symptoms with their healthcare provider for appropriate management.