Parathyroid surgery can cause a temporary hyperthyroidism (overactive thyroid). Here is what can happen: during manipulation of the thyroid, which occurs during parathyroid surgery, the thyroid can get irritated. It then starts dumping out too much thyroid hormone. This is called postoperative thyroiditis. It is a self-limiting process, which generally resolves within several weeks. In the first few days, you can see elevated T3 and T4, with low TSH (a low TSH means an overactive thyroid). Then after a few days you will just see the low TSH persist a little longer, with normal T3 and T4, then resolve. Most people will have mild symptoms like a feeling of jitteriness. You can also get all of the symptoms of hyperthyroidism like racing heart, cardiac arrhythmias, loose stools, anxiety, etc. There isn’t a specific treatment for the thyroid, since it will resolve on its own, and other treatments for hyperthyroidism are not effective. We just treat the symptoms, to relieve racing heart or anxiety.   Hyperthyroidism caused by parathyroid surgery will generally appear within a few days or week after the operation, and resolve within a month. If the hyperthyroidism occurred after that time period, then it is likely unrelated to the parathyroid operation.   It seems that patients with some underlying thyroid conditions are more likely to experience a postoperative thyroiditis. Some patients have a chronic thyroiditis such as Hashimoto’s, which predisposes the thyroid to be more easily irritated. These patients with active chronic thyroiditis may be more likely to experience the acute thyroiditis caused by manipulation of the thyroid.