People taking synthetic thyroid hormone after thyroid removal have higher levels of FT4 but similar levels of FT3 compared to people with overactive thyroid glands due to Graves' disease or toxic adenoma, suggesting different ways the body processes these two thyroid hormones.

In people with thyrotoxicosis caused by taking too much thyroid hormone medication, the brain's control system for thyroid hormone levels behaves differently than in people whose thyroid gland overproduces hormones on its own. Specifically, TSH stays low even when thyroid hormone levels are higher than normal.

When someone has Graves’ disease, their thyroid hormones get too high, which tells the brain to stop making a hormone that stimulates the thyroid — so that hormone ends up being very low or even missing in blood tests.

After thyroid removal, some patients have high TSH and high FT4 but low FT3, which suggests their bodies are not efficiently converting the available thyroid hormone (T4) into its active form (T3).

In people with thyrotoxicosis caused by taking thyroid hormone medication, the rise in active thyroid hormone (FT3) is smaller and less proportional to the increase in precursor hormone (FT4) than in people whose thyroid gland overproduces hormones, because of lower enzyme activity and lack of natural signaling from the thyroid.

In patients who have had their thyroid removed and are taking levothyroxine, those whose TSH levels remain above 2.0 mU/L show higher indicators of reduced pituitary sensitivity to thyroid hormones in the blood.