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.

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).

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.

Your thyroid can still work normally even if you're not getting quite enough iodine, thanks to built-in backup systems that kick in for months or years.

When high thyroid hormone levels are caused by taking too much thyroid medication (exogenous thyrotoxicosis), the underlying biochemical changes differ from those caused by an overactive thyroid gland (endogenous hyperthyroidism). Because of these differences, relying only on TSH levels to assess risk can lead to incorrect conclusions in one condition versus the other.

In people taking thyroid hormone medication who have normal thyroid function, the pituitary gland releases less TSH in response to TRH, but releases the same amount of prolactin as before, indicating that the two hormones are regulated differently during hormone replacement.