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 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.

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.

Taking high doses of thyroid hormone to treat thyroid cancer might weaken your bones over time and increase the chance of getting osteoporosis.

All premenopausal women with a certain type of thyroid tumor have very low levels of a key hormone (TSH), no matter how severe their thyroid symptoms are — but that low hormone level alone doesn’t tell us if their bones are being affected.

In people with normal thyroid function, taking L-dopa does not result in a detectable change in thyroid-stimulating hormone levels because any changes are too small to be measured by standard tests.