A blood test called the thyroid-stimulating hormone index (TSHI) can moderately predict which patients continue to have high TSH levels after thyroid removal and levothyroxine treatment, even when the treatment dose is considered adequate.

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.

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 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.

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.

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.