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 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.

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.

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.

In patients who have had their thyroid removed and are taking levothyroxine, those with high TSH levels despite high FT4 levels show reduced activity of an enzyme that converts T4 to T3, lower T3 relative to T4, and increased sensitivity to thyroid hormones in the brain, suggesting a biological mechanism may limit the effectiveness of hormone replacement therapy.