Is it possible to have a very overactive thyroid even if your TSH levels are in normal range?

Very unlikely. A very overactive thyroid is virtually always associated with a very low or undetectable tsh. Of course a laboratory error could be responsible and the test should always be repeated if it does not fit with the clinical picture. Lastly, stress and/or an underactive adrenal gland can sometime mimic and overactive thyroid.
Very rarely. With resistance to thyroid hormone, such a rare patient could have elevation in thyroid hormone levels independent of a suppressed pituitary tsh. This disorder is very rare and is the result of a thyroid hormone receptor defect making the pituitary less responsive to suppression from thyroid hormones. Under normal circumstances, hyperthyroidism suppresses tsh.

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