Which laboratory values are indicative of hyperthyroidism?

In hyperthyroidism, the body's metabolism is significantly increased due to an excess of thyroid hormones. The laboratory findings that characterize this condition typically show a low thyroid-stimulating hormone (TSH) level along with elevated free thyroxine (FT4) and/or triiodothyronine (FT3) levels.

The rationale behind the low TSH level is that when thyroid hormone levels are high, the pituitary gland reduces the production of TSH in an attempt to decrease thyroid hormone synthesis and secretion. The high levels of FT4 and/or FT3 further confirm the diagnosis of hyperthyroidism, as they represent the active thyroid hormones circulating in the body.

In contrast, high TSH levels would indicate a state of hypothyroidism or a pituitary gland problem rather than hyperthyroidism. Normal levels of FT4 and/or FT3 would suggest that the patient's thyroid function is normal. While a low TSH combined with normal FT4/T3 levels might suggest a subclinical form or early stage of thyroid dysfunction, it would not definitively indicate hyperthyroidism.

Thus, the combination of low TSH with high FT4 and/or FT3 levels is the standard laboratory finding associated with hyperthyroidism, validating the choice as