Which laboratory result supports a diagnosis of primary hyperthyroidism?

Primary hyperthyroidism is characterized by an overproduction of thyroid hormones, particularly thyroxine (T4), due to an issue originating in the thyroid gland itself. In this condition, the feedback mechanism that regulates thyroid hormones becomes disrupted. When the thyroid gland produces excess T4, the pituitary gland senses the high levels of thyroid hormone and responds by reducing the secretion of thyroid-stimulating hormone (TSH).

Therefore, the laboratory result indicating low TSH levels combined with high T4 levels supports a diagnosis of primary hyperthyroidism. This finding reflects the inability of the pituitary gland to produce sufficient TSH in response to the elevated levels of thyroid hormones, signaling that the hyperactivity is due to an intrinsic problem with the thyroid rather than a secondary condition influenced by the pituitary gland.

Other laboratory results associated with primary hyperthyroidism, such as normal TSH levels or high TSH levels, would not align with the physiological response expected in this condition. Thus, the combination of low TSH and high T4 is a crucial indicator of primary hyperthyroidism.