What laboratory values are characteristic of primary hyperthyroidism?

In primary hyperthyroidism, the thyroid gland overproduces thyroid hormones, leading to specific changes in hormone levels that are critical for diagnosis. In this condition, the levels of free T4 (thyroxine) are elevated because of the excessive production of thyroid hormones.

As the thyroid hormone levels rise, they exert negative feedback on the pituitary gland, which results in decreased levels of Thyroid-Stimulating Hormone (TSH). This feedback mechanism is crucial because normally, TSH stimulates the thyroid gland to produce more T4. However, in primary hyperthyroidism, the gland is already overactive, resulting in the suppression of TSH levels while free T4 remains elevated.

Thus, the characteristic laboratory findings in primary hyperthyroidism include decreased TSH levels alongside increased free T4 levels, making this combination indicative of the condition.