Which thyroid condition is characterized by decreased TSH and increased free T4?

The condition characterized by decreased TSH (thyroid-stimulating hormone) and increased free T4 (thyroxine) levels is indeed primary hyperthyroidism. In this condition, there is an overproduction of thyroid hormones (T3 and T4) from the thyroid gland itself, which leads to the suppression of TSH due to negative feedback mechanisms. The increase in free T4 levels indicates enhanced thyroid activity, while the lowered TSH reflects the pituitary gland's attempt to reduce stimulation of the thyroid due to the already elevated levels of circulating thyroid hormones.

In primary hyperthyroidism, the excess production of T4 results from intrinsic thyroid pathology—most commonly seen in conditions like Graves' disease or toxic adenomas. The body senses high levels of T4 and thus reduces TSH secretion from the pituitary gland. This is a hallmark of primary hyperthyroid conditions.

Understanding these feedback mechanisms is crucial in differentiating hyperthyroidism from other thyroid disorders, such as hypothyroidism or myxedema, where TSH would typically be elevated if T4 were low, or secondary hyperthyroidism, where TSH levels would also be elevated due to a problem originating from the pituitary gland itself.