What is a classic laboratory finding in hypothyroidism?

In the context of hypothyroidism, the classic laboratory finding is characterized by high levels of thyroid-stimulating hormone (TSH) and low levels of free thyroxine (free T4). This occurs because, in hypothyroidism, the thyroid gland is underactive and fails to produce adequate amounts of thyroid hormones. As a result, the pituitary gland responds by increasing the secretion of TSH in an attempt to stimulate the thyroid to produce more hormones.

High TSH levels indicate that the body is signaling for more thyroid hormone production due to the low levels of circulating free T4. The low free T4 confirms that thyroid hormone production is insufficient, aligning with the diagnosis of hypothyroidism. This feedback loop between the pituitary gland and the thyroid gland is essential for understanding thyroid function and disorders.

In summary, the combination of high TSH and low free T4 is a hallmark of hypothyroidism, validating the choice as the correct answer.