Which factor is indicative of primary hypothyroidism?

The presence of elevated TSH with low free T4 is a hallmark of primary hypothyroidism. In this condition, the thyroid gland is underactive and fails to produce sufficient thyroid hormones, primarily thyroxine (T4). As a consequence, the body detects low levels of T4 and responds by increasing the production of thyroid-stimulating hormone (TSH) from the pituitary gland in an attempt to stimulate the thyroid to produce more hormones.

In primary hypothyroidism, the issue lies within the thyroid itself, which leads to decreased hormone production, causing an increase in TSH. This relationship is crucial for diagnosis: elevated TSH indicates that the pituitary gland is working actively to overcome the underactivity of the thyroid. The low free T4 confirms the inadequate hormone production.

The other options describe different scenarios of thyroid function and do not correlate with primary hypothyroidism as clearly as the correct answer does.