What does high TSH and low T4 indicate?

High levels of Thyroid Stimulating Hormone (TSH) along with low levels of Thyroxine (T4) indicate hypothyroidism. In this condition, the pituitary gland produces more TSH in an attempt to stimulate the thyroid gland to produce more thyroid hormones, which include T4. However, if the thyroid gland is unable to produce adequate amounts of T4, the TSH levels will remain elevated while T4 levels are low.

This scenario is a classic illustration of primary hypothyroidism, where the issue lies with the thyroid gland itself, causing insufficient hormone production. Identifying this pattern is crucial in diagnosing hypothyroidism, as it helps guide further testing and treatment options, such as hormone replacement therapy.

In contrast, other conditions like hyperthyroidism would present with low TSH and high T4 levels, as the body would suppress TSH production due to excess thyroid hormones. Nodular thyroid disease may also demonstrate different patterns depending on whether nodules are functioning or not, while normal thyroid function would show both TSH and T4 levels within the normal range.