What condition is diagnosed based on elevated serum TSH in combination with a low T4 level?

A diagnosis of hypothyroidism is made when there is an elevated serum thyroid-stimulating hormone (TSH) level alongside a low thyroxine (T4) level. This pattern indicates that the pituitary gland is producing more TSH in an attempt to stimulate the thyroid gland to produce sufficient thyroid hormones. However, in hypothyroidism, the thyroid fails to respond adequately, resulting in low T4 levels.

In a healthy individual, TSH and T4 levels are regulated through a feedback loop. When T4 levels are low, TSH levels rise to prompt the thyroid to increase T4 production. Therefore, elevated TSH with low T4 is a clear indicator of insufficient thyroid hormone production, characteristic of hypothyroidism.

Evaluating other options: Hyperthyroidism involves low TSH levels along with elevated T4. A euthyroid state is characterized by normal levels of both TSH and T4. Thyroiditis may cause fluctuations in thyroid hormone levels, but the specific combination of elevated TSH and low T4 is most closely associated with straightforward hypothyroidism.