What does protein-bound iodine (PBI) measure?

Protein-bound iodine (PBI) is a laboratory test that measures the amount of iodine in the blood that is bound to proteins, primarily thyroxine (T4). This test reflects the levels of hormones produced by the thyroid gland and is used to assess thyroid function. Specifically, it provides insights into how much thyroxine is being produced in the body, as thyroxine is a thyroid hormone that is critical for regulating metabolism and energy levels.

Monitoring PBI levels can be particularly useful for diagnosing and managing conditions related to thyroid function, such as hypothyroidism and hyperthyroidism. When PBI levels are within a normal range, it typically indicates that the thyroid is functioning adequately and producing an appropriate amount of thyroxine. This context reinforces the significance of option D, making it the correct answer in this scenario.