: Thyroxine (T4) 24-Hour Urinary Excretion Rate

: What This Test Measures

: Thyroxine, often called T4, is one of the main hormones produced by the thyroid gland. It circulates in the blood and is gradually broken down and removed from the body, partly through the urine. This test collects all urine produced over 24 hours and measures the total amount of T4 excreted, then reports it as a rate (amount per unit time).

: Why a 24-Hour Collection Is Used

: Hormone levels in urine change throughout the day, so a single sample may not reflect overall output. Collecting urine for a full 24 hours gives a more complete picture of how much T4 the body is eliminating. Proper collection—keeping the container refrigerated and including all urine—is important for accurate results.

: How It Relates to Thyroid Function

: The amount of T4 excreted in urine generally reflects the amount of thyroid hormone being produced and processed by the body. Results are usually interpreted alongside blood thyroid tests such as TSH, free T4, and total T4, because urine measurements alone do not show how well the thyroid is regulated.

: What to Expect During Testing

: You will be given a collection container and instructions. You typically discard the first urine of the collection period, then collect all urine for the next 24 hours, ending with a final sample at the same time the next day. No special preparation is usually needed, but tell your provider about any thyroid medications, since they can affect results.