What is the typical specimen type for measuring free thyroxine levels in patients?

The measurement of free thyroxine (fT4) levels is typically conducted using a serum specimen. Serum, which is the liquid portion of blood after clotting has occurred, provides a stable and concentrated medium for assessing hormone levels, including thyroid hormones. This is essential because free thyroxine is a critical hormone produced by the thyroid gland that regulates metabolism and is involved in numerous physiological processes.

Testing serum samples allows for the accurate measurement of hormones that circulate in the blood, unbound to proteins, which is what free thyroxine represents. Serum collection is a standard procedure in clinical settings due to the reliability and stability of the samples, making it feasible to assess fT4 levels effectively.

Other specimen types like urine, saliva, or cerebrospinal fluid are less suitable for this specific measurement due to their different compositions and the fact that hormone levels may not reflect the same biological activity as found in serum. For instance, urine primarily reflects metabolites and excretory products rather than free hormone levels. Saliva can sometimes be used for certain hormone assessments, but it is not the standard for thyroid hormones, and cerebral fluid is typically reserved for assessing conditions affecting the brain and nervous system rather than routine endocrine evaluations. Thus, serum is the preferred specimen for