Which of the following statements is true regarding reverse T3 (rT3)?

The statement that reverse T3 (rT3) is formed in the blood by degradation of T4 is accurate. rT3 is a metabolite of thyroxine (T4) and is produced as part of the body's process of converting T4 into its biologically active form, triiodothyronine (T3). During this conversion, T4 can deiodinate at different sites, resulting in the production of T3 or rT3. rT3 is considered an inactive form of thyroid hormone and thus has no significant physiological action like T3 does.

Understanding the metabolism of T4 and the role of rT3 gives context to how thyroid function is assessed. While rT3's production can be influenced by various factors, including illness and stress, its primary relationship is with T4 metabolism, establishing why the formation of rT3 from T4 is the correct choice.

The other options present alternative statements that do not accurately reflect the role or characteristics of rT3. For instance, rT3 is generally classified as being biologically inactive rather than physiologically active, and its levels can be affected but are typically not decreased in euthyroid sick syndrome—rather, they can be either normal