gland. In contrast, only about 20% of T 3 (triiodothyronine) is produced by
the thyroid gland. T3 is formed in the liver and kidney by the action of a
specific enzyme, 5’-deiodinase enzyme that converts T 4 to T 3. That enzyme
also converts T4 to metabolically inactive thyroid hormone, rT 3 (reverse
T 3 ). T 3 is three to five times more physiologically active than T 4. Both T 4
and T 3 are bound to thyroxine-binding globulin (TBG), transthyretin, and
albumin(answer a),with only about 1% of free circulating hormone. Lev-
els of available binding proteins affect measurable levels of total T 4 and T 3.
When those binding proteins are found in high concentrations, total T 4 and
T 3 levels are also high, but free T4 and T3 values remain normal. The free
fractions of T 4 and T 3 are responsible for the feedback mechanism at the
level of the hypothalamus and the thyrotrophs in the anterior pituitary. T 3
and T 4 are regulated by TSH from the thyrotrophs (answers c and d).
There is some cross-reactivity of all immunoassays (answer e),but that is
not the reason for the possible inaccuracy of extrapolating from serum T 3
levels to thyroid function.
358 Anatomy, Histology, and Cell Biology