Fig. 15. 14 Thyroid function tests in various states The distribution of isotope between binding agents (resin or Sephadex and the free sites on thyroid hormone binding proteins is shown. (a) In the euthyroid state, levels of free T4, total T4, and TSH are within the normal range. Using indirect testing, 125I-T3 attached to a binding agent (BA), normally resin or Sephadex, competes for free binding sites on the thyroid hormone binding proteins (THBP), mainly TBG, and competition between BA and THBP is approximately even. (b) In the hyperthyroid state, levels of free T4 and total T4 are increased, and TSH levels are suppressed. On indirect testing, few binding sites are unoccupied on THBP, and most 125I-T3 remains attached to the binding agent. (c) In primary hypothyroidism, levels of free T4 and total T4 are low, and TSH is increased. On indirect testing, many binding sites are unoccupied on THBP and will therefore take up 125I-T3, leaving only a small quantity on the BA.
