end-organ resistance
End-organ resistance is when the organ that is supposed to act on a hormone — the end of the chain, the place where the message is meant to take effect — fails to respond. The signal is sent and even received at the door, but the room behind it stays dark. The hormone exists, yet the tissue behaves as though it does not.
The point of failure is downstream of the gland, in the responding tissue itself. It might be a defective receptor, too few receptors, or a broken link in the internal cascade that normally turns the hormone's arrival into an action. Because the gland and the hormone are fine, blood hormone levels are often normal or high — and yet the body shows the signs of having too little of that hormone's effect.
This concept is the key to a class of disorders that would otherwise be baffling. In pseudohypoparathyroidism, the kidneys ignore parathyroid hormone, so calcium falls even though PTH is high. In androgen insensitivity, target tissues cannot read testosterone, so a genetically male individual develops along female lines. Identifying that the failure is at the end organ — not the gland — points treatment toward the receiving tissue rather than the source.
In nephrogenic diabetes insipidus, the kidney's collecting ducts ignore vasopressin; the hormone is present in normal amounts, but the end organ cannot conserve water, so the person passes large volumes of dilute urine.
Normal hormone, deaf target tissue.
End-organ resistance and hormone resistance describe the same phenomenon from different angles; end-organ resistance points to where the failure is (the responding tissue), while hormone resistance names what fails (the hormone's effect).