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Three Ways an Axis Breaks: Excess, Deficiency, Resistance

Almost every endocrine disorder is one of three stories. Learn the three patterns and how a feedback loop tells you which one you're looking at.

The whole field in three patterns

When you first meet endocrinology it can feel like an endless list of glands and diseases. But step back and almost everything collapses into three patterns. A gland can make too much hormone (hormone excess). It can make too little (hormone deficiency). Or it can make a perfectly normal amount that the target tissue simply cannot respond to (hormone resistance). That's it. Once you can name which of the three you're seeing, the rest of the workup follows.

The symptoms usually mirror the physiology. Excess of a hormone looks like that hormone's normal action turned up to a damaging degree; deficiency looks like its action switched off. Resistance is the sneaky one: the patient looks deficient, yet hormone levels are high or normal, because the body keeps shouting at a tissue that won't listen.

Reading the feedback loop

Most glands sit on an axis: the hypothalamus and pituitary release a tropic hormone that drives a peripheral gland, and the gland's product feeds back to switch the tropic signal off. This negative feedback is your diagnostic instrument. Measure the gland hormone and its tropic partner together, and the pair tells you where the lesion is.

Worked trace — thyroid axis (gland hormone = T4, tropic = TSH)

NORMAL          high T4 --| pituitary --> low TSH      (loop balanced)

PRIMARY excess  gland runs wild: T4 HIGH  -> feedback -> TSH LOW
   (problem in the gland; tropic suppressed)

PRIMARY deficiency  gland fails: T4 LOW   -> no feedback -> TSH HIGH
   (gland broken; pituitary shouting louder)

SECONDARY (central) deficiency  pituitary fails: TSH LOW -> T4 LOW
   (both low together = problem is upstream, not the gland)

RESISTANCE  target ignores hormone: T4 HIGH/normal AND TSH HIGH/normal
   (high signal, no effect = receptor/post-receptor problem)

Rule of thumb: gland-hormone and tropic move OPPOSITE -> primary gland problem.
               They move the SAME direction        -> central problem OR resistance.
Pairing a gland hormone with its tropic hormone localizes the lesion.

From pattern to plan

  1. Name the clinical syndrome: does the patient look like too much, too little, or unresponsive?
  2. Confirm with a hormone level — but always paired with its tropic or feedback partner.
  3. Localize: primary (the gland), secondary/central (pituitary), or tertiary (hypothalamus)?
  4. Match the fix to the pattern: block/remove for excess, replace for deficiency, bypass or sensitize for resistance.