A single hormone number is trivia; a pair is a diagnosis. Move the trophic hormone against its target hormone and watch the decoder name the pattern, point at the broken part of the axis and hand you the nurse's next move. Start with the preset cases, then break the axis yourself.
1 · Choose the axis
Trophic hormone — TSH (from the pituitary)
18mIU/Lhigh
0.010.44.045
Reference roughly 0.4 to 4.0 mIU/L.
Target hormone — free T4 (from the thyroid)
0.4ng/dLlow
0.10.81.84.0
Reference roughly 0.8 to 1.8 ng/dL.
Trophic hormone — plasma ACTH (from the pituitary)
The chapter reads ACTH qualitatively: suppressed means the adrenal is driving; normal or high alongside excess means pituitary or ectopic.
Target hormone — 8 am serum cortisol (from the adrenal cortex)
2.0mcg/dLlow
031840
Below 3 mcg/dL is essentially diagnostic of insufficiency; above 15 to 18 mcg/dL essentially excludes it. Anything between goes to a cosyntropin test.
2 · Preset pairs from the chapter
The rule in one line. Primary disease = the two hormones move in opposite directions (broken gland, screaming pituitary). Secondary or central disease = they move in the same direction (the pituitary cannot shout at all).
3 · Where the lesion sits
Set a pair to beginMove either slider, or tap a preset case.
Cushing disease; ectopic ACTH from small-cell lung cancer
Provoke the gland when a single number will not settle it. Suspect too little → stimulate: cosyntropin 250 mcg IV or IM with cortisol at 30 and 60 minutes; a healthy cortex jumps above roughly 18 to 20 mcg/dL. Suspect too much → suppress: 1 mg dexamethasone at 11 pm, cortisol at 8 am; a normal axis falls below 1.8 mcg/dL. The nursing job is the validity of the result — exact draw times written on the tubes, interfering drugs held, and biotin supplements stopped several days ahead because they distort TSH and free T4 immunoassays.