Endocrine Function Tests
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Direct answer
No endocrine number stands alone, because glands reveal disease only when pushed: dynamic tests probe reserve with stimulation and autonomy with suppression. A morning cortisol of 140-690 nmol/L that fails to fall at midnight, and that escapes suppression below 1.8 micrograms per decilitre (50 nmol/L) after overnight 1-mg dexamethasone, screens for Cushing's syndrome. A 250-microgram Synacthen injection that cannot lift cortisol above 500 nmol/L at 30-60 minutes documents primary adrenal insufficiency, while paradoxical growth hormone rise during a 75-g oral glucose tolerance test confirms acromegaly, and urine osmolality rising more than 50 per cent after desmopressin separates cranial from nephrogenic diabetes insipidus during water deprivation.
What you must remember
- TSH first (about 0.4-4.0 mIU/L): the most sensitive screen of primary thyroid disease — high with low T4 in primary hypothyroidism, low with low T4 in pituitary disease; TRH stimulation peaks TSH at 30 minutes.
- Cushing pathway: overnight 1-mg dexamethasone failing to suppress below 1.8 µg/dL screens; low-dose 2 mg/day (Liddle's test) confirms; high-dose 8 mg suppresses pituitary corticotroph adenomas more than ectopic ACTH sources.
- Adrenal reserve: short Synacthen test (250 µg) with cortisol above 500 nmol/L at 30 or 60 minutes is normal; the insulin tolerance test, driving glucose below 2.2 mmol/L, remains a gold standard for the whole axis but is contraindicated with seizures or ischaemic heart disease.
- Acromegaly: growth hormone should suppress during a 75-g OGTT; failure to fall below about 1 ng/mL (some centres use lower cut-offs) or a paradoxical rise is diagnostic.
- Short stature: growth hormone is pulsatile, so single levels are worthless; a peak below 10 ng/mL on two separate stimulation tests supports deficiency.
- Diabetes insipidus: deprive water until plasma osmolality exceeds about 295 mOsm/kg — urine still dilute (below 300) proves DI, and desmopressin then lifts urine osmolality over 50 per cent only in the cranial form.
- Hyperaldosteronism screen: aldosterone-to-renin ratio above 30 (aldosterone in µg/dL over renin activity), with aldosterone not suppressed.
- Hypoglycaemia panel: during a witnessed episode, insulin, C-peptide and beta-hydroxybutyrate together separate insulinoma (all three inappropriately poised) from injected insulin (high insulin, crushed C-peptide).
A short-stature workup in five steps
A 12-year-old boy at minus 3 standard deviations for height walks through the protocol in order. First exclude the mundane: dietary history, coeliac serology (a classic hidden cause of faltering growth in Indian children), thyroid function starting with TSH, and a bone-age radiograph of the left hand compared against chronological age and mid-parental height. Second, an IGF-1 level: low for age supports deficiency, normal argues against. Third, two provocative growth hormone tests on separate days — insulin tolerance, arginine, clonidine or GHRH — because a single blunted response is commonplace in an anxious child. Fourth, if peaks stay below 10 ng/mL, the deficiency is real. Fifth, MRI the hypothalamo-pituitary region looking for a craniopharyngioma or stalk anomaly before starting therapy. The teaching pearl is why dynamic testing exists at all: a random growth hormone sample in the afternoon may be unmeasurable in perfectly normal children.
Where the viva actually goes
The examiner asks why TSH is ordered before T4 — because the axis responds logarithmically, small T4 changes move TSH severalfold — and why free T4 must replace total in pregnancy (oestrogen raises thyroxine-binding globulin). Expect the high-dose hook effect that falsely lowers very high prolactin levels, the midnight salivary cortisol as a Cushing screen, and ACTH drawn alongside cortisol to split primary from secondary adrenal failure. The Indian convention of 8 a.m. fasting sampling matters: a 4 p.m. cortisol means nothing against morning reference ranges.
Frequently asked questions
Which test screens for Cushing's syndrome?
The overnight 1-mg dexamethasone suppression test; cortisol failing to fall below 1.8 µg/dL (50 nmol/L) the next morning warrants confirmatory testing.
Why are two growth hormone stimulation tests required?
Growth hormone is secreted in pulses, so single stimulated values are unreliable; two subnormal peaks on separate provocations define deficiency.
How does the water deprivation test separate cranial from nephrogenic diabetes insipidus?
After proven concentration failure, desmopressin raises urine osmolality more than 50 per cent in cranial DI but barely in the nephrogenic form, where renal V2 receptors are defective.
Which single test best screens primary hypothyroidism?
TSH — it rises early and steeply when the gland fails, and falls when the pituitary fails.
What does the insulin tolerance test assess and when is it contraindicated?
Hypoglycaemia-driven reserve of the entire hypothalamo-pituitary-adrenal and growth hormone axes; contraindicated in epilepsy and ischaemic heart disease.