Endocrine assessment, diagnostics and nursing management

The hypothalamic–pituitary–target gland arrangement is repeated for almost every hormone, and each level has its own cell type and its own hormone. The hypothalamus releases releasing factors such as thyrotropin-releasing hormone and corticotropin-releasing hormone into a small portal circulation that carries them directly to the anterior pituitary, so they are not diluted in the general bloodstream. There, specialised cells answer: thyrotrophs make thyrotropin, corticotrophs make corticotropin, gonadotrophs make the gonadotropins and lactotrophs make prolactin, which is unusual because it is restrained rather than stimulated by hypothalamic dopamine.

The pituitary hormones are called tropic because their target is another gland, and the target gland hormones — thyroxine, triiodothyronine, cortisol, the sex steroids — are the ones that act on the tissues. The site of the fault is named by its level. Primary failure is disease of the target gland itself: the tropic hormone rises because the brake has gone, as in Hashimoto’s thyroiditis or autoimmune destruction of the adrenal cortex, where the high corticotropin also produces the characteristic bronzing of the skin. Secondary failure is disease of the pituitary: the tropic hormone and the target hormone fall together. Tertiary failure is disease of the hypothalamus above it.

The distinction changes treatment and monitoring. In primary hypothyroidism the replacement dose is followed with thyrotropin; in secondary hypothyroidism thyrotropin is useless as a guide because the pituitary cannot respond, so free thyroxine must be followed instead. The same trap exists for cortisol, where the morning value is interpreted against a rhythm: cortisol is highest at about 08:00 and lowest around midnight, and each laboratory sets its own reference range, which is why ranges travel poorly between hospitals and why the patient’s own previous results are the best comparison.

Each dynamic test has a protocol, an interpretation and a hazard, and the nurse owns all three. The overnight low-dose dexamethasone suppression test gives one milligram of dexamethasone at about 23:00 and measures cortisol in the morning; a healthy axis is switched off, while autonomous production is not. Obesity, alcohol excess, depression and oestrogen-containing medicines can cause a false abnormal result, so interfering drugs are held only on a written order and the reason is documented.

The 24-hour urinary free cortisol measures total daily production and depends entirely on a complete collection with the exact start and finish times recorded and stored as the laboratory requires; a missed specimen invalidates the test, which is why the nurse, not the patient, controls the timing. The cosyntropin stimulation test gives 250 micrograms of synthetic corticotropin and measures cortisol at 30 and 60 minutes: a normal adrenal cortex rises, whereas a failing one does not, and the test must be done before steroid treatment is started because exogenous steroid suppresses the response.

The water deprivation test with desmopressin is the most dangerous of the group. Fluids are withheld under continuous supervision while weight, urine volume, urine osmolality and serum sodium are followed; the test is stopped if the patient loses about three to five per cent of body weight or if the sodium rises too far, and vasopressin is never given during the deprivation phase in the classic protocol. Interpretation separates central diabetes insipidus, in which the kidney concentrates urine once the hormone is supplied, from nephrogenic diabetes insipidus, in which it cannot, and from primary polydipsia, in which chronic overdrinking has washed out the medullary gradient.

The prolonged glucose tolerance test used when an insulinoma is suspected carries the opposite hazard: the patient is fasted and may become hypoglycaemic, so glucose is drawn up at the bedside and symptoms are watched continuously. Finally, imaging has its own rules: MRI requires screening for metal and for claustrophobia and the patient must lie still for a long scan, thyroid ultrasound is painless but fine-needle aspiration is explained first, and radioiodine scanning is avoided in pregnancy and breastfeeding. Thyroid medicines, antithyroid drugs and iodine-containing contrast can all distort an isotope study, so the ordering physician must know what the patient is taking.

Safety in endocrine nursing rests on a handful of quantitative rules that the nurse applies before the physician arrives. A litre of water weighs a kilogram, so daily weight is the reference measurement for fluid balance; urine output below about half a millilitre per kilogram per hour in an adult is a warning, and the intake and output chart must add up because the arithmetic is the assessment. In the syndrome of inappropriate antidiuretic hormone secretion, water is retained and the sodium is diluted, so the signs are neurological — headache, nausea, confusion, personality change, and ultimately seizure and coma — and the speed of the fall matters more than the absolute value. Fluid restriction is the first measure, which means documenting every mouthful, offering ice chips that are also counted, and giving frequent mouth care within the restriction.

Hypertonic saline is given only on a specific order, at a controlled rate, with sodium checked at the prescribed intervals, because correcting hyponatraemia too quickly can cause permanent osmotic injury to the brain; isotonic saline alone can even worsen the problem, because the kidney excretes the salt and keeps the water. In diabetes insipidus the risk is the opposite: free access to water, a rising sodium, and teaching about desmopressin. In adrenal insufficiency the dangers are hypotension on standing, hypoglycaemia and hyperkalaemia, and the single most important teaching point is that steroid replacement must never be stopped abruptly. The body cannot increase its own cortisol output during stress, so the dose is increased during fever, vomiting, diarrhoea, dental work or surgery — the rule nurses call steroid cover — and the patient carries a card or bracelet explaining it. Adrenal crisis with profound weakness, vomiting, hypotension and low glucose is a medical emergency in which intravenous hydrocortisone and fluid are given without waiting for confirmatory tests.

In Cushing’s syndrome and in patients treated with corticosteroids, the nurse watches the glucose because steroids raise it, inspects the skin because healing is delayed, looks for the subtle signs of infection that the anti-inflammatory effect conceals, and protects the patient from falls because the bones are losing calcium.

After thyroid surgery the priorities are different again: keep oxygen, suction and a tracheostomy set at the bedside, place the patient semi-upright to reduce tension on the neck, inspect the wound for bleeding and for swelling that could obstruct the airway, and watch for the tingling that announces hypocalcaemia. Postoperative education closes the loop: lifelong replacement, the reason it is never omitted, the laboratory work that adjusts the dose, and the medical alert jewellery that speaks for the patient when the patient cannot.