Biochemistry — Calcium, Bone Chemistry and Thyroid Tests, NMC MBBS licence examination syllabus (Nepal Medical Council).
A calcium result read without the albumin beside it is not a calcium result. It is a number that might mean anything.
The first two biochemistry chapters covered metabolism — how the body switches between fed and fasting, what happens when a step is blocked, and how nitrogen and bilirubin are handled. This one covers two panels a candidate will be handed repeatedly and asked to interpret.
Calcium circulates in two forms. A portion is free and physiologically active; much of the rest is bound to albumin, where it has no effect. The laboratory usually reports the total, which adds the two together.
The consequence is the single most useful point in this chapter: a low albumin lowers the total calcium without lowering the active calcium. That patient is not calcium-deficient, and treating them as though they were is treating an artefact. Low albumin is common — in acute illness, in malnutrition, in liver disease — so this is not a rare trap.The habit to build is simple: never read a calcium without looking at the albumin on the same report. Most laboratories publish an adjusted figure alongside the total; where they do, that is the number to act on.
Calcium sets the excitability of nerve and muscle membranes, so its disturbance produces symptoms in both directions.
Low calcium makes tissue too excitable. Tingling around the mouth and in the fingers, cramps, and muscle spasm; fits when severe. Spasm can sometimes be provoked at the bedside — by tapping over the facial nerve, or by inflating a blood pressure cuff on the arm.
High calcium makes everything too quiet. Thirst and passing large volumes of urine, constipation, abdominal pain, and confusion; kidney stones over time. These are vague symptoms, and the diagnosis is often made on a blood test rather than suspected clinically.
An overactive parathyroid gland tends to be found by chance in a patient who is otherwise well, with a long history and mild symptoms.
Malignancy gives a shorter history in a patient who is unwell — weight loss, bone pain, or a cancer already known about — and the calcium is often higher.
What separates them is measuring the parathyroid hormone. If calcium is high, the hormone should be suppressed; a hormone that is not suppressed when calcium is high is inappropriate, and points to the gland itself. In malignancy the hormone is usually suppressed, as it should be.
The same principle governs thyroid tests, and it makes them far easier than they look. The pituitary hormone and the gland's own hormone move in opposite directions in ordinary disease.
An underactive gland produces less hormone, so the pituitary pushes harder and its hormone rises. An overactive gland produces more, so the pituitary backs off and its hormone falls. Reading the pair together is what makes the result interpretable; either alone can mislead.
When both move in the same direction, the problem is above the gland. A low gland hormone with a low pituitary hormone is not a thyroid that has failed — it is a pituitary that is not driving it. NMCMD17 covers that.
One practical warning: thyroid tests done during acute illness are unreliable. Serious illness disturbs the results in ways that mimic disease, and treatment started on that basis is often wrong. Unless the clinical picture is compelling, repeat once the patient has recovered.An inpatient with sepsis has a low total calcium and a low albumin. Look at the adjusted value before treating. A low albumin lowers the total without lowering the active fraction, and this patient may need nothing.
A well 55-year-old has a high calcium found on a routine test. Long history, well patient — an overactive parathyroid is likely. Measure the hormone: if it is not suppressed at a high calcium, that is the answer.
A patient with known lung cancer has a high calcium, confusion and weight loss. Malignancy-associated hypercalcaemia. The parathyroid hormone should be suppressed here, and its being so supports rather than excludes the diagnosis.
An unwell inpatient has a mildly abnormal thyroid panel. Repeat once recovered rather than starting treatment. Acute illness disturbs these tests, and the abnormality may resolve entirely.
Total calcium and active calcium. The laboratory reports the total; only the free fraction acts. Albumin is what reconciles the two.
A high hormone level and an appropriate one. A parathyroid hormone within the normal range can still be inappropriate if the calcium is high, because it should have been suppressed.
A failed thyroid and a failed pituitary. Distinguished by whether the two hormones move in opposite directions or the same one.
An abnormal test and a disease. Especially during acute illness, where the abnormality is often a consequence rather than a cause.
Read calcium with albumin. A low albumin lowers the total without lowering the active calcium.
Low calcium — excitable: tingling, cramps, spasm. High calcium — quiet: thirst, constipation, confusion, stones.
High calcium is usually parathyroid or malignancy, and the parathyroid hormone separates them.
Ask whether the hormone is appropriate for the calcium, not whether it is within range.
Thyroid hormones move in opposite directions in gland disease; the same direction points above the gland.
Do not act on thyroid tests taken during acute illness — repeat after recovery.
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