Endocrinology and Diabetes Mellitus
Core concepts
Diabetes is diagnosed by HbA1c ≥48 mmol/mol, fasting glucose ≥7.0 mmol/L, or 2-hour glucose ≥11.1 mmol/L; HbA1c is unreliable in haemoglobinopathy, pregnancy, advanced CKD and recent transfusion. DKA (ketosis, acidosis, glucose usually >11 mmol/L) versus HHS (very high glucose, hyperosmolality, minimal ketosis) both need fluids, insulin and potassium replacement guided by the potassium level. Thyroid: primary hypothyroidism is high TSH with low free T4; subclinical is high TSH with normal T4; central hypothyroidism is low or inappropriately normal TSH with low T4. Thyrotoxicosis: Graves' (diffuse goitre, ophthalmopathy, TSH-receptor antibodies), toxic multinodular, thyroiditis (painful, low uptake), amiodarone-induced. Adrenal: Addison's (postural hypotension, hyperkalaemia, hyponatraemia, hyperpigmentation; short Synacthen test), Cushing's (overnight dexamethasone suppression, ACTH to localise), Conn's (hypertension with hypokalaemia, aldosterone-to-renin ratio), phaeochromocytoma (paroxysmal hypertension, metanephrines). Pituitary: prolactinoma (dopamine agonist first line), acromegaly (OGTT growth hormone suppression, IGF-1), hypopituitarism (glucocorticoid before thyroxine). Calcium: primary hyperparathyroidism (high calcium with high or inappropriately normal PTH) versus malignancy (suppressed PTH).
Traps
Assuming a normal TSH excludes thyroid disease. Giving thyroxine before steroids in suspected panhypopituitarism or adrenal crisis - this can precipitate collapse. Interpreting a Synacthen test soon after exogenous steroids, or a random cortisol during acute illness.
Rule of thumb
When aldosterone is the problem, sodium and potassium move in opposite directions: high aldosterone gives high sodium, low potassium and a metabolic alkalosis.