Endocrinology, Vitamins, And Nutrition

Thyroid function testing

Thyroid items follow negative-feedback logic. The hypothalamus releases TRH, the pituitary releases TSH (thyroid-stimulating hormone), and the thyroid releases T4 and T3. When thyroid hormone is high, TSH is suppressed; when it is low, TSH rises. TSH is the most sensitive screening test. Interpret with free T4:

Graves disease is confirmed by thyroid-stimulating immunoglobulin (TSI) and anti-TSH-receptor antibodies; Hashimoto by anti-thyroid peroxidase (anti-TPO) antibodies.

Adrenal and reproductive hormones

Cortisol from the adrenal cortex shows diurnal variation -- highest around 8 AM, lowest near midnight, so collection time matters. Cushing syndrome (excess cortisol) is screened with a low-dose dexamethasone suppression test (failure to suppress = positive) and 24-hour urine free cortisol. Addison disease (primary adrenal insufficiency) is confirmed with the ACTH (cosyntropin) stimulation test -- no cortisol rise confirms the adrenal cannot respond. Catecholamines and metanephrines (urine/plasma) detect pheochromocytoma.

Reproductive testing: a markedly elevated hCG (human chorionic gonadotropin) confirms pregnancy and, with AFP and unconjugated estriol, forms the maternal triple screen. FSH and LH rise sharply in menopause and ovarian failure.

Vitamins and nutrition

Classify vitamins by solubility because it drives storage, deficiency speed, and toxicity risk.

• Fat-soluble (A, D, E, K): stored in liver/fat, deficiency develops slowly, and they can reach toxic levels. Vitamin A excess causes hepatotoxicity; vitamin K is essential for clotting factors II, VII, IX, X (warfarin antagonizes it).
• Water-soluble (B-complex, C): minimally stored, depleted quickly, rarely toxic. B12 and folate deficiency cause megaloblastic anemia -- check methylmalonic acid (raised only in B12 deficiency) to distinguish them. B1 (thiamine) deficiency causes beriberi/Wernicke; vitamin C deficiency causes scurvy.

Vitamin D status is measured as 25-hydroxyvitamin D (the storage form), not 1,25-dihydroxy. Deficiency causes rickets in children and osteomalacia in adults, and is linked to low calcium, low phosphate, and a compensatory rise in parathyroid hormone (PTH). PTH raises serum calcium by mobilizing bone, increasing renal reabsorption, and activating vitamin D; calcitonin opposes it.

Nutritional/visceral protein status: prealbumin (transthyretin) has a half-life of ~2 days and reflects recent nutrition far better than albumin (half-life ~20 days). A falling prealbumin is an early malnutrition flag. Iron studies (ferritin = storage), and trace elements (zinc, copper) round out nutritional assessment.

Calcium-phosphate-PTH axis

The exam tests the reciprocal regulation tightly. Primary hyperparathyroidism (parathyroid adenoma) shows high calcium with low phosphate and high PTH -- the gland is autonomously overactive. Secondary hyperparathyroidism in chronic kidney disease shows low calcium, high phosphate, and high PTH, because failing kidneys cannot excrete phosphate or activate vitamin D, so PTH rises to defend calcium. Hypoparathyroidism shows low calcium with high phosphate and low PTH, producing tetany and a positive Chvostek sign. Always interpret calcium with albumin and with PTH and phosphate together, never alone.

Hormone testing pitfalls

Many hormones are pulsatile or diurnal, so a single random value can mislead. Cortisol drawn at noon is uninterpretable without the collection time; growth hormone and gonadotropins are pulsatile and often require stimulation or suppression tests rather than a single level. Catecholamine and metanephrine collection requires the patient to avoid certain foods and drugs that cause false elevations. Beta-hCG doubles roughly every 48 hours in early normal pregnancy; a plateau or slow rise suggests ectopic pregnancy or nonviability, a correlation favored on the exam.

For the maternal triple/quad screen, a low AFP with high hCG suggests Down syndrome risk, while a high AFP suggests a neural tube defect -- pattern recognition rather than single thresholds is the skill being measured.

Vitamin deficiency correlations and tumor markers

Match deficiency to clinical picture: vitamin A deficiency causes night blindness, vitamin D causes rickets and osteomalacia with the low-calcium/high-PTH pattern, vitamin E causes hemolytic anemia and neuropathy, and vitamin K causes a prolonged PT/INR because factors II, VII, IX, and X are not carboxylated.

Among water-soluble vitamins, thiamine (B1) deficiency causes Wernicke-Korsakoff and beriberi, niacin (B3) causes pellagra (dermatitis, diarrhea, dementia), B6 deficiency causes sideroblastic anemia, and B12 versus folate is resolved with methylmalonic acid -- elevated only in B12 deficiency, because B12 is a cofactor in the conversion of methylmalonyl-CoA to succinyl-CoA.

Endocrine-related tumor markers also appear here: calcitonin for medullary thyroid carcinoma, catecholamines/metanephrines for pheochromocytoma, and the broader markers (CEA, CA 19-9, CA 125, AFP, PSA) that the exam expects you to pair with the correct malignancy rather than treat as screening tests in healthy people.

Methods, interferences, and integrated endocrine cases

Most hormones are measured by immunoassay (competitive for small analytes like cortisol and thyroid hormones, sandwich/non-competitive for large ones like TSH). Two interference traps recur: the hook effect, where an extremely high analyte concentration paradoxically gives a low sandwich-assay result unless the sample is diluted, and heterophile antibodies, which cause falsely high or low results that do not dilute linearly. An integrated case the exam favors: a patient with weight gain, cold intolerance, fatigue, a high TSH, low free T4, and positive anti-TPO antibodies is autoimmune (Hashimoto) primary hypothyroidism.

Contrast a patient with weight loss, heat intolerance, tremor, exophthalmos, a suppressed TSH, high free T4, and positive thyroid-stimulating immunoglobulin -- that is Graves disease. Recognizing the full constellation, choosing the confirmatory antibody, and accounting for assay interferences together are the layered skills that distinguish a passing chemistry performance on these endocrine items.