Organ function test and hormones for biochemistry PCT
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I now have comprehensive content from Harper's Biochemistry on organ function tests, and from Tietz on hormones classification. I have enough to write a thorough, well-organized answer for a biochemistry PCT (Practical/Course Topic). Let me compile the full response.
Biochemistry PCT: Organ Function Tests & Hormones
(Sources: Harper's Illustrated Biochemistry 32nd Ed; Tietz Textbook of Laboratory Medicine 7th Ed)
PART 1 - ORGAN FUNCTION TESTS
Tests grouped by the organ they assess are called organ function tests. The major panels covered in biochemistry PCT are kidney, liver, thyroid, and adrenal function tests.
1. Tests of Kidney Function
Urinalysis (Routine)
A complete urinalysis checks:
- Physical: volume (24-hr timed), colour, odour, appearance (clear/turbid), specific gravity, pH
- Chemical: protein, glucose, blood, ketone bodies, bile salts, bile pigments
Serum Urea and Creatinine
| Marker | Notes |
|---|---|
| Serum urea | Increases as renal function declines; affected by non-renal factors (diet, catabolism) |
| Serum creatinine | More specific - not significantly affected by non-renal factors; only rises when GFR falls ~50% (poor sensitivity for early disease) |
Proteinuria
- Normal: <150 mg protein / 24 hr; <30 mg albumin / 24 hr (below routine detection)
- Proteinuria = >150 mg protein / 24 hr; sign of renal disease
- Main cause: loss of glomerular basement membrane integrity → glomerular proteinuria (albumin is the predominant protein)
- Microalbuminuria: 30-300 mg albumin / 24 hr - early marker of renal damage in diabetes mellitus
Creatinine Clearance (GFR Estimate)
Clearance (mL/min) = (U × V) / P
- U = urinary concentration of analyte (timed sample)
- P = plasma concentration
- V = urine volume per minute (24-hr volume ÷ 1440 min)
Creatinine clearance slightly overestimates GFR because creatinine is also secreted by the renal tubules.
Inulin Clearance is the gold standard: freely filtered, not reabsorbed or secreted, constant blood level - but requires IV infusion (exogenous).
2. Liver Function Tests (LFTs)
LFTs assess diagnosis, prognosis, and therapy monitoring of liver disease.
| Test | What it Assesses | Clinical Interpretation |
|---|---|---|
| Serum bilirubin | Bilirubin metabolism | Obstructive jaundice: mainly conjugated ↑; Hepatocellular disease: both conjugated + unconjugated ↑ |
| Total protein & albumin | Synthetic function | Low in chronic liver disease (cirrhosis) |
| Prothrombin time (PT) | Coagulation factor synthesis | Prolonged in acute liver disorders (impaired coagulation factor synthesis) |
| ALT (alanine aminotransferase) | Hepatocyte damage | More specific for liver disease; elevated significantly in acute viral hepatitis even before jaundice |
| AST (aspartate aminotransferase) | Hepatocyte damage | Also elevated in cardiac/skeletal muscle injury - less specific |
| Alkaline phosphatase (ALP) | Biliary obstruction / bone | Elevated in obstructive jaundice AND bone disease |
Key rule: ALT > AST elevation = liver disease; AST elevation can be cardiac/skeletal muscle.
3. Thyroid Function Tests
The thyroid secretes T4 (thyroxine / tetraiodothyronine) and T3 (triiodothyronine).
| Test | Purpose |
|---|---|
| Serum TSH (thyrotropin) | Best first-line screening test for thyroid dysfunction |
| Free T4 (fT4) | Confirms hypo- or hyperthyroidism |
| Free T3 (fT3) | Measured when T3 thyrotoxicosis suspected |
- Total T4 is rarely measured now because thyroid-binding globulin (TBG) changes alter it even without true thyroid disease
- Free hormone assays are preferred
| Condition | TSH | fT4 |
|---|---|---|
| Hypothyroidism | ↑ | ↓ |
| Hyperthyroidism | ↓ | ↑ |
| Secondary hypothyroidism | ↓ | ↓ |
4. Adrenal Function Tests
Tests for Cushing syndrome (hyperfunction) and Addison disease (hypofunction).
- Cortisol secretion shows diurnal variation: highest early morning (~8 AM), lowest at midnight
- Loss of diurnal variation = earliest sign of adrenal hyperfunction
| Test | Method | Interpretation |
|---|---|---|
| Diurnal cortisol | Serum at midnight + 8 AM | Loss of variation → Cushing syndrome |
| Dexamethasone suppression test (DST) | 1 mg dexamethasone given at midnight; measure cortisol at 8 AM | Failure to suppress cortisol = Cushing syndrome confirmed |
5. Markers of Cardiovascular Risk / Myocardial Infarction
| Marker | Purpose |
|---|---|
| Total cholesterol | Lipid risk assessment |
| LDL:HDL ratio | Best predictor of atherosclerosis risk |
| Cardiac troponin | Gold standard for MI confirmation |
| CK-MB isoenzyme | Cardiac-specific - confirms MI when ECG is inconclusive |
PART 2 - HORMONES (Biochemistry Classification)
Classification by Chemical Structure
| Class | Examples | Solubility | Transport | Half-life | Receptor Location |
|---|---|---|---|---|---|
| Polypeptide/Protein | ACTH, Insulin, PTH, GH | Water-soluble | Free in plasma | Short (10-30 min) | Cell surface membrane receptors |
| Amino acid derivatives | Thyroxine (T4, T3), Epinephrine, Norepinephrine | Water-soluble | Bound to transport proteins (T4) or free (catecholamines) | T4: 7-10 days; Catecholamines: <1 min | Nuclear receptors (thyroid); G-protein coupled receptors (catecholamines) |
| Steroids / Lipid-derived | Cortisol, Estrogen, Testosterone, Aldosterone | Hydrophobic, water-insoluble | Bound to specific carrier proteins (CBG, SHBG) | 30-90 min | Intracellular nuclear receptors (cytoplasm/nucleus) |
Frequently Measured Hormones by Endocrine Organ
| Gland | Hormone | Type | Principal Action |
|---|---|---|---|
| Hypothalamus | TRH | Peptide (3aa) | Releases TSH and Prolactin from anterior pituitary |
| GnRH | Peptide (10aa) | Releases LH and FSH | |
| CRH | Peptide (41aa) | Releases ACTH and β-LPH | |
| GHRH | Peptide (40/44aa) | Releases growth hormone | |
| Somatostatin | Peptide | Inhibits GH, TSH, insulin, glucagon release | |
| Anterior pituitary | TSH | Glycoprotein | Stimulates thyroid hormone synthesis/release |
| LH, FSH | Glycoproteins | Gonadal function, reproduction | |
| ACTH | Polypeptide | Stimulates adrenal cortex → cortisol | |
| GH | Protein | Growth, IGF-1 release | |
| Prolactin | Protein | Lactation | |
| Adrenal cortex | Cortisol | Steroid | Gluconeogenesis, anti-inflammatory |
| Aldosterone | Steroid | Na⁺ retention, K⁺ excretion | |
| Androgens (DHEA) | Steroid | Secondary sexual characteristics | |
| Adrenal medulla | Epinephrine, Norepinephrine | Catecholamines (tyrosine derivatives) | Fight-or-flight response |
| Thyroid | T4, T3 | Tyrosine derivatives | Metabolic rate, growth |
| Pancreas | Insulin | Polypeptide | Glucose uptake; lowers blood glucose |
| Glucagon | Polypeptide | Raises blood glucose; promotes glycogenolysis | |
| Gonads | Estradiol, Testosterone | Steroids | Reproduction, secondary sex characteristics |
| Parathyroid | PTH | Polypeptide | Raises serum Ca²⁺; activates vitamin D |
Physiologic Functions of Hormones (3 Categories)
- Growth, development, and maturation - GH, IGF-1, thyroxine, gonadal steroids, cortisol
- Systemic homeostasis, energy balance, integrated metabolism - insulin, glucagon, cortisol, aldosterone
- Regulation of reproduction - LH, FSH, GnRH, estrogen, testosterone, prolactin
Hormone Action Mechanisms
| Receptor Type | Hormone Class | Signal Transduction |
|---|---|---|
| Cell surface G-protein coupled receptors (GPCR) | Catecholamines, Glucagon | cAMP → PKA cascade |
| Cell surface tyrosine kinase receptors | Insulin, IGF-1 | PI3K/Akt, MAPK pathways |
| Intracellular nuclear receptors (NR) | Steroids, Thyroid hormones, Vitamin D | Direct gene transcription regulation |
Sources:
- Harper's Illustrated Biochemistry, 32nd Ed - Organ Function Tests (p. 586)
- Tietz Textbook of Laboratory Medicine, 7th Ed - Classification of Hormones (Ch. 38)
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Organ Function Tests Study Guide
Word Document · DOCX
What's in the document (8 sections):
| Section | Content |
|---|---|
| Cover Page | Title, topic overview, source attribution |
| 1. Kidney Function Tests | Urinalysis (physical + chemical), serum urea vs creatinine, proteinuria + microalbuminuria, clearance formula, inulin vs creatinine |
| 2. Liver Function Tests | Full LFT panel table, bilirubin fractionation by jaundice type, ALT vs AST comparison, ALP, albumin and PT as synthetic markers |
| 3. Thyroid Function Tests | T3/T4/TSH overview, test panel, interpretation table for all thyroid conditions |
| 4. Adrenal Function Tests | Cortisol diurnal variation, overnight dexamethasone suppression test (step-by-step), Addison vs Cushing comparison |
| 5. Cardiac Markers | Lipid risk profile, MI biomarkers (troponin, CK-MB, timing) |
| 6. Quick Review Table | All 5 organs - key tests, markers, and must-know facts in one table |
| 7. MCQ Practice | 6 exam-style questions with colour-coded answers |
| 8. Formulae + Reference Values | Creatinine clearance formula, normal ranges for all key tests |
The document uses colour-coded headings, highlighted exam tips (orange banners), shaded key points, and striped tables throughout for easy visual scanning.
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