Overview
A hormone is a chemical messenger that transmits a signal from one cell to another, altering target-cell growth, metabolism or function upon binding. Hormones are classified by the distance over which they act β autocrine, paracrine and endocrine β and by their chemical nature β steroids, amino acid derivatives and polypeptides.
This lesson explores the synthesis, regulation and functional importance of key hormones β insulin, glucagon, thyroid hormones, parathyroid hormone and growth hormone β and the diseases that result from their excess or deficiency, all of which are frequently assessed in the clinical laboratory.
Learning Objectives
After this lesson you will be able toβ¦- Classify hormones by site of action (autocrine, paracrine, endocrine) and chemical nature.
- Describe insulin and glucagon synthesis, secretion and their opposing metabolic effects.
- Explain thyroid hormone synthesis, transport and the disorders of excess and deficiency.
- Describe the regulation and action of parathyroid hormone on bone, kidney and intestine.
- Describe growth hormone regulation, effects and the clinical consequences of excess/deficiency.
Clinical Story
Why This MattersA 16-year-old boy presents with excessive thirst, frequent urination and unexplained weight loss over three weeks. The physician suspects new-onset diabetes and orders fasting blood glucose along with HbA1c. The laboratory technologist's understanding of insulin's role in glucose regulation is essential to interpreting these results and flagging a critical value for urgent clinical review.
Core Concepts
Hormones are classified by distance of action: autocrine (acts on the cell that released it), paracrine (acts on nearby cells, e.g. interleukin-1 released by white blood cells acting on T cells), and endocrine (released into the bloodstream to act on distant cells, e.g. insulin acting on liver and muscle).
By chemical structure, hormones fall into three classes: steroids (cholesterol derivatives), amino acid derivatives (e.g. thyroid hormones, catecholamines), and polypeptides (chains of amino acids, e.g. insulin, glucagon, growth hormone). Second messengers such as cAMP mediate the intracellular effects of many surface-receptor hormones; atrial natriuretic factor is unique in using cGMP.
Insulin is a polypeptide hormone synthesised by pancreatic Ξ²-cells as proinsulin; enzymatic excision of C-peptide yields active insulin, which is co-released with C-peptide via exocytosis. When blood glucose rises, GLUT2 transporters carry glucose into Ξ²-cells, glucokinase (hexokinase IV) phosphorylates it, ATP rises, KβΊ channels close, the membrane depolarises, voltage-gated CaΒ²βΊ channels open, and insulin is released.
Insulin stimulates glucose uptake (via GLUT4 in muscle/adipose), glycogen synthesis, and fat/protein synthesis, while inhibiting their breakdown. Deficiency or resistance causes diabetes mellitus β Type I (IDDM, absolute deficiency, early onset) and Type II (NIDDM, insulin resistance, typically older/obese). Classic symptoms are polyuria, polydipsia and glucosuria.
Glucagon is secreted by pancreatic Ξ±-cells in response to hypoglycemia, epinephrine, arginine, alanine, acetylcholine and cholecystokinin; it is inhibited by somatostatin, insulin and free fatty acids. Glucagon raises blood glucose by stimulating hepatic glycogenolysis and gluconeogenesis, and mobilises free fatty acids from adipose tissue, sparing glucose for the brain. Insulin and glucagon together form a feedback system maintaining stable blood glucose.
T4 (thyroxine) and T3 (triiodothyronine) are tyrosine-based hormones produced by thyroid follicular cells and regulated by pituitary TSH. Iodide is actively trapped and bound to tyrosine residues on thyroglobulin (via thyroperoxidase) to form MIT and DIT, which combine to form T3 and T4. Iodine deficiency causes goitre.
Thyroid hormones increase basal metabolic rate, regulate protein/fat/carbohydrate metabolism, support growth and neural maturation, and increase catecholamine sensitivity. Most circulating hormone is bound to Thyroxine-Binding Globulin (TBG); only the free fraction is biologically active. Hyperthyroidism (e.g. Graves' disease) results from excess free hormone; hypothyroidism (e.g. Hashimoto's thyroiditis) results from deficiency.
PTH, secreted by the chief cells of the parathyroid glands, raises blood calcium by enhancing bone resorption (via osteoclast stimulation), increasing renal reabsorption of calcium (while decreasing phosphate reabsorption), and stimulating renal activation of Vitamin D, which increases intestinal calcium absorption. Secretion is controlled by negative feedback from serum calcium.
Primary hyperparathyroidism is due to autonomous PTH hypersecretion, while secondary hyperparathyroidism is an appropriate response to hypocalcemia. Hypoparathyroidism most commonly follows accidental damage during thyroid surgery.
Growth hormone (GH/somatotropin) is secreted by somatotrophs in the anterior pituitary, regulated by hypothalamic GHRH (stimulatory) and somatostatin (inhibitory). GH increases amino acid transport, protein synthesis, lipolysis, and calcium retention; it opposes insulin by reducing glucose uptake and promoting gluconeogenesis.
Excess GH (usually from a somatotroph adenoma) causes gigantism in children and acromegaly in adults. Deficiency causes growth failure and short stature (dwarfism) in children, and reduced muscle mass/energy in adults.
Laboratory Principle
Hormones circulate at very low concentrations (pmolβnmol/L range), so they are measured by immunoassay techniques such as chemiluminescent immunoassay (CLIA), electrochemiluminescence immunoassay (ECLIA) or ELISA. These methods use labelled antibodies that specifically bind the hormone of interest; the amount of signal generated (light, colour, or electrochemical current) is proportional to hormone concentration, allowing precise quantification even at trace levels.
Equipment Required
Reagents & Materials
| Reagent / Material | Concentration / Grade | Purpose | Storage |
|---|---|---|---|
| Anti-insulin / anti-TSH antibody-coated microparticles | Manufacturer-specific | Immunoassay capture antibody | 2β8 Β°C |
| Chemiluminescent substrate | Ready-to-use | Signal generation | 2β8 Β°C, protect from light |
| Calibrators (multi-level) | Traceable to WHO reference standard | Assay calibration | 2β8 Β°C or frozen per insert |
| Glucose oxidase reagent | Working reagent | Blood glucose estimation | 2β8 Β°C |
Step-by-Step Procedure
Collect a fasting venous blood sample in an appropriate tube (serum for most hormone assays; fluoride-oxalate for glucose) as clinically indicated.
Centrifuge promptly and separate serum/plasma; freeze if the assay is not run immediately, especially for labile hormones like PTH.
Load the sample onto the immunoassay analyzer along with antibody-coated reagent; incubate per the assay protocol to allow antigen-antibody binding.
Wash unbound material, add substrate, and measure the chemiluminescent or colorimetric signal generated.
The analyzer calculates hormone concentration from a stored calibration curve; verify against QC before releasing the report.
Flow Diagram
Quality Control
Run two-level immunoassay controls with every analytical run; monitor with Levey-Jennings charts and Westgard multi-rules. Repeat calibration when a new reagent lot is introduced.
Enrol in an EQAS programme for endocrinology/hormone assays and review peer-group performance reports each cycle to detect calibration drift.
Reference Values
Normal Rangesβ οΈ Reference ranges may vary between laboratories and assay platforms. Always apply your laboratory's established reference intervals.
Clinical Interpretation
| Finding | Possible Significance | Action / Follow-up |
|---|---|---|
| Fasting glucose > 126 mg/dL | Diabetes mellitus | Confirm with repeat testing/HbA1c; refer clinically |
| Low TSH, high free T4 | Hyperthyroidism (e.g. Graves' disease) | Correlate with clinical signs; consider thyroid antibody panel |
| High TSH, low free T4 | Primary hypothyroidism (e.g. Hashimoto's thyroiditis) | Confirm with repeat testing; assess anti-TPO antibodies |
| Low calcium, low PTH | Hypoparathyroidism | Urgent clinical correlation, especially post-thyroid surgery |
Common Errors & How to Avoid Them
Cause: Recent food intake elevates glucose and insulin, invalidating fasting reference ranges.
Prevention: Confirm 8β12 hour fasting status before collection.
Cause: PTH and some hormones degrade at room temperature.
Prevention: Process and freeze promptly per assay-specific instructions.
Cause: Extremely high hormone concentrations can saturate antibodies and cause falsely low readings.
Prevention: Dilute and re-test samples when results are discordant with clinical picture.
Laboratory Tips from the Bench
Always check TSH first when screening for thyroid dysfunction β it is the most sensitive single test for primary thyroid disease.
C-peptide is co-secreted with insulin in equal amounts and can help distinguish endogenous insulin production from exogenous insulin administration.
Remember "Insulin In, Glucagon Gone" β insulin drives glucose INto cells, glucagon makes glucose GONE from storage (glycogen) into blood.
Important Notes
Most circulating thyroid hormone is protein-bound and inactive; only the free fraction is biologically active and clinically meaningful, so free T4/T3 assays are preferred over total hormone measurement.
Most peptide hormones act via surface receptors and intracellular second messengers like cAMP; steroid and thyroid hormones instead diffuse into cells and act directly on nuclear receptors.
Interactive Quiz
Test Your KnowledgeFlashcards
Tap to flipClick or tap any card to reveal the answer.
Clinical Case Study
Apply Your KnowledgeThree-week history of excessive thirst, frequent urination and 4 kg unintentional weight loss, with no prior medical history.
Markedly elevated fasting glucose and HbA1c with glucosuria in a young patient is consistent with new-onset Type I (insulin-dependent) diabetes mellitus due to absolute insulin deficiency.
- βPolyuria, polydipsia and weight loss are classic hyperglycemia symptoms.
- βHbA1c reflects average glycemic control over 2β3 months.
- βKetones should always be checked to rule out diabetic ketoacidosis.
Frequently Asked Questions
TSH is far more sensitive to small changes in thyroid function due to the log-linear feedback relationship between TSH and free T4, making it the most sensitive first-line test.
Type I diabetes results from absolute insulin deficiency due to Ξ²-cell destruction and requires insulin therapy. Type II diabetes involves insulin resistance with relative insulin deficiency, typically in older, obese individuals.
The parathyroid glands sit close to the thyroid and can be accidentally removed or devascularised during thyroidectomy, causing sudden PTH deficiency and hypocalcemia.
Quick Revision
10-Minute ReviewKey Takeaways
- Hormones are classified by distance of action and by chemical structure.
- Insulin and glucagon maintain glucose homeostasis through opposing actions.
- Thyroid hormones regulate metabolic rate and are controlled by the pituitary TSH feedback loop.
- PTH is the primary regulator of blood calcium, acting on bone, kidney and intestine.
- Growth hormone excess and deficiency cause distinct clinical syndromes depending on age of onset.
- Immunoassay techniques allow precise measurement of hormones present at very low concentrations.
Competency Checklist
Track Your MasteryReferences
- Vasudevan DM, Sreekumari S, Vaidyanathan K. Textbook of Biochemistry for Medical Students. 8th ed.
- Lehninger AL, Nelson DL, Cox MM. Principles of Biochemistry. 7th ed.
- NIOS Medical Laboratory Technology curriculum β Biochemistry Module, Lesson 12: Hormones.