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How do 1 write 1st question to score maximum marks Question I for 15 marks
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MODEL ANSWER: Thyroid Hormones (15 Marks)
INTRODUCTION (1 mark)
Thyroid hormones are the principal hormones secreted by the thyroid gland - Thyroxine (T4) and Triiodothyronine (T3). They regulate basal metabolic rate, growth, and development throughout the body.
1. FORMATION / SYNTHESIS (3 marks)
Thyroid hormones are formed in the follicular colloid of the thyroid gland using iodine and thyroglobulin.
Steps of Synthesis:
Step 1 - Iodide Trapping:
- Iodide (I-) is actively transported from blood into thyroid follicular cells by the Sodium-Iodide Symporter (NIS) - this is the iodide pump.
- Normal concentration ratio: 30x blood (can rise to 250x when maximally active).
- Controlled by TSH.
Step 2 - Iodide to Iodine (Oxidation):
- Iodide is transported into the follicular lumen via Pendrin (chloride-iodide counter-transporter).
- Enzyme thyroid peroxidase (TPO) oxidizes I- to active iodine (I0 or I3-).
Step 3 - Thyroglobulin synthesis:
- Thyroid cells synthesize Thyroglobulin (TG) - a large glycoprotein (MW ~335,000) with ~70 tyrosine amino acid residues - via endoplasmic reticulum and Golgi apparatus.
- Secreted into follicular lumen.
Step 4 - Iodination of Tyrosine (Organification):
- Active iodine attaches to tyrosine residues in thyroglobulin:
- 1 iodine + Tyrosine = Monoiodotyrosine (MIT)
- 2 iodines + Tyrosine = Diiodotyrosine (DIT)
Step 5 - Coupling:
- MIT + DIT = T3 (Triiodothyronine)
- DIT + DIT = T4 (Thyroxine)
- Catalyzed by thyroid peroxidase.
- T3 and T4 remain bound to thyroglobulin and stored as colloid in follicles (enough for 2-3 months supply).
2. SECRETION (1 mark)
When TSH stimulates the thyroid:
- Follicular cells engulf thyroglobulin-colloid by endocytosis (pinocytosis).
- Lysosomes fuse with the colloid vesicle - proteases cleave T3 and T4 from thyroglobulin.
- Free T3 and T4 are released into the blood.
- MIT and DIT are deiodinated by deiodinase - iodine is recycled within the cell.
- Daily secretion: ~93% T4 and ~7% T3 (but T3 is 4x more potent).
3. TRANSPORT (1 mark)
In the blood, thyroid hormones are highly protein-bound (>99%):
| Protein | Binds |
|---|---|
| Thyroxine-Binding Globulin (TBG) - main carrier | ~70% of T4 |
| Thyroxine-Binding Prealbumin (TBPA / Transthyretin) | ~10-15% |
| Albumin | small amount |
- Only free (unbound) hormone is biologically active.
- T4 half-life: ~6 days; T3 half-life: ~1 day.
4. METABOLISM (1 mark)
- Most T4 is converted to T3 (active form) by 5'-deiodinase in peripheral tissues (liver, kidney, muscle). This accounts for 80% of circulating T3.
- T4 can also be converted to Reverse T3 (rT3) - an inactive form.
- T3 enters cells and binds to nuclear thyroid hormone receptors (interacting with a retinoid X receptor as heterodimer) on DNA, increasing gene transcription → protein synthesis.
- Excretion: Conjugated in liver with glucuronate/sulfate → excreted in bile.
5. FUNCTIONS (3 marks)
| System | Effects |
|---|---|
| Metabolism | Increases BMR (calorigenic effect), increases O2 consumption, stimulates Na+/K+ ATPase |
| Cardiovascular | Increases HR, cardiac output, blood flow, upregulates beta-adrenergic receptors |
| Growth & Development | Essential for normal bone growth, brain development (critical in fetus and neonate) |
| CNS | Increases alertness, reflexes, and psychomotor activity |
| GIT | Increases gut motility and appetite |
| Carbohydrate | Enhances glucose absorption, glycogenolysis, gluconeogenesis |
| Fat | Increases lipolysis; lowers serum cholesterol and triglycerides |
| Protein | Anabolic at normal levels; catabolic at high levels |
| Temperature | Increases body temperature (thermogenic) |
| Onset/Duration | T4 has latent period of 2-3 days, peak at 10-12 days; T3 acts 4x faster |
6. REGULATION OF SECRETION (3 marks)
Hypothalamo-Pituitary-Thyroid (HPT) Axis:
Hypothalamus
↓ TRH (Thyrotropin-Releasing Hormone)
Anterior Pituitary
↓ TSH (Thyroid Stimulating Hormone)
Thyroid Gland
↓ T3 / T4
(Negative feedback to Hypothalamus & Pituitary)
TRH (Thyrotropin-Releasing Hormone):
- Secreted by hypothalamus → stimulates anterior pituitary to release TSH.
- Stimulated by: cold, stress, low T3/T4.
TSH (Thyroid Stimulating Hormone):
- Acts on thyroid via Gs-protein → increases cAMP → stimulates ALL steps of thyroid hormone synthesis and secretion (trapping, organification, coupling, endocytosis).
- Also causes thyroid hypertrophy and hyperplasia.
Negative Feedback:
- Rising T3/T4 inhibits both TRH (hypothalamus) and TSH (pituitary) - primary regulator.
- T3 is the main inhibitor at pituitary level.
Other regulators:
- Cold exposure - stimulates TRH → TSH → T3/T4 (thermoregulatory mechanism).
- High iodide - Wolff-Chaikoff effect (temporarily inhibits thyroid synthesis).
- Autoregulation - thyroid can adjust iodide trapping based on iodide availability.
7. NOTE ON HYPOTHYROIDISM (2 marks)
Definition: Deficiency of thyroid hormones.
Types:
| Type | Cause |
|---|---|
| Primary | Thyroid gland failure (most common) |
| Secondary | Pituitary failure (low TSH) |
| Tertiary | Hypothalamic failure (low TRH) |
| Congenital | Cretinism |
Common Causes (Primary):
- Hashimoto's thyroiditis (autoimmune - most common worldwide)
- Iodine deficiency (most common globally)
- Post-thyroidectomy / radioiodine therapy
- Drugs (lithium, amiodarone, antithyroid drugs)
Clinical Features:
| Feature | Description |
|---|---|
| Metabolic | Weight gain, cold intolerance, fatigue |
| Skin | Dry skin, coarse hair, loss of outer eyebrow (Queen Anne's sign), myxedema (non-pitting edema due to mucopolysaccharide deposition) |
| CVS | Bradycardia, low cardiac output, pericardial effusion |
| CNS | Slow mentation, memory loss, depression, delayed relaxation of reflexes |
| Reproductive | Menorrhagia, infertility |
| Labs | Low T3/T4; High TSH (in primary); hypercholesterolemia |
Myxedema Coma:
- Severe life-threatening hypothyroidism.
- Features: hypothermia, hypoventilation, altered consciousness, hyponatremia.
- Treatment: IV T3/T4, supportive care, treat precipitants.
Cretinism (Congenital Hypothyroidism):
- Due to maternal iodine deficiency or thyroid agenesis.
- Features: Mental retardation, short stature, deaf-mutism, pot belly, protruding tongue.
- Preventable with neonatal screening and early T4 replacement.
Treatment:
- Oral Levothyroxine (T4) - daily, lifelong.
- Monitor TSH levels to guide dose.
EXAM WRITING TIPS FOR MAX MARKS:
- Start with a definition/introduction - examiners appreciate structured opening.
- Use numbered steps for synthesis - shows clarity and earns step marks.
- Draw the diagram: Draw the HPT axis feedback loop - this alone can earn 1-2 marks.
- Use a table for functions and for hypothyroidism features - organized, easy to read.
- Bold key terms like TSH, TRH, T3, T4, thyroid peroxidase, thyroglobulin.
- Mention clinical correlations: Wolff-Chaikoff effect, myxedema coma, cretinism - shows higher-order understanding.
- End with treatment for hypothyroidism - this section is often a mark-grabber.
Content based on Guyton and Hall Textbook of Medical Physiology, Goldman-Cecil Medicine, and Costanzo Physiology.
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