Thyroid disorders: hypothyroidism vs hyperthyroidism, TFT interpretation and treatment
RezzyThyroid disorders are a bread-and-butter topic for the UKMLA AKT, and getting a handle on Thyroid Function Test (TFT) interpretation is the absolute key to scoring well here. Think of the pituitary and the thyroid like a thermostat and a heater—they're constantly talking to each other to keep the body's "temperature" (metabolism) just right.
Let's start by breaking down the clinical pictures of hypothyroidism and hyperthyroidism, as the exam loves to give you a "spot diagnosis" based on symptoms.
Clinical Comparison: Hypo vs. Hyper
| Feature | Hypothyroidism (Slow & Low) | Hyperthyroidism (Fast & High) |
|---|---|---|
| Metabolism | Weight gain, cold intolerance | Weight loss, heat intolerance |
| Skin/Hair | Dry skin, thinning hair (outer 1/3 of eyebrow) | Sweating, pretibial myxoedema (Graves') |
| Heart | Bradycardia | Tachycardia, Palpitations, AF |
| Bowels | Constipation | Diarrhoea/Frequent BM |
| Neuro | Slowed reflexes, "mental fog", depression | Anxiety, tremor, brisk reflexes |
I've got a great comparison table that goes into a bit more detail on these clinical features to help you differentiate them in a clinical vignette.
| System | Hypothyroidism | Hyperthyroidism |
|---|---|---|
| General | Fatigue, weight gain, cold intolerance | Weight loss, heat intolerance, anxiety |
| Cardiac | Bradycardia, hypertension (diastolic) | Tachycardia, palpitations, Atrial Fibrillation |
| GI | Constipation | Increased frequency, diarrhoea |
| Skin/Hair | Dry, cool skin; thinning hair; brittle nails | Warm, moist skin; fine hair; palmar erythema |
| Neuromuscular | Delayed tendon reflexes, carpal tunnel | Brisk reflexes, fine tremor, proximal myopathy |
| Menstrual | Menorrhagia | Oligomenorrhea or amenorrhea |
TFT Interpretation: The "Thermostat" Logic
When interpreting TFTs, always look at the TSH first. It’s the most sensitive marker.
- Primary Hypothyroidism: High TSH, Low T4 (The pituitary is screaming at a thyroid that isn't listening).
- Primary Hyperthyroidism: Low TSH, High T4/T3 (The thyroid is overproducing, so the pituitary shuts down).
- Subclinical Hypothyroidism: High TSH, Normal T4.
- Subclinical Hyperthyroidism: Low TSH, Normal T4/T3.
High-Yield Tip: If you see a Low TSH and Low T4, think Secondary (Central) Hypothyroidism—this is a pituitary problem, not a thyroid one!
Management Essentials (NICE Guidelines)
1. Hypothyroidism
- First-line: Levothyroxine (T4).
- Monitoring: Check TSH every 6–12 weeks after dose changes until stable, then annually.
- Pregnancy: Increase dose by ~25-50% as soon as pregnancy is confirmed because the demand for T4 goes up.
2. Hyperthyroidism (e.g., Graves' Disease)
- First-line: Carbimazole (titration or block-and-replace).
- Symptom control: Beta-blockers (Propranolol) are great for the "fast" symptoms like tremors and tachycardia.
- Definitive: Radioiodine or Surgery.
- Warning: Patients on Carbimazole must be warned about agranulocytosis. If they get a sore throat or fever, they need an urgent FBC!
To help you visualize the diagnostic pathway, let me pull up a flowchart for TFT interpretation.
That flowchart covers the basics of how to approach these cases in the exam.
Would you like to try some practice questions on this topic to see how it's tested in the UKMLA, or should I pull up some flashcards for memorising the drug side effects and monitoring?
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