IM12.8-10 | Thyroid Diagnostic Testing — Summary & Reflection

KEY TAKEAWAYS

The diagnostic workup for thyroid disease is a tiered, sequenced process. Investigation sequence: Tier 1 — serum TSH (screen); Tier 2 — FT4 ± FT3 (confirm and characterise); Tier 3 — anti-TPO, TRAb (identify aetiology); Tier 4 — ultrasound ± RAIU/scintigraphy ± FNAC (structural/functional imaging).

TFT patterns:
- Primary overt hypothyroidism: TSH high, FT4 low → treat with levothyroxine
- Subclinical hypothyroidism: TSH 4–10, FT4 normal → treat if TSH >10 or risk factors
- Primary overt hyperthyroidism: TSH suppressed (<0.1), FT4/FT3 high → antithyroid drugs
- T3 thyrotoxicosis: TSH suppressed, FT4 normal, FT3 high → requires FT3 measurement
- Secondary hypothyroidism: TSH low, FT4 low → pituitary disease; treat cortisol first
- Sick euthyroid: TSH variable, FT3 low, acute illness → do NOT treat; recheck after recovery

CBC findings: Macrocytic or normocytic anaemia in hypothyroidism (check B12 for pernicious anaemia association). Mild leukopenia in thyrotoxicosis. Agranulocytosis with carbimazole/PTU — fever + sore throat + mouth ulcers = stop drug, urgent CBC.

ECG patterns:
- AF: absent P waves, irregularly irregular RR — in thyrotoxicosis; rate-control with beta-blockers; anticoagulate (CHA₂DS₂-VASc); defer cardioversion until euthyroid
- Bradycardia + low-voltage complexes + flat T waves: hypothyroidism
- Electrical alternans: pericardial effusion (confirm by echocardiography)

REFLECT

Look back at the three patients from the opening hook. Patient C — the one with AF at 136 bpm and suppressed TSH — needed immediate rate control and anticoagulation assessment before anything else. The TFT confirmed the mechanism; the ECG determined the urgency. Patients A and B needed structured investigation and outpatient management plans, but no emergency intervention. The diagnostic skill that separated these patients was not knowledge of normal ranges — it was the ability to integrate TFT pattern recognition with ECG interpretation and clinical context in real time. Now think about a scenario you might encounter in your clinical posting: an elderly patient admitted for 'falls and confusion' who is found to have a TSH of 24 mIU/L on routine bloods. The admitting team attributes the confusion to her age. What would you do differently? How would the diagnostic sequence — TSH confirmed, FT4 confirmed, CBC checked, ECG reviewed for bradycardia, echocardiography for pericardial effusion — change her diagnosis from 'falls in the elderly' to 'myxoedema complicating hypothyroidism'? The distinction between a routine finding and a medical emergency lies in systematic interpretation, not in the number itself.