A 6-month-old infant is brought with a history of constipation and excessive sleepiness. On examination, he is lethargic, has periorbital puffiness, large tongue and umbilical hernia. The investigation which will help to diagnose this condition is:

- History given in question shows typical presentation of hypothyroidism, so thyroid hormone assay is required. DIAGNOSIS OF HYPOTHYROIDISM: T4 levels - low Primary hypothyroidism - TSH usually > 100 mu/L Central / secondary hypothyroidism - low TSH levels TSH T4 Thyroid dyshormonogenesis ELEVATED LOW Thyroid dysgenesis ELEVATED LOW Central hypothyroidism LOW LOW TREATMENT - Oral Levothyroxine(early morning with empty stomach)