<prompt> 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:KaryoptypingT4, TSH assayRectal mucosal biopsyKnee X-ray  <response>: - 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 muL Central secondary hypothyroidism - low TSH levels TSH T4 Thyroid dyshormonogenesis ELEVATED LOW Thyroid dysgenesis ELEVATED LOW Central hypothyroidism LOW LOW TREATMENT - Oral Levothyroxineearly morning with empty stomach 