The most sensitive and specific test to confirm primary hypothyroidism in the newborn is which finding?

• A. Depressed TSH
• B. Elevated TSH
• C. Elevated T4 and FT4
• D. Normal TSH

Answer: (B)

Primary hypothyroidism in a newborn means the thyroid gland isn’t making enough hormone, so the pituitary responds by ramping up TSH to try to stimulate the thyroid. That compensatory rise in TSH is why an elevated TSH is the most sensitive and specific indicator for confirming primary hypothyroidism in the neonate. A depressed TSH would point to a central problem, not primary thyroid failure; elevated T4 or FT4 would argue against hypothyroidism; and a normal TSH would not explain the hormone deficiency. In practice, this pattern is confirmed by high TSH with low free T4.