Which investigation is considered the initial screening for congenital hypothyroidism?

The initial screening for congenital hypothyroidism involves measuring serum thyroid-stimulating hormone (TSH) and thyroxine (T4) levels. This is a critical step because congenital hypothyroidism is often detected in newborns through newborn screening programs, which typically assess levels of TSH and T4 from a blood sample.

When TSH levels are elevated, it indicates that the pituitary gland is increasing its production of TSH in response to low levels of circulating T4, suggesting that the thyroid is underactive. This physiological response is what makes the TSH measurement particularly informative in diagnosing congenital hypothyroidism. Following this initial screening, if abnormal results are found, further confirmatory testing is conducted to establish the diagnosis.

Other options like radioisotope scans, thyroid ultrasounds, and urinalysis are not suitable for initial screening. A radioisotope scan is more of a diagnostic test used to visualize the function of the thyroid gland, while a thyroid ultrasound provides structural information about the gland but does not give functional status or hormone levels. Urinalysis does not provide any helpful information regarding thyroid function. Therefore, assessing serum TSH and T4 levels is the most appropriate and effective method for the initial diagnosis of congenital hypothyroidism.