Algorithm for screening and diagnosis of congenital hypothyroidism^[1,2]

T4: thyroxine; TSH: thyroid-stimulating hormone (thyrotropin); T3: triiodothyronine.

* Filter paper blood spot TSH cutoffs are expressed as serum values. To get equivalent whole blood values, divide the serum value by 2. Screening programs in North America typically report serum values, whereas screening programs in Europe often report whole blood values.

¶ For newborn screening programs that employ primary T4 screen, samples with T4 values below a certain concentration (usually less than the 10^th percentile for the laboratory's samples on the same day) undergo a TSH measurement on the same filter paper blood spot.

Δ Prematurity or nonthyroidal illness may cause false-positive results on serum testing and/or the initial screen, with low T4 and normal TSH.

◊ This result usually indicates that the screen was a false positive, especially if the screening TSH was only mildly elevated. However, a study suggests that infants with these findings are at increased risk for developing subclinical hypothyroidism later in life^[3].

§ Rarely, infants with low TSH and low free T4 are ultimately proven to have primary hypothyroidism (rather than central hypothyroidism) because the expected rise in serum TSH is delayed.