There are two newborn screening tests performed in blood to detect hypothyroidism: thyroid stimulating hormone (TSH) and thyroxine (T4). When the thyroid gland is defective, known as Primary CH, TSH values are elevated and T4 values usually are low, although T4 values may be within the normal range in mild CH. Primary CH accounts for > 95% of cases. When there is defective hypothalamic or pituitary regulation of the thyroid gland, known as Central CH, T4 values are low; the TSH values may be low, normal, or mildly elevated.
Serum confirmatory tests usually are limited to 2 tests: TSH and free thyroxine, or FT4 (the small fraction of T4 that is not bound to serum proteins and represents the more biologically relevant measurement). An elevated serum TSH test is diagnostic of primary CH, the most common form of CH. A low serum FT4 with a normal or low TSH is diagnostic of central CH. Infants with central CH should have other tests to evaluate hypothalamic-pituitary function since ACTH/cortisol, growth hormone and/or gonadotropin deficiencies are associated with hypoglycemia (typically within hours of birth), and in male neonates, hypogonadism (micropenis, small testicles, cryptorchidism).
An image of the thyroid gland is often obtained by thyroid ultrasound and/or a radionuclide thyroid scan. These tests determine whether it is enlarged (goiter), absent (athyreosis), small (hypoplasia), or malformed and not in the normal location in the neck (ectopia). In these situations, the infant most often has a permanent form of primary CH. The thyroid gland may be normal in size and in the normal location in the neck (eutopic) on ultrasound, especially in familial causes of CH and transient CH.