Why Is Thyroid Screening Essential in Newborns?

A newborn shows muscle weakness and is not as engaged in everyday baby activities as peers. As time passes, differences in intellectual development also become apparent. These symptoms could be signs of insufficient thyroid hormone production, an easily detectable and preventable reason for intellectual disability.

Thyroid hormone is essential in the development of several organs, most importantly the brain. If not detected and treated promptly, congenital hypothyroidism may lead to severe and irreversible neurocognitive impairment and intellectual disability. About 1 in 2,000 to 4,000 newborns is born with insufficient thyroid hormone production. Before the application of hormone screening in newborns, congenital hypothyroidism was one of the most common reasons for preventable intellectual disability.

What is congenital hypothyroidism?

Congenital hypothyroidism is a condition when the newborn can’t produce enough thyroid hormones. Thyroid hormones play a major role in several metabolic reactions that provide the body with energy. In addition, they are essential in brain development and function.

During pregnancy, the fetus receives the required thyroid hormone from the mother. Therefore, the lack of fetal hormonal production is mostly not noticeable until birth.

Congenital hypothyroidism may occur for several reasons:

• Defective thyroid gland development. In 80% of cases, the thyroid gland might be absent (agenesis), developed minimally (hypoplasia), or could be located somewhere other than the usual spot on the neck area (ectopy). Development defects are mostly not genetically inherited.
• Impairment in hormone synthesis. Defects in any step of hormone synthesis and secretion may result in a deficiency of hormones. This condition is mainly inherited genetically.
• Central hypothyroidism. Defects in the thyroid-stimulating hormone (TSH) production from the pituitary gland will lead to hypothyroidism.
• Resistance to TSH hormone. TSH receptors located on the thyroid gland and responding to the stimulatory activity might be altered, thus, becoming unresponsive to the brain stimulus to produce hormones.
• Transient congenital hypothyroidism. In iodine-deficient regions, the most common reason for temporary low thyroid hormone levels is the lack of iodine required for hormone synthesis. In iodine-sufficient areas, other causes, such as antithyroid drug use during pregnancy, maternal autoimmune thyroid diseases with thyroid-blocking antibodies, and high-level iodine exposure, which shuts down the gland activity in high doses, might cause temporary low thyroid hormone levels.

What are the symptoms of hypothyroidism?

Ninety-five percent of newborns do not show symptoms at birth due to the maternal hormones passed to the baby, and although reduced, secreted hormones delay the appearance of symptoms. When present at birth, symptoms include increased birth weight and head circumference.

In the later months of life, several symptoms appear gradually:

Congenital hypothyroidism may present with abnormalities in other organs, such as the heart, urinary organs, digestive organs, and skeleton.

Most significant symptoms in untreated cases are related to delayed intellectual and neurological development. Untreated newborns and those with delayed treatment initiation and inadequate medication adherence develop intellectual functional limitations and hearing and attention deficits.

Importance of newborn screening

Hypothyroidism symptoms might develop slowly and easily could be missed by parents, especially in families without previous experience. Time until diagnosis might be definitive for the severity of neurological development deficits. Therefore, mandatory newborn screening is implemented in most countries worldwide.

For newborn screening, blood samples from a heel prick are collected between 2 and 4 days after birth. With this method, thyroxine hormone, thyroid-stimulating hormone (TSH), or both are measured.

Initially measured hormones:

Some preterm newborns experience a delayed elevation in TSH level. Therefore, American and European guidelines recommend a second measurement in preterm, low-birth-weight babies.

Is screening enough?

As the name suggests, screening is merely to catch suspected findings of some pathologies. If there is a suspicion of hypothyroidism, further tests might be required to confirm the diagnosis.

• Thyroid autoantibodies. This analysis helps diagnose transient congenital hypothyroidism that occurred due to maternal antibodies.
• Serum thyroglobulin. Low level or absence may indicate agenesis of the thyroid gland.
• Urine iodine measurement. This method might help diagnose low and high levels of iodine.
• Thyroid ultrasound. Imaging methods might give an understanding on the structure of the gland.
• Knee X-ray imaging. In the late stages of pregnancy, bones on the knee level gain more bone material due to thyroid hormone. Hence, knee radiography shows fewer bone structures (ossification centers) in congenital hypothyroidism cases, which could diagnose diseases.
• Genetic testing. Used when genetic inheritance or defects in the specific gene is suspected.

Thyroid hormones are essential in neonatal growth, and missed cases of hypothyroidism might result in intellectual and cognitive developmental delays. A simple screening test might have a life-changing impact on the newborn's life, and a timely treatment initiation may prevent neurological consequences.