How Is Hypothyroidism Diagnosed?

In the United States, Canada, and much of the Western world, newborns are routinely screened for thyroid hormone deficiency. Infants with abnormal screening tests receive follow-up evaluation for hypothyroidism. Such testing commonly leads to the correct diagnosis within the first four weeks of age, and treatment can begin immediately.

Diagnosis later in childhood is usually based on information from blood tests, which check levels of thyroid hormones T4, T3, and TSH, among other related substances. Abnormally low levels of T4 and T3 indicate hypothyroidism is present.

• If TSH is present at a higher-than-normal level, the abnormality is within the thyroid gland. It is not responding properly to TSH.

• If TSH is low, the abnormality is within the brain or pituitary gland. The pituitary is not releasing TSH despite levels of thyroid hormone low enough that it should do so.

Among babies with hypothyroidism, roughly 95% or more of cases represent problems in the thyroid gland. In less than 5% of cases, the abnormality is found in the brain or in the pituitary gland, the small gland at the base of the brain, almost always the pituitary gland.

This is the same for hypothyroidism that develops in older children and young adults, although the exact causes of hypothyroidism are different for the different age groups.