Thyroid Disorders
Thyroid physiology and congenital hypothyroidism
Thyroid hormone regulates growth and neurodevelopment. Newborn screening uses TSH or T4. Primary congenital hypothyroidism is most often thyroid dysgenesis, followed by dyshormonogenesis. Transient causes include maternal antithyroid antibodies, iodine excess or deficiency, and maternal antithyroid drugs. Start levothyroxine immediately when screening is abnormal after confirmatory testing, because delayed treatment impairs cognition. In central hypothyroidism, free T4 is low with low or normal TSH; screening TSH alone can miss it. In sick euthyroid syndrome, TSH and T4 may be abnormal transiently, so repeat when the child is well.
Acquired disease
Hashimoto thyroiditis causes goiter and hypothyroidism with antithyroid peroxidase antibodies. Graves disease causes hyperthyroidism, goiter, exophthalmos, and tachycardia; neonatal Graves occurs from transplacental TSH-receptor antibodies and resolves over months. Thyroid nodules are uncommon but more often malignant in children; medullary thyroid carcinoma is associated with MEN2 and RET mutations. Trap: biotin supplements interfere with immunoassays, and free T4 immunoassays can be unreliable in severe illness.
Worked example
A 2-week-old has screening TSH 45 mU/L and low free T4. Repeat testing shows the same. Start levothyroxine and recheck TSH and free T4 in 4–6 weeks; do not wait for a scan. If TSH is high but free T4 is normal on repeat, consider transient TSH elevation and retest rather than treating indefinitely. Monitor growth and development.