Division of Chemistry and Toxicology
	Congenital Hypothyroidism Unit

	Thyroxine (T4)
		Thyroxine or T4 is an iodinated amino acid secreted primarily by the thyroid gland. The thyroid hormones are powerful regulators of the body's metabolic rate, acting as catalysts in oxidative reactions. Absence of the thyroid gland or reduced levels of T4 can lead to severe mental retardation in the newborn inflant before any clinical signs of hypothyroidism are apparent. T4 measurement, therefore, is an extremely valuable indicator of primary thyroid status. Congenital hypotyroidism is probably the most common preventable cause of mental retardation. It's incidence is one in four thousand births.

 

	Principles of the Test
		The T4 test is a solid phase immunoassay (www.wallac.fi) based on the competitive binding of europium-labeled and unlabeled thyroxine. An enhancement reagent dissociates the europium ion from labeled T4. The resulting highly fluorescent chelate is measured and is inversely proportional to the quantity of T4 in the sample.

 

	Thyroid Stimulating Hormone (TSH)
		Thyroid Stimulating Hormone or TSH is a glycoprotein secreted from the anterior pituitary gland. It stimulates production of the major thyroid hormones, T4 and T3, by the thyroid gland. Immediately after birth there is a TSH surge, followed by stabilization several days later. However, an elevated TSH level is also an early indicator of hypothyroid status in the newborn. Since clinical signs may be absent and early intervention necessary to prevent irreversible mental retardation, the TSH test is an important part of Newborn Screening.

 

	Principles of the Test
		The TSH assay is a solid phase, two-site immunoassay (www.wallac.fi). Europium-labeled monoclonal antibodies are directed against antigenic sites on the TSH molecule. Enhancing reagent dissociates the europium ion into a fluorescent solution which is directly proportional to the concentration of TSH in the infant's blood. This laboratory utilizes both T4 and TSH assays to obtain an accurate picture of thyroid status in Michigan newborns. Marilyn Boucher, Endocrine Manager Phone: 517-335-9795 boucherm@state.mi.us