Neonatal Thyroxine (T4); Neonatal Screen for Hypothyroidism
Normal brain growth and development cannot take place without adequate thyroid hormone. Congenital hypothyroidism (cretinism) is characterized by low levels of T4 and elevated levels of TSH. Screening for congenital hypothyroidism is done in all 50 states. If hypothyroidism is undetected, growth and intellectual disability occur, as well as, in some cases, death.
This is a screening test of T4 activity to detect neonatal hypothyroidism. Specimens should be obtained after the first 24 hours of protein feeding or within the first week of life. T4 is obtained from whole blood blotted on filter paper using a radioimmunoassay technique.
Peaks in 24–36 hours then decreases
Neonates (1–3 days): 12–22 μg/dL or 152–292 nmol/L
Neonates (1–2 weeks): 10–17 μg/dL or 126–214 nmol/L
Clinical Alert
Critical Value7 days or younger: T4 6.5 g/dL or 84 nmol/L8 days and older: T4 5.0 g/dL or 64 nmol/L.
Cleanse the infant’s heel with an antiseptic and puncture the skin with a sterile disposable lancet. To help blood flow, warm the foot or massage the leg.
If bleeding is slow, it helps to hold the leg dependent for a short time before blotting the blood on the filter paper. Wipe away the first drop of blood.
Completely fill in the circles on the filter paper. This can best be done by placing one side of the filter paper against the infant’s heel and watching for the blood to appear on the front side of the paper and to fill the circle completely. Do not damage the filter paper. Apply a sterile dressing to the wound.
Air-dry for 1 hour, fill in all requested information, and send to the laboratory immediately. Protect specimen from extreme heat and light.
Low values are associated with hypothyroidism.
A number of nonthyroid conditions can result in depressed T4 levels (e.g., low birth weight, prematurity, twinning, fetal distress, deficient TBG levels).
Pretest Patient Care
Refer to neonatal TSH testing for care. The same protocols prevail for neonatal T4.
Be aware that T4 is usually collected at the same time as the phenylketonuria specimen.
The optimal collection time is 3–7 days after birth; the baby must be on protein feeding for at least 24 hours. For low birth weight or premature babies, the recommended time is 4–10 days old.
Posttest Patient Care
Refer to neonatal TSH testing for care. The same protocols prevail for neonatal T4. Also, see newborn screening in Chapter 11. Review test results; report and record findings. Modify the nursing care plan as needed.
If the baby is released early, the baby must be brought back for testing.
Clinical Alert
Do not interpret this test in terms of the adult serum T4 values. This is an entirely different procedure using a different type of specimen.
Notify the attending healthcare provider and the infant’s parent or parents of positive results within 24 hours.
If T4 results are abnormal, a TSH test should be performed.
Normal T4 and, in some cases, normal TSH screening results do not ensure against failure of normal development because of the presence of hypothyroidism. Of all cases of infantile hypothyroidism, 6%–12% have normal screening hormone levels.