Thyroxine (T<sub>4</sub>), Neonatal
| Thyroxine (T4), Neonatal | | | |
| Number | | 001446 |
| CPT | | 84437 |
| Synonyms | | T4, Neonatal |
| Special Instructions | | State patient's exact age. Please indicate if cord serum is used. Optimal collection time is 3-7 days after birth (ie, usually just before discharge). Four to 10 days after birth is recommended for low birth-weight infants. Special instructions or pertinent information should be included on the form. Please indicate if only certain tests are to be performed. Include information regarding blood transfusions and antibiotics or other medications administered to the infant which may influence initial test results. |
| Specimen | | Blood |
| Volume | | Three blood soaked circles |
| Container | | Filter paper |
| Collection | | Do a finger or heelstick and thoroughly saturate the circles on the filter paper (Schleicher and Scheull No 903). Label the card with the patient's name and exact age. The prompt and accurate collecting and processing of infant blood samples is crucial to early detection of these disorders when the infant is 48-120 hours of age. Specimens must be collected on filter paper kits. Specimens should be mailed on the day of collection. The method for collecting specimens is relatively simple but care must be taken to ensure the sample is collected properly in order to avoid inaccurate test results. Steps for collection are as follows: Stimulate blood flow by warming the foot and/or massaging the leg. Clean the puncture site with an alcohol swab, then dry with a sterile sponge to remove alcohol. Hold the infant's heel securely and puncture the medial aspect of the heel with a sterile lancet at least 2.5 mm in length. Wipe away the first drop of blood and absorb the next drops on the filter paper. Be sure to allow large drops of blood to form. Lightly touch the filter paper to the drops and allow them to flow onto the filter paper and diffuse through the circles. Apply a bandaid or similar sterile covering to the site. |
| Storage Instructions | | Refrigerate. Care in handling the specimens is essential because exposure to extreme heat or light or touching the filter paper can cause erroneous test results. Store and transport in a ziplock bag. |
| Causes for Rejection | | Filter paper not thoroughly saturated; radioactive tracer given to baby before this sample is obtained; specimen respotted (several drops of blood applied to the same circle) |
| Reference Interval | | 1-5 days: >7.5 μg/dL; 6 days and older: >6.5 μg/dL (abnormal paper spot results should be confirmed by submitting 0.5 mL serum) |
| Use | | Detect congenital hypothyroidism |
| Limitations | | Congenital thyroglobulin deficiency will result in low T4 values even though the patient is euthyroid. TSH is also low in TBG deficiency but T3 uptake is high.1 TSH is more sensitive for primary hypothyroidism. The risk of a false-negative result is increased when subjects with incomplete absence of thyroid parenchyma are detected only by T4.1 |
| Methodology | | Radioimmunoassay (RIA) |
| Additional Information | | If low values are obtained, the patient must have confirmatory tests run: T4, TSH, sometimes T3 uptake (THBR). For retesting, combined T4 and TSH is recommended when the initial T4 is low.2 Excessive quantities of TBG result in increased T4, while deficiency in TBG has the opposite effect. Extrathyroidal conditions resulting in depressed T4 levels include low birth weight (LBW). In normal as well as LBW infants, the T4 will be lower between 5 and 9 days compared to 3-5 days after birth.3 The incidence of permanent abnormalities leading to hypothyroidism is approximately 1:3600-5000 live births (as determined by tests in the United States).2 |
| Footnotes | | - Fisher DA and Klein AH, “Thyroid Development and Disorders of Thyroid Function in the Newborn,” N Engl J Med, 1981, 304(12):702-12 (review).
- American Academy of Pediatrics, Committee on Genetics, “Newborn Screening Fact Sheets: Congenital Hypothyroidism,” Pediatrics, 1989, 83(3):454-6.
- Dussault JH, Morisette J, and Laberge C, “Blood Thyroxine Concentration Is Lower in Low-Birth-Weight Infants,” Clin Chem, 1979, 25(12):2047-9
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| References | | Delange F, Dalhem A, Bourdoux P, et al, “Increased Risk of Primary Hypothyroidism in Preterm Infants,” J Pediatr, 1984, 105(3):462-9 (review). Fisher DA, “Euthyroid Low Thyroxine (T4) and Tri-iodothyronine (T3) States in Premature and Sick Neonates,” Pediatr Clin North Am, 1990, 37(6):1297-1312. Gravdal JA, Meenan A, and Dyson AE, “Congenital Hypothyroidism,” J Fam Pract, 1989, 29(1):47-50. LaFranchi SH, Hanna CE, Krainz PL, et al, “Screening for Congenital Hypothyroidism With Specimen Collection at Two Time Periods: Results of the Northwest Regional Screening Program,” Pediatrics, 1985, 76(5):734-40. Mitchell ML, Larsen PR, Levy HL, et al, “Screening for Congenital Hypothyroidism. Results in the Newborn Population of New England,” JAMA, 1978, 239(22):2348-51. Sadler WA and Lynskey CP, “Blood-Spot Thyrotropin Radioimmunoassay in a Screening Program for Congenital Hypothyroidism,” Clin Chem, 1979, 25(6):933-8. |
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