Serum Free Thyroxine and Free Triiodothyronine Concentrations in Healthy Fullterm, Preterm and Sick Preterm Neonates

Abstract

SUMMARY. There are few data available on free thyroid hormone concentrations in the early neonatal period. With the widespread application of screening procedures for detecting congenital hypothyroidism there is a need for reference ranges in neonates. In this study we have evaluated thyroid function in healthy fullterm and preterm neonates, and sick neonates all within one to to days postnatal age. Our data indicates that free thyroxine but not free triiodothyronine is higher in fullterm neonates than the adult reference range and that both free thyroid hormone concentrations are reduced in healthy and sick preterm neonates as compared to fullterm neonates. Assessment of thyroid function in the early neonatal period needs to take into account these changes particularly in preterm and sick preterm neonates.The sequence of events leading to the development of the thyroid and the control and secretion of the thyroid hormones in the fetus has been well reviewed by Fisher. I At birth, an acute release of thyrotrophin (TSH) occurs which stimulates an increase of both thyroxine (T 4 ) and triiodothyronine (T 3 ) concentrations in serum. The peak TSH concentration occurs within an hour of birth but rapidly declines during the first 24 h with a slower decrease over the following two days. Serum T 4 and free T 4 concentration after peaking at 24 h then slowly decrease during the first few weeks of Iife. • 3Essentially similar changes occur in serum T3 and free T3 concentrations. In premature neonates without serious illness, both T 4 and TSH follow the same pattern as fullterm neonates although at a lower level.Although there are published indices of thyroid function in healthy babies and children,": 5. 6. 7 in premature babies using cord blood ll • 9 • 10 and serum, II. 12. 13 and in prematures with respiratory distress syndrome;" information on free thyroid hormone concentrations measured by the newer direct technique in the early neonatal period is more limited. With the widespread adoption of screening proceCorrespondence: Rhys John. dures for congenital hypothyroidism using the filter paper blood sample collected between three and seven days postnatally, there has been an increased need to evaluate thyroid function in neonates. In most instances, using TSH as the screening assay, the diagnosis of neonatal hypothyroidism is easily made but in a proportion who show only moderately increased TSH concentrations, full thyroid function tests may need to be done to determine whether any thyroid dysfunction is present. These moderately increased TSH concentrations are usually only transient elevations and are seen particularly in preterm infants or in acutely ill infants. 15 The aim of this study was to define normal serum free thyroid hormone concentrations in fullterm, preterm and sick preterm neonates. Materials and methodsA total of 104 neonates, with ages ranging from one to 10 days were studied. Five were delivered with the aid of forceps, 20 by caesarian section and the remaining were delivered vaginally. The fullter...

Neonatal Hypothyroxinemia: Effects of Iodine Intake and Premature Birth

“…Our results are in general good agreement with those reported in other studies for both PT and T infants. T 4 , FT 4 , and T 3 levels are lower in PT neonates compared to T newborns, increasing progressively with PMA, whereas TSH levels are the same (27)(28)(29)(30)(31). To our knowledge, the possibly independent effects of I intake vs. PT birth on these parameters of thyroid function have not been assessed by others.…”

The Relation of Transient Hypothyroxinemia in Preterm Infants to Neurologic Development at Two Years of Age

“…13 Although the authors of that study did not report on thyroxine concentrations, other studies have shown that triiodothyronine and thyroxine concentrations are directly correlated in preterm infants. [8][9][10][11]33 In another study, in which finemotor development and coordination, passive language, and gross-motor skills, but not cerebral palsy, were assessed at the age of two years, blood thyroxine concentrations more than 3 SD below the mean shortly after birth were associated with a risk of failing to achieve one or more developmental milestones that was 3.5 times the normal risk. 14 The 479 subjects in that study were premature infants with gestational ages of less than 32 weeks or birth weights below 1500 g. The cohort was reevaluated at five and nine years of age 15 ; each decrease of 1 SD in the thyroxine concentration in the first weeks of life was associated with a 30 percent increase in the risk of handicap and neurologic dysfunction at five years of age and a 30 percent increase in the likelihood that the child would need special education or repeat a grade in school by the age of nine.…”

“…[1][2][3][4][5][6][7][8][9][10][11] Until recently, it was not thought to have sequelae or to require thyroid-hormone replacement. 3,4,12 Recently, however, studies have linked hypothyroxinemia in the neonatal period to subsequent problems in motor and cognitive development.…”

Developmental Follow-Up of Very Low Birthweight Premature Infants with Low Free Thyroxine

“…5 Free T 4 concentration has been shown to be reduced in premature infants compared with full-term neonates during the first 10 days of life. 7 The nature of this reduction and its long-term consequences are not fully understood. A reduction in T 4 with normal TSH (sick euthyroid syndrome) is a normal response in a critically ill person, be it an adult or premature infant.…”

“…Both free T 4 and triiodothyronine (T 3 ) concentrations are reduced in healthy as well as sick premature neonates compared with full-term neonates during the first 10 days of life. 7 This limited and discrepant literature about free T 4 levels in low birthweight infants make it difficult to interpret. In addition, there are no published data that evaluate the relationship between screening values for thyroid disease during the first week and long-term outcome of premature infants.…”