Neonates thyroid hormone levels

Longnecker MP, Gladen BC, Patterson DG, et al. 2000. Polychlorinated biphenyl (PCB) exposure in relation to thyroid hormone levels in neonates. Epidemiology 11 249-254. 

Interesting changes have been found in thyroid hormone levels in the neonatal period. It has been known for many years that serum T4 levels are normally higher in infancy than at any other time. Such elevations could well obscure a diminution in thyroid hormone levels associated with hypothyroidism, particularly partial hypothyroidism associated, for example, with dyshormonogenesis. Early detection of hypothyroidism is essential as treatment is simple and the consequences of delay in treatment are severe and to some extent, irreversible. 

The effect of intravenous iodinated contrast material administration to the neonatal thyroid levels in pregnant women were investigated in a retrospective study of 64 newborns. All 64 newborns, except for one premature and one septic patient were found to have normal thyroid hormone levels. Although the patient group was small, the study found that there was no significant risk of thyroid function abnormalities to the neonates after intravenous administration of contrast material to the mothers [5 ]. 

Li FX, Byrd DM, Deyhle GM, Sesser DE, Skeels MR, Katkowsky SR, Lamm SH (2000) Neonatal thyroid-stimulating hormone level and perchlorate in drinking water. Teratology... 

It is usually performed either at birth, soon after the finding of elevated neonatal TSH levels, at 3 years of age after 1 month of L-thyroxine withdrawal, or on administration of RhTSH, which allows a precise diagnosis to be made and thus avoids the hypothyroid state. Finally the severe hypothyroidism resulting from TPO mutations should be promptly tteated with the thyroid hormone, in order to maintain TSH levels at the lower limit of the normal range. 

We conclude that brain Na,K-ATPase responds to thyroid hormone status at around 14 days of age. The onset of thyroid hormone responsivity might be earlier if the very low (rather than zero) levels of T3 detected in some of the neonates was sufficient to induce Na,K-ATPase. 

Yamazoe Y, Murayama N, Shimada M, Kato R (1989) Thyroid hormone suppression of hepatic levels of phenobarbital-inducible P-450b and P-450e and other neonatal P-450s in hypophysectomized rats. Biochem Biophys Res Commun 160 609-614... 

Clinical concern Transient hypothyroxinemia of prematurity (THOP) occiu s in 50% of extremely low gestational age neonates (ELGANs 24-28 weeks) and is a major factor of neurodevelopment abnormalities (cognitive delay, cerebral palsy, hearing loss, mental retardation, blindness or epilepsy) [9 ]. THOP characterised by very low total and free T4 levels and normal TSH may be safely treated with continuous infusion of 4 gg/kg/day levothyroxine for 42 days to produce a biochemical euthyroid state (phase I/II trials) [1(P]. There is a pressing need for a phase III trial of thyroid hormone that is of sufficient duration and size to determine whether a clinically important reduction in risk of developmental impairments in ELGANs can be achieved [11 ]. 

The first one is the observation that in areas with persisting borderline iodine deficiency, transient hypothyroidism in the newborns and also some anomalies of thyroid func on in pre iant women can still be linked to such an easily preventable factor as iodine deficiency. In some of these areas it is now clear that in screening, for neonatal hypothyroidism, roughly 2 out of 3 newborns rec alled for abnormal thyroid hormone or TSH levels are false positive. Apart from the psycholomcal and economic al costs linked to the rectal proc ess per-se, this transient hypothyroidism might induce around birth, brain maturation retardation of unknown long term... 

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