To finalize this litany of the unknown… We also need to know whether low thyroxine levels are common in the preterm (they are) what the consequences are (associated with worse long term outcomes) whether they are causative (don’t know) and whether replacement with exogenous thyroxine is a good idea or not (haven’t got the slightest).
In older patients they talk about the sick euthyroid syndrome: critically ill older patients have a suppression of thyroid function, which appears to be adaptive, but maybe not. In any case as a recent review for adults (free access) notes, “there is currently no convincing evidence to suggest that restoring physiological thyroid hormone concentrations in unselected patients with NTIS would be beneficial” NTIS being another term for the sick euthyroid syndrome.
This review article from last year (La Gamma E, Paneth N. Clinical importance of hypothyroxinemia in the preterm infant and a discussion of treatment concerns. Current Opinion in Pediatrics. 2012;24(2):172-80) is pretty complete. Ed LaGamma and Nigel Paneth review the data regarding thyroid dysfunction in the very preterm infant. I reproduce the table of key points below, but please get access to the full article if you can, it is well written (as you would expect from those two) and quite complete. I think investigating preterm infants thyroid function could be very important for improving preterm outcomes.
Also don’t forget, dopamine is very efficient at suppressing thyroid function, as is well known in adults, and also in babies, as Istvan Seri showed many years ago (Seri I, Tulassay T, Kiszel J, Ruppert F, Sulyok E, Ertl T, et al. Effect of low-dose dopamine infusion on prolactin and thyrotropin secretion in preterm infants with hyaline membrane disease. Biol Neonate. 1985 1985;47(6):317-22) and confirmed several times since, (Filippi L, et al. Dopamine infusion: a possible cause of undiagnosed congenital hypothyroidism in preterm infants. Pediatric critical care medicine. 2006 May;7(3):249-51). So many very sick babies are getting dopamine, and this has often been ignored in the reports of the studies of thyroid function in the preterm infant, we often do not know how many were on dopamine, which really makes interpretation of the published data complicated.
All of which makes me think that, for a neonatal fellow looking for an academic career path, you could do worse than neonatal endocrinology… if you speak French (you don’t have to be fluent, just the basics will do, you will become fluent very quickly), drop me a line, we have great endocrinologists at my hospital, Sainte Justine in Montreal, and we could create a new super-fellowship! In addition to the neonatology and neonatology/clinical ethics fellowships that are already very active and very high quality.