I don’t know if, like me, you are surprised by how often when admitting a baby, or doing an antenatal consult, mothers are taking thyroxine. It seems that there is an epidemic of hypothyroidism during pregnancy, and I am not sure what paediatricians should do about it.

A large proportion of these mothers were not on thyroid supplement prior to pregnancy, and seem to have so-called “sub-clinical hypothyroidism”. It is defined by a normal T4 with an elevated TSH and seems to occur with extremely high frequency, from 15 to 28% of all mothers in areas of the world where there is sufficient iodine.

I tend to have doubts about anything that occurs that frequently among otherwise healthy people as being a disease, it seems likely that, for most women with this “condition”, it is just a variant of normal, a review article from the BMJ from a few years ago (Wiles KS, et al. Are we overtreating subclinical hypothyroidism in pregnancy? BMJ. 2015;351:h4726.) stated the following:

 Evidence that this will cause adverse pregnancy outcome is inconsistent and conflicting. Equally, treatment with thyroxine has not been shown to be beneficial. While results of ongoing trials are awaited, thyroxine treatment is recommended in the absence of evidence of harm. However, the possibility of overtreatment in pregnancy should be considered. Monitor for iatrogenic hyperthyroidism with a repeat TSH four to six weeks after any change in thyroxine dose and be aware that most of these women will not need ongoing thyroid replacement after pregnancy.

I checked for recent systematic reviews of treatment of this and the latest ones that I could find include many observational studies, and a few small RCTs: the best SR, I think, includes only the RCTs, and is quite inconclusive (Yamamoto JM, et al. Impact of levothyroxine therapy on obstetric, neonatal and childhood outcomes in women with subclinical hypothyroidism diagnosed in pregnancy: a systematic review and meta-analysis of randomised controlled trials. BMJ Open. 2018;8(9):e022837). There is possibly a reduction in preterm delivery, but the confidence intervals are wide and include a 21% increase in preterm delivery. Some of the observational studies seem to show a reduction in pregnancy loss, but the RCTs have not really investigated this. There is a possible increase in NICU admission among babies from treated pregnancies by 23%, but again confidence intervals are wide and include a reduction in NICU admission. The total sample size from the 2 articles that were meta-analyzed is only just over 1000, which is disappointing for a “condition” that is so incredibly frequent and should be relatively easy to study. The overall conclusion of the SR is “no evidence of benefit of levothyroxine therapy on obstetrical, neonatal, childhood IQ or neurodevelopmental outcomes. Current trial evidence does not support the treatment of subclinical hypothyroidism diagnosed in pregnancy”.

To return to my main question, if a baby is born after maternal thyroxine treatment during pregnancy, should the paediatrician (or family doc) do anything different for the baby?

In this study from Sydney the authors had noted that many babies were getting thyroid function screening, despite the presence of a universal thyroid screen (Churcher LM, et al. Reducing unnecessary neonatal testing in infants of mothers with thyroid disease. J Paediatr Child Health. 2020) they instituted a new guideline based on the most recent evidence which is summarized on a card including these major points regarding babies born from a mother who was getting thyroid supplementation:

• Additional tests are very rarely needed
• The only groups that need additional screening tests for the baby are:
  • Maternal thyroid disease with orbitopathy
  • Maternal past/current treatment of Graves’ disease (radio-iodine or thyroidectomy)
  • Maternal anti-thyroid drugs – Carbimazole or PTU
• Even these babies only need additional tests if maternal Thyroid Receptor Antibody is positive or unknown
• The Newborn Screen is enough for all others

After producing this they noted a dramatic reduction in unnecessary thyroid testing of newborns. Without having to argue with the obstetricians about the need for 1 in 5 otherwise healthy mothers to be treated with thyroxine during pregnancy, it appears to have no adverse neonatal effects and does not require further testing or treatment of the baby.