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There have been notable advances in terms of what biochemistry laboratories can measure, how quickly they can measure and how much sample is required to do the measuring. This has improved many aspects of clinical care but ease of access and rapid feedback may increase the likelihood of babies undergoing blood tests unnecessarily. Checking thyroid-stimulating hormone (TSH) and thyroid hormone concentrations in neonates is a good case in point. Many different neonatal (and adult) symptoms and signs can be linked to abnormal levels of thyroid hormone and so it is easy to understand why doctors are keen to check thyroid function. The potential impact of maternal thyroid disease on neonatal thyroid status also needs to be considered. Approximately 10% of adult women have autoimmune thyroid disease and many babies may be considered eligible for thyroid function testing because of this.1
What is normal?
Interpreting thyroid biochemistry requires the clinician to know what normal is in the first place. The biochemistry report of TSH and thyroxine concentrations may be accompanied by an ‘adult’ reference range which can lead to concerns about possible hypothyroidism or thyroid gland overactivity in healthy babies. Discussions about the threshold above which TSH concentrations can be said to be significantly raised are highly pertinent in the neonatal period.2 ,3 Is a serum TSH of 9 mU/L of any clinical significance? What is the differential diagnosis? Is this result within the normal spectrum at this age? Should the baby undergo further investigation? Should the baby be treated with thyroxine?2 ,3 If the biochemistry is thought to be mildly abnormal, then thyroid function tests are sometimes repeated regularly even though the likelihood of a recognised abnormality of thyroid development is remote.
Delivery is associated with a profound change in circulating TSH and thyroid hormone concentrations. Mean free thyroxine …