medwireNews: The likelihood of newborn babies exceeding a particular thyroid stimulating hormone (TSH) cutoff varies by ethnicity, a study suggests.
“These results imply that there are ethnic differences in thyroid physiology, which may be due to genetic factors, and which should be considered when examining TSH cut-offs in different populations”, the researchers write in The Journal of Clinical Endocrinology & Metabolism.
Their study of 824,588 newborn babies screened at a single centre between 2006 and 2012 revealed that, overall, the proportion of babies exceeding the cutoff levels ranged from 1.06% for a TSH level of 4 mU/L to 0.10% for a level of 10 mU/L.
Currently, babies in the UK are screened at age 5–10 days and any with TSH exceeding 20 mU/L are immediately referred. But in case of a delayed physiological TSH surge, those with a borderline result undergo a repeat test a week later.
Catherine Peters (Great Ormond Street Hospital, London, UK) and study co-authors say that the cutoff for a repeat test in their centre is 6 mU/L, but this varies nationally and internationally. They found that the proportion of children at their centre exceeding 4 mU/L was influenced by year of testing, becoming larger in more recent years, and a cutoff of 10 mU/L was influenced by gender, with girls more likely than boys to exceed it.
The team therefore suggests that 6 or 8 mU/L would be better thresholds, although cautioning that this “is based on year and gender differences rather than screening sensitivity and specificity”, which are available only for historical cases.
Babies of White or White/mixed ethnicities, comprising about 60% of the patient population, had the lowest likelihood of exceeding the cutoffs. Then there was a group of babies mainly of Indian, African and Caribbean ethnicity, who were at least twofold more likely than White British babies to exceed the cutoffs. And a group of Pakistani, Bangladeshi and Chinese babies were around three- to fivefold more likely to exceed the cutoffs.
The researchers did not look at repeat test results for these children, but note that 34% of referrals involved Asian babies, despite these comprising only 19% of babies tested, suggesting that the findings cannot be explained by differences in the postnatal TSH surge.
Peters and team also observe that ethnic groups with a high likelihood of exceeding the TSH thresholds originate from “regions of the world with historical records of iodine deficiency”, and suggest that iodine insufficiency in British newborns should be investigated further, as a preventable cause of congenital hypothyroidism.
By Eleanor McDermid
J Clin Endocrinol Metab 2016; Advance online publication
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