Background: The UK Newborn Screening Program Centre recommend that a blood-spot TSH cut-off of 10mU/l is used to detect congenital hypothyroidism. The value used varies from 5-10mU/l and so we examined the implications of altering this threshold.
Methods: Our regional blood spot TSH cut-off is 6mU/l. Positive or suspected cases were defined as a TSH >6mU/l throughout the study period (1/4/2005 to 1/3/2007). All term infants (>35 weeks) whose first TSH was 6-20mU/l had a second TSH measured. The biochemical details of infants with a TSH between 6.1-10.0mU/l and then >6mU/l on second sampling were sent to Paediatric Endocrinologists to determine approaches to management.
Results: 148 of 65446 infants (0.23%) had a first blood spot TSH >6.0mU/l. 120 were term infants with 67 of these (0.1% of all infants tested) having a TSH between 6.1-10.0mU/l and 53 a TSH >10.0mU/l. Of the 67 term infants with a TSH between 6.1-10.0mU/l on initial testing, 4 continued to have a TSH >6mU/l. One with a TSH greater than 10 mU/l and 1 infant with a TSH <10mU/l on the second blood spot have been diagnosed with congenital hypothyroidism. The survey of endocrinologists highlighted significant differences in practice.
Conclusions: A reduced threshold of 6mU/l will increase the number of false positive ‘term’ infants by 126% but abnormalities of thyroid function requiring treatment will be detected. We suspect that the additional expense involved in setting a lower threshold is justified.