Unlike the other conditions, hypothyroidism is a congenital rather than an inherited disorder in the majority of cases. This is an endocrine condition, which results from failure of the thyroid gland to produce thyroxine. There are a number of different forms of the condition. Some infants may have a very small thyroid gland in their neck or no gland at all while others may not be able to make the hormone thyroxine. It is important to know the cause; this can be determined by performing a scan of the thyroid gland soon after the diagnosis has been made. Approximately one in every 3,500 infants born in Ireland may have the condition; early detection allows early treatment and the prevention of the onset of symptoms.
The diagnosis is made by measuring blood thyroid stimulating hormone (TSH), high levels of which are suggestive of the condition. However, this hormone rises in blood immediately after birth and then falls to normal by about the second day of life. This is one of the reasons why the heel-prick sample should not be taken before about 72 hours after birth otherwise a false-positive result could be produced.
The majority of infants with congenital hypothyroidism require treatment with thyroxine. Some will be reviewed at about three years of age at which time a small number may be able to discontinue treatment under medical supervision. Otherwise treatment is for life and the dose of thyroxine will be adjusted as the infant grows.
Compared to some of the other conditions the frequency of false positive results is relatively high. Consequently the number of requests for repeat blood samples is also high. Possible reasons include:
- a transiently raised plasma TSH concentration which returns to normal in time. These infants may require a number of repeat samples to be collected. As hypothyroidism is more common in infants and children with Down syndrome, a disproportionate number of repeat samples may be requested from these infants as they too may have a transiently elevated plasma TSH level during the newborn period before developing hypothyroidism later;
- infants who may have required surgery before the screening sample was taken. Some antiseptic skin preparations contain iodine and this may be absorbed through the skin causing transient hypothyroidism with an elevated blood TSH level.
Action following detection of raised result
If the bloodspot TSH is raised, thyroid functions tests will be requested by the staff in the NNBSL. If these demonstrate a raised plasma TSH and a low normal or low Free T4 then congenital hypothyroidism is suspected. The following action will be implemented but may be modified depending on the age of the baby and the day of the week
1. The test results are discussed with the On-call Paediatric Endocrinologist and a decision made as to when the baby should be admitted to the Day Ward at the Children’s University Hospital, Temple Street.
2. The Clinical Liaison Officer (NNBSL) will:
- Arrange for a technetium thyroid scan with Nuclear Medicine in the Radiology Department, Temple Street
- book a bed in the Day ward through the Admission Office
- contact the Maternity Unit in which the baby was born. Clear instructions will be given to include the child’s demographic details, the condition suspected and the arrangements for admission. They will be asked to contact the parents to arrange admission for the appointed time
- contact the Endocrine Registrar to inform them of the admission and the Clinical Biochemistry Laboratory in case urgent TFTs are required
3. On arrival in the Day ward the, the child will be examined and an indwelling cannula will be inserted. Blood will be drawn for repeat TFTs
4. Following the thyroid scan the baby and the parents will meet with the Consultant paediatric Endocrinologist and the results of the investigations will be discussed and treatment commenced if required
5. The diagnosis will be based on the results of the thyroid scan; some children will be re-evaluated at three years to age to determine whether they have permanent dyshormonogenic hypothyroidism or transient hypothyroidism of the newborn
Infants born in the Cork area will be referred to the Cork University Hospital for clinical evaluation, thyroid scan and treatment. This follows a similar procedure.
On occasion there may be deviations from this protocol to ensure that those babies with unambiguous congenital hypothyroidism are started on treatment by 10 day of life. For these, a thyroid ultrasound examination may replace the thyroid scan as the baby might have been started on treatment before attending the hospital.
Newborn Screening for Congenital Hypothyroidism is complete following review by the consultant endocrinologist and a firm diagnosis made