The Practice and limitations of screening for congenital hypothyroidism in selected hospitals in Sri Lanka
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Date
2007-04
Authors
Nanayakkara, Damayanthi Kusum Kumari
Journal Title
Journal ISSN
Volume Title
Publisher
University of Peradeniya
Abstract
The global incidence of congenital hypothyroidism (CH) is approximately 1:3000 of all births. Geographical regions with recognised iodine deficiency show a higher incidence of CH (1:700). However, clinical features of CH are subtle at birth and need biochemical methods for early detection. Neonatal screening for CH is not a routine practice in Sri Lanka. Hence, the diagnosis is based on “clinical features”. This thesis aims to determine the major limitations in the current practice, to find out the burden of thyroid dysfunction in early infancy and finally to assesses the feasibility of conducting a routine neonatal screening program in Sri Lanka. The initial field experience and the pilot study results are reported here. , The first was a retrospective study that looked at the current practice of using clinical features for the diagnosis CH. Confirmatory tests were done on serum. The second was a prospective study that was carried out at the first triple vaccination to measure serum thyroid stimulating hormone (TSH) and free thyroxin in clinically normal infants. The third study was a pilot study carried out at the time of BCG vaccination using filter paper blood spot assay technique. After obtaining consent from the parents filter paper blood samples were collected by heel-stick or venepuncture method. Air-dried blood samples were tested for TSH using neonatal blood spots TSH kits Gmmunoradiometric assay method, skyBio.Ltd.UK). Neonates with blood spot TSH above 20mU/L were identified as at risk and recalled for confirmation with serum analysis. Hypothyroidism was confirmed by elevatedserum TSH associated with low free T4. Serum TSH above 20mU/L or blood spot TSH above 6.0mU/L was considered as hyperthyrotropinemia. Infants with abnormal thyroid functions were closely followed up. Both first and second studies found a high frequency of thyroid dysfunction Le., hypothyroidism and marginal hyperthyrotropinemia. The first study also found that prolonged neonatal jaundice was significantly associated with CH. However, in the absence of a routine biochemical screening programme in Sri Lanka, delay in diagnosis of CH is inevitable. The pilot study screened 4352 neonates.Total coverage from each hospital was over 90% with sample rejection rate at 1.5% (95%CI 1.1%-1.9%). The average sampling time was 1.2+1.5 days and 75% (95% CI 74%-76%) were screenedwithin 48hours of birth. The mean TSH value was 5.9mU/L in this study group. Using TSH cut off (blood spot) at 20mU/L, 52 neonates were identified at risk and 1.2% (95% cl 0.9%-1.5%) were recalled. This confirmed 4 true, 5 unclassified cases of CH. The incidence of true CH was 1:1088, whereas including transient cases; the total incidence was high as 1: 483. These studies showed that thyroid dysfunction in neonates in this area were higher compared to global figures. It is comparable to the figures seen in countries with iodine deficiency. Prolonged neonatal jaundice was found to be a significant clinical tool in the diagnosis of CH in the absence of a routine screening program. However, there are some limitations in the current clinical practice.
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Keywords
Medicine , Congenital hypothyroidism , Iodine deficiency , Thyroid stimulating hormone