Relation of basic and clinical research on foetal and neonatal thyroid pathology to neonatal thyroid screening

Optimal Assessment and Quantification of Iodine Nutrition in Pregnancy and Lactation: Laboratory and Clinical Methods, Controversies and Future Directions

Iodine intake must be boosted during pregnancy to meet the demands for increased production and placental transfer of thyroid hormone essential for optimal foetal development. Failure to meet this challenge results in irreversible brain damage, manifested in severity from neurological cretinism to minor or subtle deficits of intelligence and behavioural disorders. Attention is now being focused on explaining observational studies of an association between insufficient iodine intake during pregnancy and mild degrees of intellectual impairment in the offspring and confirming a cause and effect relationship with impaired maternal thyroid function. The current qualitative categorisation of iodine deficiency into mild, moderate and severe by the measurement of the median urinary iodine concentration (MUIC) in a population of school-age children, as a proxy measure of dietary iodine intake, is inappropriate for defining the degree or severity of gestational iodine deficiency and needs to be replaced. This review examines progress in analytical techniques for the measurement of urinary iodine concentration and the application of this technology to epidemiological studies of iodine deficiency with a focus on gestational iodine deficiency. We recommend that more precise definitions and measurements of gestational iodine deficiency, beyond a spot UIC, need to be developed. We review the evidence for hypothyroxinaemia as the cause of intrauterine foetal brain damage in gestational iodine deficiency and discuss the many unanswered questions, from which we propose that further clinical studies need to be designed to address the pathogenesis of neurodevelopmental impairments in the foetus and infant. Agreement on the testing instruments and standardization of processes and procedures for Intelligence Quotient (IQ) and psychomotor tests needs to be reached by investigators, so that valid comparisons can be made among studies of gestational iodine deficiency and neurocognitive outcomes. Finally, the timing, safety and the efficacy of prophylactic iodine supplementation for pregnant and lactating women needs to be established and confirmation that excess intake of iodine during pregnancy is to be avoided.

Thyroid dysfunctions in children detected in mass screening for congenital hypothyroidism

BACKGROUND

Congenital hypothyroidism (CH) affects approximately 1:3000-1:4000 infants.

OBJECTIVES

To determine the prevalence of CH and isolated hyperthyrotropinemia (IHT) in newborns selected in mass screening for CH.

METHODS

Mass screening of 233,120 neonates born in southeastern Poland was carried out and CH-suspected children were identified. Serum thyroid-stimulating hormone and free thyroxine levels were determined during first confirmation and diagnosis re-evaluation in 118 and 34 children, respectively. Additionally, the patients were subjected to thyroid ultrasonography (n=53) and/or scintiscan (n=28).

RESULTS

Out of 118 children, first confirmation indicated CH in 58 neonates and IHT in 4 neonates. Out of these, 34 were re-evaluated with regard to diagnosis. A final diagnosis of permanent CH was reported in 34 children with thyroid dysgenesis (n=27) or dyshormonogenesis (n=7), transient CH affected 15 children, and permanent IHT was diagnosed in 6 children. CH prevalence was 1:4570 (permanent 1:6475, transient 1:38,853) and permanent IHT 1:38,853.

CONCLUSIONS

The prevalence of CH and IHT corresponds to the prevalence of the condition in iodine-sufficient and borderline iodine-deficient areas.

[Utility of congenital hypothyroidism screening in neonates for monitoring iodine deficit disorders in the Canary Islands (Spain)]

BACKGROUND

Newborns in European cities where iodine intake is low have been demonstrated to present high frequencies of transitory hypothyroidism. Because the neonatal period is critical for cerebral development, this is a cause for concern. Published studies (WHO/UNICEF/ICCIDD) indicate that neonates with a thyroid-stimulating hormone (TSH) concentration of more than 5 mU/ml revealed by screening for congenital hypothyroidism present mild iodine deficiency.

OBJECTIVE

To analyze the utility of TSH values as an indicator of the prevalence of iodine deficiency in the general population.

MATERIAL AND METHODS

We prospectively evaluated 19 809 neonates, corresponding to all the neonates screened from May 2001 to April 2002 in the Canary Islands.TSH determination in whole blood dried on filter paper was performed using immunofluorescence (Delphia) in the Center for the Detection of Metabolic Disorders in the Canary Islands. The percentage of neonates in each island with TSH values of > 5 mU/l was calculated. Samples of cord blood were not used.

RESULTS

A total of 19 809 infants were analyzed. Of these 1800 had values of TSH > 5 mU/L, representing 9.08 % of neonates. The mean age at blood extraction was 4.31 6 3.78 days (range: 0.5-40). The percentage of neonates with values of THS > 5 mU/L in each island was 13.1 % in Gran Canaria, 5.1 % in Lanzarote, 7.3 % in Fuerteventura, 6.0 % in Tenerife, 6.2 % in La Palma, 6.6 % in Gomera and 10.1 % in Hierro. In 77.5 % of neonates in Gran Canaria blood was extracted for screening within the first 72 hours of life and 15.2 % of these neonates had TSH concentrations of > 5 mU/L. In 22.5 % of neonates blood was extracted on the third day of life or later and 7.9 % of these neonates had TSH values of > 5 mU/L.

CONCLUSIONS

In the Canary Islands, the percentage of neonates with iodine deficiency, according to elevated TSH levels detected screening for congenital hyperthyroidism, was small. The validity of TSH level as an indicator of the prevalence of iodine deficiency in the general population is influenced by the days of life at which the blood sample is taken.