First, to date, no data are available on the thyroid volume of full-term newborns in Italy, making it essential to conduct studies to establish reference values for normality. This allows these values to be correlated with maternal and fetal variability, providing a scientific basis for better understanding thyroid development in the first days of life. Thyroid ultrasound is useful for supplementing newborn screening for congenital hypothyroidism, helping to identify early abnormalities. Furthermore, thyroid volume is a sensitive indicator of iodine intake, essential for assessing the population's iodine nutritional status. It is equally valuable for studying and monitoring the effects of maternal and environmental factors, for which thyroid volume itself serves as a marker. Having normal values allows for a more precise comparison and contextualization of data from children with pathologies, who undergo ultrasound for pathological screening. In summary, these regulatory references improve the diagnosis, prevention, and management of neonatal thyroid dysfunction, contributing to a more informed public health.

The thyroid is a fundamental endocrine gland that regulates metabolism throughout the life cycle. Thyroid's function is particularly crucial in fetuses, newborns, and children, as thyroid hormones play a key role in the development of the central nervous system. A deficiency of thyroid hormones in childbearing age women due to untreated hypothyroidism, autoimmune disorders or iodine deficiency poses a threat to both fertility and pregnancy progression and can negatively impact fetal and neonatal neurological outcomes later in life. Since the introduction of neonatal TSH- screening method the prognosis of newborn with congenital hypothyroidism has significantly improved due to early start of levothyroxine treatment. The findings of reduced or increased thyroid size in newborn with TSH elevation can support the need for further investigation such as genetic analysis. For this reason, having normative data of thyroid volume is fundamental in newborn population. According to the current guidelines for congenital hypothyroidism, the primary imaging techniques for evaluating neonatal thyroid include ultrasound and radionuclide scintigraphy. Ultrasound is the preferred modality due to its non-invasive nature and ability to provide detailed information about the gland's structure, size, position, vascularization, and echogenicity. Thyroid volume is tightly regulated during fetal development, and variations in size may indicate pathological maternal and neonatal dysfunctions. Therefore, thyroid volume measurement is a crucial indicator for monitoring the gland's development and function in newborns and is strongly associated to environmental factor such as iodine status and maternal factors. International studies conducted in countries with different iodine status and national salt iodization programs (Poland, Germany, Belgium, Turkey, United Kidgdom, United States China, Brazil) have shown significant variations in average thyroid volume values in euthyroid term newborn ranging from 0.47 in the Unites States to and 1.62 ml in the United Kingdom. These findings highlight the importance of geography and iodine supplementation in determining neonatal thyroid volume. Ultrasound of the thyroid gland was generally recorded during the first three weeks of life. However, no reference data are currently available for Italian newborns. Italy is considered an iodine sufficient area but there is an increased risk of deficiency during pregnancy. Establishing specific reference values of newborn thyroid volume for the Italian population is therefore necessary. In addition to the geographical region of origin, several maternal factors may influence neonatal thyroid volume, including levothyroxine intake in cases of hypothyroidism, nutritional iodine supply/intake, use of medications/disinfectants containing iodine, gestational diabetes and smoking during pregnancy. One study found a negative correlation between high maternal levothyroxine doses and neonatal thyroid volume. Although this correlation is weak, multivariate logistic models confirmed the impact of maternal therapy on neonatal thyroid volume. Excess maternal thyroid hormones can suppress fetal TSH secretion, limiting follicular cell proliferation, leading to a reduced neonatal thyroid volume. Studies about the association between gestational diabetes and thyroid neonatal volume reported difference results. Smoking during pregnancy has also been associated with variations in neonatal thyroid volume. Studies on adults have shown a 25% increase in thyroid volume in smokers compared to non-smokers. The high prevalence of smoking among young women in certain regions could explain an increased neonatal thyroid volume. This study is essential for establishing national reference values for thyroid volume in healthy term and preterm Italian newborns, addressing a gap in the available data. Understanding geographical differences, maternal influences, and the newborn characteristics of preterm newborns will improve the monitoring and management of neonatal thyroid conditions, with potential implications for public and neonatal health.

Inclusion Criteria:

• Healthy full term newborns (born between 37 and 42 weeks of gestation, with no antennal issue or complications at birth)
• Consent obtained by parent(s) or legal guardian(s)

Exclusion Criteria:

• Congenital malformations
• TSH abnormalities in neonatal screening
• Pregnancy not carried out in Italy, due to different iodine status
• Inability or unwillingness of the parent(s) or legal guardian(s) to provide informed consent or to comply with study procedures

400 healthy term newborns, born between 37 to 42 weeks of gestational age