Nowadays, taking care of preterm birth is associated with an important increase in survival. This increased survival comes with impairment in neurodevelopmental outcomes in long term evaluation. Thyroid hormones are essentials for brain development, especially for neuronal differentiation. Transient hypothyroxinaemia of prematurity (THOP) is a frequent condition defined by decreased thyroid hormones without the expected rise in thyroid stimulating hormone. Various studies have showed various results regarding the consequences of THOP on neurodevelopment in premature neonates. However, the biggest and most powerful studies agree to say that THOP impair neurodevelopment. On the other hand, only a few studies evaluated the impact of treatment of THOP, and only two focused on treating exclusively the neonates with a biological diagnosis of THOP (Suzumura and co. in 2010 and Nomura and co. in 2014) and their results are inconsistent.

In this study, we aim to show that a treatment with L-thyroxine at a dose of 7.5 µg/kg/j for neonates diagnosed with THOP (defined as a level of l-T4 < 12 pmol/L and a level of TSH < 15 mUI/L before 15 days of life or < 85 mUI/L after 15 days of birth) is associated with an increased neurodevelopmental prognosis.

Inclusion Criteria:

• Premature infants born before or at 3032 weeks of gestation
• For whom blood sample for thyroid examination has been performed for routine care during his stay in neonatology unit.

Exclusion Criteria:

• Other type of thyroid dysfunction (including, but not exclusively: mother with Basedow disease, congenital hypothyroidism, hyperthyroidism)
• Associated polymalformative sindrome

Reanimation and intensive care units of neonatology in Auvergne-Rhone Alpes region (Lyon Croix Rousse hospital, Lyon Femme mère Enfant hospital, St Etienne Hopital Nord, Grenoble Hopital couple enfant).