What is likely to cause the neoate's thyroxine levels to fall below normal reference ranges?

The decrease in a neonate's thyroxine levels falling below normal reference ranges due to inadequate protein feeding is grounded in the relationship between nutrition and thyroid hormone synthesis. Thyroxine (T4) is synthesized from the amino acid tyrosine and requires adequate protein intake for proper production. Inadequate protein feeding can lead to insufficient building blocks necessary for hormone synthesis, subsequently impairing the thyroid's ability to produce adequate amounts of thyroxine.

In neonates, the thyroid gland may already be less active or immature compared to older children or adults; therefore, the effects of inadequate nutrition can lead to even more pronounced deficiencies and an inability to maintain hormone levels within the normal range. This situation highlights the importance of adequate nutrition, particularly protein, in maintaining normal endocrine function and metabolic balance in newborns.

While congenital hypothyroidism may also explain low thyroxine levels, as it is a primary issue with thyroid function that is present at birth, the option refers specifically to inadequate protein feeding as a reversible cause of low thyroxine levels, making it a more direct factor related to nutrition rather than an intrinsic glandular failure.