Thyroid hormone transporters and deiodinases in the developing human hypothalamus

Thyroid hormone (TH) signaling in brain cells is dependent on transport of TH across the plasma membrane followed by intracellular deiodination and binding to the nuclear TH receptors. The aim of this study is to investigate the expression of the specific TH transporters monocarboxylate transporter 8 (MCT8 (SLC16A2)), MCT10, organic anion transporting polypeptide 1C1 (OATP1C1 (SLCO1C1)), and the types 2 and 3 deiodinases (D2 and D3) in the developing human hypothalamus.

Fifteen postmortem brain samples of fetuses and young children ranging between 17 weeks of gestation and 29 months of postnatal age including one child (28 months) with central congenital hypothyroidism were studied.

Sections of the different hypothalami were stained with polyclonal rabbit antisera against MCT8, MCT10, OATP1C1, D2, and D3.

We found MCT8 and D3 but not D2 protein expression to be present in our earliest sample of 17 weeks of gestation, indicating triiodothyronine degradation, but not production at this time of development. At term, expression of TH transporters and D2 decreased and D3 expression increased, suggesting decreased TH signaling just before birth. The child with central congenital hypothyroidism showed higher MCT8 and D2 expression compared with the other children of similar age.

This study reports the developmental timing of expression of components crucial for central TH signaling in the human hypothalamus. In general, during fetal hypothalamic development, the coordinated expression of D2 and D3 in combination with the different TH transporters suggests that proper TH concentrations are regulated to prevent untimely maturation of brain cells.