Inhibitory effects of thyroxine on cytokine production by T cells in mice

As a sensitive factor of endocrine system, thyroxine exerts regulatory activities on a variety of cell types in immune system including T lymphocytes. Cytokines secreted by T lymphocytes are widely involved in numerous mechanisms, which are crucial for homeostasis at various statuses. Here we investigated the effects of thyroxine on the cytokine production of T lymphocytes in vivo and in vitro, using BALB/c mice with thyroxine treatment. We examined the IFN-γ, IL-2, IL-4 and IL-10 of T-cell type cytokines transcriptions and the proportions of IFN-γ, IL-4 and IL-10-secreting CD4+/CD8+ T cells. The results showed that the administration of high-level thyroxine induced weakly expression of T-cell type cytokine in transcriptional level together with impaired responsibility to mitogen (Con A) stimulation. Along with the severe suppression on T-cell type cytokine transcription, the proportions of IFN-γ, IL-4 and IL-10-producing T lymphocytes were markedly attenuated and the productions of serum IFN-γ and IL-10 were significantly decreased synchronously. The repressive effects of thyroxine on T-cell type cytokine were seen both in mRNA and protein level. The results suggested that T lymphocytes under normal condition appear to be sensitive to suppression of high-level thyroxine administration.