Thyroxine – Inhibitor of Migration Inhibitory Factor
A study published in the Proceedings of the National Academy of Sciences of the United States of America (PNAS) suggests that thyroxine (T4) is a strong endogenous inhibitor of the migration inhibitory factor (MIF) pro-inflammatory activities. Importantly, the study also indicates that low T4 and high MIF levels may contribute to the lethal systemic inflammatory response during severe sepsis.
Low circulating thyroid hormone levels are common in critically ill patients with severe infections and about 60% of these patients have abnormally low plasma T4 levels, with the lowest levels being observed in patients with sepsis.
The mechanism behind this phenomenon, often referred to as “euthyroid sick syndrome” remain poorly understood. Euthyroid sick syndrome (ESS) is a state of adaptation or dysregulation of thyrotropic feedback control wherein the levels of T3 and/or T4 are abnormal, but the thyroid gland does not appear to be dysfunctional. The classical phenotype of this condition is often seen in starvation, critical illness, or patients in the intensive care unit. Similar endocrine phenotypes are observed in fetal life and in hibernating mammals.
This condition occurs in the setting of a nonthyroidal illness (NTI), without preexisting hypothalamic-pituitary and thyroid gland dysfunction. The most prominent alterations are low serum triiodothyronine (T3) and elevated reverse T3 (rT3), leading to the general term “low T3 syndrome. But approximately 60% of critically ill patients have abnormally low plasma T4 levels, with the lowest levels being observed in patients with sepsis. These low thyroid hormone levels are good indicators of disease severity and predictors of mortality.
Macrophage migration inhibitory factor (MIF), also known as glycosylation-inhibiting factor (GIF), L-dopachrome isomerase, or phenylpyruvate tautomerase is a protein that in humans exerts important regulatory effects on innate immunity. Bacterial antigens stimulate white blood cells to release MIF into the blood stream. The circulating MIF binds to CD74 on other immune cells to trigger an acute immune response.
Migration inhibitory factor is a cytokine involved in cell-mediated immunity, immunoregulation, inflammation, and is also known to be induced by glucocorticoids, and being able to override their anti-inflammatory actions.
Glucocorticoids are known to stimulate white blood cells to release MIF and hence MIF partially counteracts the inhibitory effects that glucocorticoids have on the immune system. Importantly, trauma activates the anterior pituitary gland to release MIF.
Migration inhibitory factor is implicated in the pathogenesis of systemic infections, autoimmune diseases, cancer, metabolic disorders and atherosclerosis. Of note, plasma MIF levels are significantly elevated in non-survivors of severe sepsis, compared with survivors, and administration of antibodies against MIF improves survival in experimental sepsis.
The authors of the PNAS study review the literature and previous work indicating that during sepsis, plasma MIF levels can be higher than 180 ng/mL, and the increased accumulation of MIF can have a profound effect on organ function and mortality.
In the PNAS study, Yousef Al-Abed and colleagues from the Feinstein Institute for Medical Research, Manhasset, NY, USA, provide evidence for an inverse correlation between plasma T4 and MIF levels during the progression of sepsis, and identify T4 as a potential endogenous antagonist of MIF inflammatory activity.
Consistent with these human data, plasma free-T4 levels decreased and plasma MIF levels increased within hours after the onset of severe sepsis in rats induced by cecal ligation and punctur. Moreover, administration of exogenous D-T4 significantly improved survival in mice with severe sepsis.
The data suggest that low T4 and high MIF may be primary effectors in the lethal systemic inflammatory response during severe sepsis as modulation of either (T4 replacement or MIF antagonism) can improve survival.
According to the authors the low plasma free T4 levels “could result from a significant endogenous T4 fraction binding the hydrophobic pocket within the MIF molecule”. As during sepsis, plasma MIF levels can reach high levels, “this binding may not be sufficient to effectively inhibit MIF pro-inflammatory activity and prevent an overwhelming inflammatory response”.
These findings indicate a previously unrecognized but clinically relevant interaction between T4 and MIF, two key molecules in the critically ill patient with severe infections.
SOURCE: Proc Natl Acad Sci USA 2011, 108:8224. Epub 2011 May 2
