Thyroid hormone increases mannan-binding lectin levels

Background: Recent studies have indicated the existence of causal links between the endocrine and immune systems and cardiovascular disease. Mannan-binding lectin (MBL), a protein of the innate immune system, may constitute a connection between these fields. Methods: To test whether thyroid hormone regulates MBL levels, we studied eight patients with Graves’ hyperthyroidism before and after methimazole therapy, eight healthy subjects before and after short-term experimental hyperthyroidism, and eight hypothyroid patients with chronic auto-immune thyroiditis before and after L-thyroxine substitution. Results: In all hyperthyroid patients, MBL levels were increased – median (range), 1886 ng/ml (1478–7344) – before treatment and decreased to 954 ng/ml (312–3222) after treatment (P = 0.01, paired comparison: Wilcoxon’s signed ranks test). The healthy subjects had MBL levels of 1081 ng/ml (312–1578). Administration of thyroid hormones to these persons induced mild hyperthyroidism and increased MBL levels significantly to 1714 ng/ml (356–2488) (P = 0.01). Two of the eight hypothyroid patients had undetectably low levels of MBL both before and after L-thyroxine substitution. The other six hypothyroid patients had decreased levels of MBL of 145 ng/ml (20–457) compared with 979 ng/ml (214–1533) after L-thyroxine substitution (P = 0.03, paired comparison: Wilcoxon’s signed ranks test). Conclusion: Our data show that thyroid hormone increases levels of MBL. MBL is part of the inflammatory complement system, and this modulation of complement activation may play a role in the pathogenesis of a number of key components of thyroid diseases.