© 2017, The Author(s).Background: Previous studies of our group among bipolar offspring and bipolar twins showed significant higher prevalence’s and levels of antithyroid peroxidase antibodies (TPO-Abs) in offspring and co-twins (without a mood disorder) compared to controls, suggesting that TPO-Abs might be considered as vulnerability factor (trait marker) for BD development. Objectives: Here we elucidate, in the same cohorts, but now after 12- and 6-year follow-up, whether TPO-abs should be considered as a ‘trait’ marker for BD. The present study aims to investigate whether TPO-Abs (1) are stable over time, (2) are associated with lithium-exposure, (3) share a common genetic background with BD and are related to psychopathology. Results: In bipolar offspring and twins, the prevalence of TPO-Abs is stable over time (rs = .72 p < .001 resp. rs = .82, p < .001) and not associated with lithium use. At follow-up, an increased prevalence of TPO-abs was again observed in bipolar offspring (10,4% versus 4%) and higher TPO-abs titers were still present in co-twins of bipolar cases compared to control twins [mean 1.06 IU/ml (SD.82) versus mean.82 IU/ml (SD.67)], although statistical significance was lost. Conclusions: Although our results show a trend toward an increased inherited risk of the co-occurrence of BD and thyroid autoimmunity, large-scale studies can only draw final conclusions. Nationwide epidemiological and GWAS studies reach such numbers and support the view of a possible common (autoimmune) etiology of severe mood disorders and chronic recurrent infections and autoimmunity, including thyroid autoimmunity.