|"Beware the bad cat bearing a grudge"|
So said the study published by Detlef Degner and colleagues . Anti-TPO antibodies by the way, refers to anti-thyroid peroxidase antibodies which, as the name suggests, are antibodies against thyroid peroxidase, an important step in the production of thyroid hormones. Said thyroid hormones have some pretty far-reaching effects on our physiology. Anti-TPO antibodies are also diagnostic for autoimmune related conditions affecting the thyroid such as Hashimoto's thyroiditis.
The Degner paper looked at a small group of participants diagnosed with depression (n=52) and analysed various thyroid related measures compared with a smaller control group made up of 19 participants diagnosed with schizophrenia. Authors reported a "pathologically increased" frequency of anti-TPO antibodies in those with depression compared with those with schizophrenia (32% vs 5% respectively). With something of a rather large confidence interval (CI) and hence the need for quite a bit more investigation, they also reported "the odds ratio of uni- or bipolar patients with depression for an autoimmune thyroiditis was ten times higher... when compared with schizophrenia patients".
Reiterating again the quite small participant numbers, one needs to be rather careful with this particular study before too many firm conclusions are reached. Added to the fact that there was no asymptomatic control group included for study, I'd like to see quite a bit more done in this area before pinning my colours to any particular mast. That being said, this is certainly not the first time that (a) thyroid function has been correlated with depressive symptoms or depressive disorder  and/or (b) elevated levels of anti-TPO antibodies have been linked to depression  also crossing different geographies . The paper by Carta and colleagues  (open-access) further extends the anti-TPO antibody link to "mood and anxiety disorders". This, complete with some discussion about how a "sub-clinical dysfunction of axis Thyrotropin Releasing Hormone (TRH) – Thyroid Stimulating Hormone (TSH) with consequent alteration of circadian rhythms of TSH" might be involved, linking an "aberrancy in the immuno-endocrine system" as a bridge between autoimmunity and psychiatry.
Autoimmune conditions have been previously discussed on this blog as potentially being a risk factor for mood disorder (see here). Under this banner, I'm minded to bring in another paper by Carta and colleagues  discussing how "Anti-TPO prevalence was significantly higher in celiac patients than in the control group" and further: "A higher frequency of PD [panic disorder] and MDD [major depressive disorder] was found in celiac patients with positive anti-TPO when compared to negative anti-TPO patients". This assumes that there may be some elevated risk of autoimmune issues impacting on the thyroid extending into other autoimmune conditions such as celiac (coeliac) disease as per other work. I could start going on about how this research might impact on other peripheral work e.g gluten exposure and feelings of depression but don't want to get too speculative at this point on any correlation with something like gluten or gut permeability.
Suffice to say that outside of just looking at thyroid-stimulating hormone (TSH) levels, the Degner results and other research suggest a whole other ballgame of autoimmune involvement affecting thyroid function and potentially impacting on psychiatry...
Music to close: I Will Wait by Mumford and Sons.
 Degner D. et al. Association between autoimmune thyroiditis and depressive disorder in psychiatric outpatients. Eur Arch Psychiatry Clin Neurosci. 2014 Sep 6.
 Demartini B. et al. Depressive Symptoms and Major Depressive Disorder in Patients Affected by Subclinical Hypothyroidism: A Cross-sectional Study. J Nerv Ment Dis. 2014 Aug;202(8):603-7.
 Pop VJ. et al. Are autoimmune thyroid dysfunction and depression related? J Clin Endocrinol Metab. 1998 Sep;83(9):3194-7.
 Muñoz-Cruzado Poce MJ. et al. Prevalence of thyroid disorders in patients diagnosed with depression. Aten Primaria. 2000 Jul-Aug;26(3):176-9.
 Carta MG. et al. The link between thyroid autoimmunity (antithyroid peroxidase autoantibodies) with anxiety and mood disorders in the community: a field of interest for public health in the future. BMC Psychiatry. 2004 Aug 18;4:25.
 Carta MG. et al. Association between panic disorder, major depressive disorder and celiac disease: a possible role of thyroid autoimmunity. J Psychosom Res. 2002 Sep;53(3):789-93.
Degner D, Haust M, Meller J, Rüther E, & Reulbach U (2014). Association between autoimmune thyroiditis and depressive disorder in psychiatric outpatients. European archives of psychiatry and clinical neuroscience PMID: 25193677