AUTHOR=Raij Tommi , Raij Kari 

TITLE=Association between fatigue, peripheral serotonin, and L-carnitine in hypothyroidism and in chronic fatigue syndrome

JOURNAL=Frontiers in Endocrinology

VOLUME=15

YEAR=2024

URL=https://www.frontiersin.org/journals/endocrinology/articles/10.3389/fendo.2024.1358404

DOI=10.3389/fendo.2024.1358404

ISSN=1664-2392

ABSTRACT=<sec><title>Background</title><p>Fatigue of unknown origin is a hallmark symptom in chronic fatigue syndrome (CFS) and is also found in 20% of hypothyroidism patients despite appropriate levothyroxine treatment. Here, we suggest that in these disorders, peripheral serotonin levels are low, and elevating them to normal range with L-carnitine is accompanied with reduced fatigue.</p></sec><sec><title>Methods</title><p>We conducted a retrospective analysis of follow-up clinical data (CFS N=12; hypothyroidism with fatigue N=40) where serum serotonin and fatigue levels were compared before <italic>vs.</italic> after 7 weeks of oral L-carnitine supplementation.</p></sec><sec><title>Results</title><p>After L-carnitine, serotonin increased (8-fold in CFS, Sig. = 0.002, 6-fold in hypothyroidism, Sig. &lt; 0.001) whereas fatigue decreased (2-fold in both CFS and hypothyroidism, Sig. = 0.002 for CFS, Sig. &lt; 0.001 for hypothyroidism). There was a negative correlation between serotonin level and fatigue (for CFS, rho = -0.49 before and -0.67 after L-carnitine; for hypothyroidism, rho = -0.24 before and -0.83 after L-carnitine).</p></sec><sec><title>Conclusions</title><p>These findings suggest a new link between low peripheral serotonin, L-carnitine, and fatigue.</p></sec>