HTR5A and migraine disorder: Chugani et al found that the synthesis capacity of 5‐HT increased in migraine patients.50 Plasma levels of 5‐HT decreased in migraine attack interphase and increased during migraine attacks.51 In one case study of a pair of sisters who had hemiplegic migraine, the systemic 5‐HT levels were low, including levels of 5‐HT metabolite 5‐hydroxyindoleacetic acid (5‐HIAA) in the cerebrospinal fluid and platelet 5‐HT.