"In humans, neuroendocrine challenge studies in hypothyroid patients have shown a reduced 5-HT responsiveness that is reversible with thyroid replacement therapy. In adult animals with experimentally-induced hypothyroid states, increased 5-HT turnover in the brainstem is consistently reported while decreased cortical 5-HT concentrations and 5-HT2A receptor density are less frequently observed. In the majority of studies, the effects of thyroid hormone administration in animals with experimentally-induced hypothyroid states include an increase in cortical 5-HT concentrations and a desensitization of autoinhibitory 5-HT1A receptors in the raphe area, resulting in disinhibition of cortical and hippocampal 5-HT release. Furthermore, there is some indication that thyroid hormones may increase cortical 5-HT2 receptor sensitivity."
So how common do you think hypothyroidism is? What do you think is the most prevalent cause? In what percentage of cases do you think low thyroid function is the cause of depression?
I think that the incidence of hypothyroidism is far greater than has been captured by the medical/scientific community. There are a great many people whose lab numbers are considered "normal" by endocrinologists, yet who have very clear symptoms of hypo (and whose other lab numbers really should cause DRs to suspect hypo). It is a sad situation for those folks with "mysterious" and debilitating physical and psychological symptoms who could be easily helped with a little thyroid replacement medication.
I'm starting to suspect that a great deal of hypo may be caused by damage of the thyroid gland resulting from chronic gluten exposure.
Autoimmune thyroid disease (Hashimotos thyroiditis) is the most common cause in Western countries, iodine deficiency worldwide. It's debatable whether being on the low end of normal is a common feature of relatively low iodine intake, and whether that's worse for paleos not eating iodised salt and eating a lot of goitrogens e.g. cruciferous vegetables. I read a case study of a Chinese women in the US who ate large amounts of Pak Choi every day and developed hypothyroid coma, but that was an extreme case!
In the large majority of depression cases there are other reasons, and thyroid function tests come back normal (though it's true this classes the whole normal range as the same and could mask a protective effect of being in the upper part of normal).