What is already known about this subject
It is known that central neurotransmitters system regulates/influences the hypothalamic–pituitary–thyroid axis. Activation of central dopaminergic and serotonergic system has been reported to inhibit hypothalamic–pituitary–thyroid axis, reduce TSH releasing and thyroid hormones and may therefore result in hypothyroxinemia (normal TSH concentrations and a disproportionately low concentration of free T4). The laboratory criteria of hypothyroxinemia, defined as “normal TSH and Low free T4”, overlaps, to a great extent, with the laboratory criteria of central hypothyroidism (low or low-to-normal TSH concentrations and a disproportionately low concentration of free T4). However, hypothyroxinemia or central hypothyroidism could be easily ignored in patients treated with mirtazapine, a noradrenergic and specific serotonergic antidepressant, if attention is paid to only patients with elevated TSH.
We have conducted a thorough search of literature on PubMed, using a combination of “mirtazapine” AND “thyrotropin OR thyroid-stimulating hormone OR TSH OR thyroxine OR free T4 OR Thyroid function OR hypothyroxinemia OR hypothyroidism”. This search found only one relevant paper that assessed thyroid function in 17 outpatients affected by major depressive disorder and treated by mirtazapine. A significant decrease in free T4 but no change in TSH was observed in that study. However, that study did not report whether free T4 was dropped to the cutoff level defining hypothyroxinemia. Most importantly, no comparison with non-mirtazapine users among patients affected by major depressive disorder was made.