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.