Comparison of baseline characteristics between the two study
groups
Table 1 compares the distribution of demographics and clinical
characteristics of two groups. There were no significant differences in
age, gender, duration of illness, and uses of antipsychotics,
benzodiazepines, anxiolytics, and selective serotonin reuptake
inhibitors (SSRIs) between the two groups. The uses of
serotonin-norepinephrine reuptake inhibitors (SNRIs) or trazodone were
higher in the non-mirtazapine group than those in the mirtazapine group
(P <0.01).
There were no differences between the two groups with respect to
baseline level of thyroid function measurements (Table 1). On the other
hand, baseline free T4 levels were negatively associated with risk of
hypothyroxinemia, with adjusted RR= 0.88 (95% CI: 0.86-0.90) for 1
pmol/L increment in baseline free T4.
Analysis of association
between mirtazapine use and occurrence of hypothyroxinemia
The rate of hypothyroxinemia in the
mirtazapine group (33/88, 37.5%)
was higher than in the non-mirtazapine group (26/132, 19.7%). After
adjusting for age, gender, duration of treatment, baseline free T4
level, use of other antidepressants (SSRIs, SNRIs or trazodone), use of
non-antidepressant medications and other potential confounders, patients
who received mirtazapine as an adjunctive antidepressant were more
likely to develop hypothyroxinemia compared with those who did not
receive mirtazapine (adjusted RR: 1.64, 95% CI: 1.31-1.78) (Table 2).
Analysis of association
between mirtazapine use and other thyroid dysfunctions
In the mirtazapine group, the reduction in T4 was 15.63 nmol/L, and was
3.22 pmol/L in free T4, which were significantly greater than those in
non-mirtazapine group (t =-3.053, P =0.003, andt =-2.861, P =0.005, for ΔT4 and Δ free T4, respectively)
(Table 3). In the multiple linear regression model, mirtazapine use was
statistically significantly associated with greater reduction in free T4
level [(Δ free T4(2–1))] after adjusting for
confounders, with adjusted B -1.14 (95% CI: -1.88~
-0.41, P =0.002). The occurrence of subclinical and overt
hypothyroidism was similar in the two groups (Table 3).