Thyroid hormones have been extensively investigated in major depression but abnormalities in thyroid hormones have not been found. Still in terms of treating depression thyroid hormones have had attractions. Meta-analyses indicate that augmentation of anti-depressants with triiodothyronine (T3) or thyroxine (T4) and can be helpful in the treatment of refractory depression.
There are definite difficulties with thyroid function in major depression but the problem is too little iodine. With dysregulation of the sodium-dependent multivitamin transporter (SMVT) iodine transport is impaired. The SMVT transports iodine. The role of the SMVT in the transport of iodine has not been emphasized to the same extent as the role of the SMVT in the transport of biotin and pantothenic acid. As it turns out the SMVT plays a key role in iodide homeostasis which if upset can result in major depression despite normal levels of thyroid hormones.
As an augmentation strategy to anti-depressants iodine would be taken three times a day. The Tolerable Upper level Limit for iodine for adults is 1100 micrograms per day. Tests on thyroid function would have to be obtained. Abnormal levels of thyroid hormones as shown by tests on thyroid function must be avoided.
If iodine can augment anti-depressants in the treatment of major depression this would point to the SMVT as playing a key role in psychiatric illnesses.