The symptoms and signs of hyperthyroidism resemble those of primary mental disorders. Overactivity of the adrenergic system caused by hyperthyroidism may explain the similarity between the clinical presentations of hyperthyroidism and mania or anxiety, as well as the precipitating role of hyperthyroidism in the development of mania or anxiety disorder. It may also explain the increased sense of well being often experienced in the early stages of hyperthyroidism.[20,21]
The relationship between hyperthyroidism and depression is less clear. Depression is usually linked to hypothyroidism, not to hyperthyroidism. However, prolonged hyperthyroidism might exhaust noradrenergic transmission and thus contribute to depression. Noradrenergic exhaustion might well occur in patients with hyperthyroidism who have bipolar disorder. In the initial phase of hyperthyroidism, thyroid hormone stimulation of the noradrenergic system may cause mania; later, when noradrenergic neurotransmission is exhausted, it may contribute to depression.[21]
Mental symptoms and disorders secondary to hyperthyroidism should be treated first by restoring euthyroidism. Most mental symptoms, including depression, usually resolve once euthyroidism has been regained. Treatment with beta-adrenergic antagonists alone may quickly relieve many symptoms, including mental symptoms, even if euthyroidism is not restored,[22] providing evidence that overactivity of the adrenergic system is largely responsible for mental symptoms in hyperthyroidism.
Thyroid deficits are frequently observed in bipolar patients, especially in women with the rapid cycling form of the disease.[24] Both subclinical hypothyroidism and subclinical hyperthyroidism increase the risk for Alzheimer's disease, especially in women.[25] However, most hypothyroid patients do not meet the criteria for a mental disorder.
A recent study evaluated brain glucose metabolism during T4 treatment of hypothyroidism. A reduction in depression and cognitive symptoms was associated with restoration of metabolic activity in brain areas that are integral to the regulation of mood and cognition.[26••]
In hypothyroidism, replacement therapy with T4 remains the treatment of choice and resolves most physical and psychological signs and symptoms in most patients. However, some patients do not feel entirely well despite doses of T4 that are usually adequate.[27] In T4-treated patients, it was found that reduced psychological well being is associated with occurrence of polymorphism in the D2 gene,[28••] as well as in the OATP1c1 gene.[29]
Thyroid hormone replacement with a combination of T4 and T3, in comparison with T4 monotherapy, improves mental functioning in some but not all hypothyroid patients,[30,31•] and most of the patients subjectively prefer combined treatment.[32] Two studies have evaluated whether D2 polymorphism is associated with changes in psychological well being after combined T4 and T3 treatment. One underpowered study[33] reported a trend toward improvement. In a second study[28••] involving a very large sample, D2 polymorphism was associated with improvement in psychological well being after T4 and T3 treatment.
Thyroid Disease and Mental Disorders: Cause and Effect or Only comorbidity?
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