A hypothesis of thyroid-catecholamine-receptor interaction. Its relevance to affective illness.

Recent prospective studies suggest that thyroid state plays a role in affective disorders. A lack of thyroid hormones can lower the threshold for depression; an excess can contribute to a state of tense dysphoria. Thyroid function in some persons also appears to influence the course of affective disorders. Adequate mobilization of thyroid hormones favors recovery from depression; excess mobilization increases the risk of mania in vulnerable individuals. Although other mechanisms may be involved, evidence suggests that the modulation by thyroid hormones of the beta-adrenergic receptor response to catecholamines may contribute to these effects. Norepinephrine stimulates such receptors; thyroid hormones increase their ability to receive stimulation. The plausibility of such interactions between catecholamines and thyroid hormones occurring in the CNS is strengthened by their common origin in the amino acid tyrosine and by their synergism in many metabolic processes.