Figure 2.

Synchronization by light input of the rhythmic variations in hypothalamic–pituitary–thyroid secretions: It acts directly, through the retino-hypothalmic tract (RHT), by exciting the light-entrainable circadian pacemaker located in the suprachiasmatic nucleus (SCN) of the hypothalamus, which then outputs the circadian signal via neural projection, exciting the rhythmic secretion of thyrotropin-releasing hormone (TRH) and consequently of thyrotropin (TSH) and thyroid hormones(T4:thyroxine; T3:triiodotyronine). Light acts also indirectly, by modulating with an inhibiting effect the variations in melatonin secretion at retina and pineal gland levels, thus further stimulating the secretions of the hypothalamic–pituitary–thyroid axis.