Thyroid function and serotonin activity are highly linked. Serotonin stimulates hypothalamic TRH (Thyroid Releasing Hormone) production, leading to an increase in TSH (Thyroid Stimulating Hormone) production from the pituitary.
Serotonin stimulates hypothalamic TRH production which leads to an increase in TSH production from the pituitary. Adequate serotonin production is necessary to maintain thyroid hormone levels. Theoretically, an excess of serotonin may lead to the opposite reaction. For example, excess serotonin can lead to a reduction in dopamine production, which can then lead to an increase in norepinephrine production, leading to an increased stress response and cortisol production.
When your serotonin levels are low, you feel anxious, depressed, and pessimistic, and you might also struggle with sleep problems — all common symptoms associated with hyperthyroidism.