"Thyroid function and serotonin activity are highly linked. Both need to be addressed to optimize function.
T3 (the active thyroid hormone created from T4 in the liver by the 2D6 enzyme) desensitizes presynaptic serotonin autoreceptors - thus leading to an increase in serotonin production. Giving T3 induces serotonin production. Similarly, in hypothyroidism, serotonin production is reduced. T3 augments the effects of serotonin-increasing medications (such as the SSRIs) by the additive effect both have on desensitizing presynaptic serotonin autoreceptors.
Serotonin stimulates hypothalamic TRH production, leading to an increase in TSH production from the pituitary. Adequate serotonin production is necessary to maintain thyroid hormone levels. Increasing serotonin levels with an SSRI may thus help improve thyroid hormone production.
A caveat: Theoretically, an excess serotonin may lead to the opposite reaction. For example, excess serotonin leads to reduction in dopamine production, which then leads to increased norepinephrine production, leading to an increased stress response and cortisol production.
High levels of cortisol (which can also be caused by high stress levels, low testosterone, etc.) can directly lead to suppression of pituitary TSH secretion, and impair conversion of T4 to T3, and can impair serotonin function (by reducing serotonin receptor density, increasing serotonin uptake via increase in serotonin transporter production, and by increasing tryptophane oxygenase production in the liver - thus reducing tryptophan, the precursor to serotonin)."