The hypothalamic-pituitary-thyroid axis (HPT axis) is a neuroendocrine system that regulates metabolism. When the hypothalamus senses low circulating levels of the hormones T3 and T4, it signals to the pituitary, which then signals the thyroid gland to release T3 and T4. T4 normally is converted to the more active T3, but T4 can also be converted to reverse T3 (rT3). Reverse T3 works against the T3 receptor, so high levels can be detrimental.
During illness, profound changes may occur in the hypothalamic-pituitary-thyroid (HPT) axis. The most consistent change is a decrease in serum tri-iodothyronine (T3) level, but in severe illness, thyroxine (T4) may also decrease. The persistence of a normal or even decreased level of thyrotropin (TSH) in the face of decreased serum thyroid hormone concentrations implies a major change in HPT axis set-point regulation. Since these abnormalities of thyroid hormone concentration usually occur without any evidence of thyroid disease and disappear with recovery, they have been referred to as the `sick euthyroid syndrome’ or the `euthyroid sick syndrome’.
TSH serum levels are lower and those of free T4 are greater at night, when melatonin levels are higher, so that the response of pituitary to hypothalamic TRH and of thyroid to pituitary TSH is influenced by the pineal hormone melatonin, which alters the hypothalamic-pituitary-thyroid (HPT) axis function. Melatonin drives the molecular clockwork in the pituitary.
Melatonin & Thyroid Function
Melatonin has a suppressing action on thyroid function. Both hypothyroid and thyrotoxic patients have disturbed pineal function, which is not the case in those with weight issues. Those with hypothyroidism were found to have higher peak melatonin levels, total nighttime melatonin secretion, and urinary elimination of melatonin than normal individuals. Although thyrotoxic patients released a normal amount of melatonin during the night, their melatonin secretion peak occurs earlier in the night.
The molecular clockwork in the pituitary is strongly dependent on melatonin. Melatonin drives the rhythmic expression of clock genes in the pituitary, and the length of daytime light as well as melatonin supplements are involved in melatonin signaling.
Melatonin plays a role in the regulation of TSH release from the pituitary. Short days and long nights are correlated with decreasing levels of TSH in the pituitary. Moreover, chronic treatment with melatonin suppresses TSH release from the pituitary.
Melatonin has an inhibitory action on the Hypothalamic-pituitary-thyroid (HPT) axis. Long nights result in reduced levels of circulating thyroxin (T4). An active pineal gland produces melatonin, which inhibits thyrotrophin-releasing hormone (TRH) release from the hypothalamus. The effects of melatonin on the Hypothalamic-pituitary-thyroid (HPT) axis are similar to its effects on the Hypothalamic-pituitary-gonadal (HPG) axis.
Melatonin supplementation inhibits the TSH content in the pituitary. However, Melatonin supplementation blocks the stimulatory effect of TSH on thyroid cells responsible for the production and secretion of thyroid hormones thyroxine (T4) and triiodothyronine (T3). Free T3, T4 and TSH levels are lower with melatonin supplementation.
Melatonin is Not Just for Sleep Anymore
While most consider melatonin to be only produced by the pineal gland. It is produced throughout the body in much greater quantity (400x more in the gut). Melatonin is much more involved than previously thought. From enhancing autoimmune flairs during cytokine storms, to playing a role with infertility, melatonin and melatonin supplements used for ‘sleep problems’ enhances these conditions and not in a good way. It even plays a role in hot flashes and night chills.
If you have questions about sleep and you are having thyroid, autoimmune or infertility concerns with your health. Please contact my office.