What is the effect of thyroid hormones on the nervous system?

From the Guidelines

Thyroid hormones have a profound impact on the nervous system, influencing development, function, and overall neurological health, with excess thyroid hormones leading to symptoms such as nervousness, tremor, and increased heart rate, as seen in conditions like thyrotoxicosis and hyperthyroidism 1. The effects of thyroid hormones on the nervous system are multifaceted, involving the regulation of neuronal development, myelination, and synaptic function. In conditions characterized by an excess of thyroid hormones, such as thyrotoxicosis, hyperthyroidism, thyroiditis, and Graves’ disease, patients may experience increased heart rate, tremor, and nervousness due to the excess hormone production and secretion 1.

Key Effects on the Nervous System

• Regulation of neuronal development and myelination
• Influence on synaptic function and neurotransmitter systems, including norepinephrine, serotonin, and GABA pathways
• Impact on cerebral blood flow and glucose metabolism in the brain
• Binding to nuclear receptors that regulate gene expression in neurons and glial cells, affecting energy metabolism, protein synthesis, and neurotransmitter function In the context of real-life clinical medicine, it is crucial to recognize the significance of thyroid hormones in maintaining optimal nervous system health, and to manage conditions of thyroid dysfunction appropriately to prevent or mitigate neurological and psychiatric symptoms.

Clinical Implications

• Proper diagnosis and treatment of thyroid dysfunction are essential for preventing long-term neurological damage
• Beta-blockers, such as propranolol, may be used to manage symptoms associated with excess thyroid hormones, including increased heart rate and tremor 1
• Individualized treatment approaches, considering medical comorbidities and specific patient needs, are necessary for optimal management of thyroid-related conditions and their effects on the nervous system.

From the FDA Drug Label

The mechanisms by which thyroid hormones exert their physiologic action are not well understood. These hormones enhance oxygen consumption by most tissues of the body, increase the basal metabolic rate and the metabolism of carbohydrates, lipids and proteins Thus, they exert a profound influence on every organ system in the body and are of particular importance in the development of the central nervous system.

The effect of thyroid hormones on the nervous system is that they exert a profound influence and are of particular importance in the development of the central nervous system.

• Key points:
  • Thyroid hormones have a profound influence on every organ system, including the nervous system.
  • They are particularly important in the development of the central nervous system. 2

From the Research

Effects of Thyroid Hormones on the Nervous System

• Thyroid hormones have a significant impact on the central nervous system, with effects on noradrenergic, serotonergic, and dopaminergic receptor function, as well as influence on second messenger, calcium homeostasis, axonal transport mechanisms, and morphology 3.
• The effects of thyroid hormones on the brain are highly specific with respect to brain area, and the biochemical mechanisms and physiological importance are not well understood 3.
• Thyroid hormones play a crucial role in the normal development of the central nervous system and have been implicated in cognitive functions in the adult brain, including learning and memory 4.
• Non-classical thyroid hormones, which were previously considered inactive breakdown products, have been shown to produce significant biological effects and may be involved in the development of Alzheimer's disease 4.

Neuroprotective Effects of Thyroid Hormones

• Thyroid hormones have been shown to exert a neuroprotective effect in brain areas involved in learning and memory, and may be involved in the development of Alzheimer's disease 4.
• Both classical and non-classical thyroid hormones act as neuroprotective agents in the brain, supporting the idea that they may be involved in the development of Alzheimer's disease 4.
• Thyroid hormone signalling in the central nervous system has been implicated in the control of energy metabolism, with effects on the integration of the sympathetic signal in target tissues 5.

Mechanisms of Thyroid Hormone Action

• Thyroid hormones affect target tissues directly via thyroid hormone receptors, but also indirectly through effects on the integration of the sympathetic signal in target tissues 5.
• The pre-autonomic neurones in the central nervous system, which reside in the hypothalamus, are involved in the modulation of sympathetic signalling to target tissues, affecting glucose and lipid metabolism 5.
• Thyroid hormones trigger biological effects both by directly affecting gene expression through the interaction with nuclear receptors and by activating protein kinases and/or ion channels 6.

Movement of Thyroid Hormones in the Central Nervous System

• Thyroid hormones in the central nervous system present a number of complexities, including a marked difference in hormone effects on the developing brain compared to the mature brain, and extensive regional heterogeneity within the brain 7.
• The blood brain barrier and the choroid plexus may work in tandem to maintain brain interstitial levels of thyroid hormone at near constant levels, with thyroid hormone entering neurons directly from the interstitial fluid or indirectly from glial cells 7.