Thyroid hormones are chemical messengers produced by a small gland in your neck, regulating various bodily functions. These hormones, primarily thyroxine (T4) and triiodothyronine (T3), influence nearly every cell and organ system. Their signaling pathway helps the body maintain balance and proper function.
Journey of Thyroid Hormones
The thyroid gland, located at the front of your neck, produces thyroid hormones. It creates two main hormones: thyroxine (T4) and triiodothyronine (T3). T4 is produced in larger quantities, making up about 80% of the total thyroid hormone released, while T3 is produced in smaller amounts but is more potent.
The release of these hormones into the bloodstream is controlled by a feedback loop involving the brain. The hypothalamus releases thyrotropin-releasing hormone (TRH), which stimulates the pituitary gland to produce thyroid-stimulating hormone (TSH). TSH then signals the thyroid gland to release T3 and T4.
Cellular Communication
Once released into the bloodstream, thyroid hormones travel to target cells. While most T4 and T3 are bound to carrier proteins in the blood, a small fraction remains unbound and active, ready to enter cells. These hormones then cross the cell membrane, often with the help of specific transporters.
Inside the cell, T4 is frequently converted into the more active T3 by enzymes called deiodinases. T3 then enters the cell’s nucleus, where it binds to specific proteins known as thyroid hormone receptors (TRs). These TRs are a type of nuclear receptor that act as transcription factors, meaning they directly influence gene activity.
The binding of T3 to a TR causes a change in the receptor’s shape, which then allows it to interact with specific DNA sequences called thyroid hormone response elements (TREs) located in the promoter regions of target genes. This interaction either activates or represses the transcription of these genes, leading to the production or inhibition of specific proteins. This precise control over gene expression is how thyroid hormones communicate instructions to cells, orchestrating cellular responses.
Widespread Influence on the Body
Thyroid hormone signaling impacts numerous physiological processes. These hormones influence metabolism, affecting how the body utilizes energy. They increase the basal metabolic rate, which is the amount of energy the body needs at rest, by influencing calorie expenditure and stimulating the breakdown of fats and carbohydrates for energy.
Thyroid hormones also play a role in growth and development, particularly during infancy and childhood, where they are essential for proper bone and brain maturation. They influence the cardiovascular system by affecting heart rate and the strength of heart contractions. They can also impact blood vessel dilation and constriction, affecting blood pressure.
Thyroid hormones contribute to regulating body temperature by influencing cellular energy production. They also affect brain function, where imbalances can impact mood and cognitive processes like memory. The digestive system is also under their influence, as thyroid hormones affect the speed at which food moves through the gastrointestinal tract and the absorption of nutrients.
Disruptions in Signaling
When the thyroid hormone signaling pathway is disrupted, it can lead to health conditions. Hypothyroidism, or an underactive thyroid, occurs when the thyroid gland produces insufficient hormones. This slowdown results in a general slowing of body functions, manifesting as fatigue, weight gain, increased sensitivity to cold, and constipation.
Conversely, hyperthyroidism, an overactive thyroid, involves the production of excessive thyroid hormones. This excess speeds up bodily processes, leading to symptoms such as rapid heart rate, nervousness, anxiety, weight loss despite an increased appetite, and heat sensitivity. Both conditions can also cause an enlargement of the thyroid gland, known as a goiter.