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B5 K) Thyroxine
B5 K) Thyroxine
Thyroxine is produced by the thyroid gland, which is found in the neck. Thyroxine controls the basal metabolic rate (BMR) – BMR is the rate at which chemical reactions happen in the body whilst the body is at rest. Thyroxine causes more chemical reactions to take place; it increases BMR. Thyroxine is also important for stimulating protein synthesis which is important for growth and repair.
The secretion of thyroxine from the thyroid gland is controlled by another hormone called thyroid stimulating hormone (TSH), which is secreted by the pituitary gland (found at the base of the brain). A greater quantity of TSH in blood results in the thyroid gland secreting more thyroxine. A lower quantity of TSH in blood results in the thyroid gland secreting less thyroxine
The concentration of thyroxine in blood is part of a negative feedback system. This is where the body undertakes different actions if the concentration of thyroxine is above or below the optimal levels. The graph for the concentration of thyroxine in blood and time is shown below.
The secretion of thyroxine from the thyroid gland is controlled by another hormone called thyroid stimulating hormone (TSH), which is secreted by the pituitary gland (found at the base of the brain). A greater quantity of TSH in blood results in the thyroid gland secreting more thyroxine. A lower quantity of TSH in blood results in the thyroid gland secreting less thyroxine
The concentration of thyroxine in blood is part of a negative feedback system. This is where the body undertakes different actions if the concentration of thyroxine is above or below the optimal levels. The graph for the concentration of thyroxine in blood and time is shown below.
Let’s now go through what happens when the concentration of thyroxine in blood is too high (the first part of the above graph). When thyroxine levels are too high, the secretion of TSH from the pituitary gland is inhibited (stopped), which results in a lower level of TSH in the bloodstream. A lower level of TSH in the bloodstream means that the thyroid gland will secrete less thyroxine into the bloodstream. This results in the thyroxine levels in blood falling back to their normal/ optimal level, which means that the BMR falls as well back to the normal level.
If the levels of thyroxine in blood are too low (the second part of the above graph), the opposite will happen. When thyroxine levels are too low, the pituitary gland will secrete more TSH into the bloodstream. A greater level of TSH in the bloodstream stimulates the thyroid gland to secrete more thyroxine into the bloodstream, which causes blood thyroxine levels to increase back to their normal/ optimal level. This increases BMR to the normal/ optimal level.
If the levels of thyroxine in blood are too low (the second part of the above graph), the opposite will happen. When thyroxine levels are too low, the pituitary gland will secrete more TSH into the bloodstream. A greater level of TSH in the bloodstream stimulates the thyroid gland to secrete more thyroxine into the bloodstream, which causes blood thyroxine levels to increase back to their normal/ optimal level. This increases BMR to the normal/ optimal level.
Underactive Thyroid Gland
Sometimes individuals can have an underactive thyroid gland, which is where their thyroid gland does not secrete enough thyroxine. This causes their blood thyroxine levels to be lower than they should be, which results in them having a lower BMR. As their BMR is lower, less glucose is broken down during respiration reactions, which means that the glucose is stored as fat instead. This can result in individuals with underactive thyroid glands gaining weight.
Sometimes individuals can have an underactive thyroid gland, which is where their thyroid gland does not secrete enough thyroxine. This causes their blood thyroxine levels to be lower than they should be, which results in them having a lower BMR. As their BMR is lower, less glucose is broken down during respiration reactions, which means that the glucose is stored as fat instead. This can result in individuals with underactive thyroid glands gaining weight.