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Thyroid Gland
The thyroid gland is located along the front portion of the windpipe, positioned right below the Adam's apple. It contains specialized cells that are responsible for synthesizing and releasing three important hormones. The two lipid-soluble hormones are triiodothyronine (T3) and thyroxine (T4) while the water-soluble polypeptide hormone is called calcitonin. The T3 and T4 hormones are both tyrosine derivatives and require carriers within the blood. They can easily travel across the membrane of the cell and enter the nucleus of the target cell, where they act at the transcriptional level. T3 and T4 hormones act in very similar ways and are responsible for resetting the basal metabolic rate of the human body. This means they can affect processes such as cellular respiration, the contraction of the heart, protein synthesis and degradation, and many more. They are also crucial in human growth and development. T3 and T4 hormones are produced in thyroid cells called follicular cells. Hypothyroidism and hyperthyroidism are two abnormalities of the thyroid that can affect the human in different ways. Calcitonin is produced by parafollicular cells (C-cells) and is a water-soluble large polypeptide. This means that calcitonin can travel within the blood without any protein carrier and it binds onto protein receptors found on the membrane of the cell. Calcitonin is stimulated and released when the blood calcium concentration is high. It can decrease the plasma concentration in three ways - by inhibiting the kidneys from absorbing calcium into the body, it can decrease the amount of calcium absorbed in the intestines and it can also cause the bone to absorb more calcium from the blood by decreasing the activity of osteoclasts and increasing the activity of osteoblasts.
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