Until a little more than one hundred years ago, the single controlling force for all of the complex processes that go on in the human body was thought to be the nervous system. But there were too many phenomena that, when carefully analyzed, seemed to have no relationship to the nervous system, too many differences in people --- in size and energy, for example --- that could not be accounted for satisfactorily in terms of nervous activity alone. The explanation was to be found in certain glands, the endocrines, of which the thyroid is one and, in fact, one of the first to be discovered. Because commonly used tests for thyroid function are not accurate particularly when it comes to mild and even some moderate forms of hypothyroidism, and many if not most of those with low thyroid function remain undiscovered.
Since the hormones of the thyroid gland regulate metabolism in every cell of the body, a deficiency of thyroid hormones can affect virtually all bodily functions. The degree of severity of symptoms in the adult range from mild deficiency states which are not detectable with standard blood tests (subclinical hypothyroidism) to severe deficiency states which can be life-threatening (myxedema). There is an old medical saying that just a few grains of thyroid hormone can make the difference between an idiot and an Einstein. It aptly characterizes the thyroid as a quickener of the tempo of life.
All of the endocrine glands play remarkable roles in the body's economy. Unlike the many millions of other glands such as the sweat glands in the skin, the salivary glands in the mouth, the tear glands in the eyes, which perform only local functions, the endocrine glands pour their hormone secretions into the bloodstream which carries them to all parts of the body. From the pea-sized pituitary gland at the base of the brain come hormones that influence growth, sexual development, uterine contraction in childbirth, and milk release afterward. The adrenals, rising like mushrooms from atop the kidneys, pour out more than a score of hormones, including hydrocortisone and adrenaline needed for the body's response to stress and injury. Also in the endocrine system are the sex glands - ovaries and testes; the pineal gland in the brain whose hormones play a role in nerve and brain functioning; the thymus behind the breastbone which appears to be involved in establishing the body's immunity function; and areas in the pancreas, the islets of Langerhans, which secrete insulin.
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