Free Thyroid for Dummies by Alan L. Rubin

pituitary releases more TSH
    to stimulate the thyroid and tell it to get a move on with making and releasing more thyroid hormone (if it can). However, if the pituitary detects excessive amounts of thyroid hormone, it cuts back on the amount of TSH it produces, so TSH levels fall. This to and fro dialogue between the thyroid and pituitary glands is called the negative feedback for TSH release.
    In short, as thyroid hormone falls, TSH rises, and as thyroid hormone rises, TSH falls. Because T3, T4, and TSH are all easily measurable in a blood sample, it’s a relatively simple task for laboratory tests to determine the state of your thyroid function (see Chapter 4).
    As TSH is also regulated by the release of thyrotrophin-releasing hormone (TRH) from the hypothalamus (see Figure 3-3), the level of TSH in your blood remains very stable throughout life, and abnormal levels usually mean some disease is present.
    Moving thyroid hormones around
    After T3 and T4 are released from the thyroid, they don’t just travel loosely in your blood to their targets – special proteins in the bloodstream carry them.
    Because 99.97 per cent of thyroid hormone is attached to proteins, only 0.03
    per cent floats freely in your circulation.
    Only the free thyroid hormone can leave your blood and enter your cells. The rest is solidly bound to proteins and is not available to perform the actions of thyroid hormone – essentially, it’s inactive. When a doctor measures the total thyroid hormone in your blood, she measures bound hormone along with the unbound hormone. If she only knows the total T4 amount in your blood, she needs to order a second test to determine the unbound T4 – the hormone that is free in your blood. This second test is important because many drugs and diseases alter the blood levels of thyroxine-binding proteins – the proteins that thyroid hormones bind to. If a drug like oestrogen, for example, increases the amount of thyroxine-binding proteins in your body, your thyroid makes 07_031727 ch03.qxp 9/6/06 10:45 PM Page 31
    Chapter 3: Discovering How Your Thyroid Works
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    Proteins that carry thyroid hormones
    Three different proteins carry thyroid hormones
    proteins are inactive and one theory is that
    around your body. The most important by having so much tied up with proteins means far is thyroxine-binding globulin, responsible that an increase in the thyroid gland’s hormone for carrying 75 per cent of thyroid hormones in
    output does not result in an increase in thyroid
    your blood. Transthyretin, previously known as
    activity. Another theory is that the combination
    thyroxine-binding prealbumin, carries 20 per of the hormone and the protein produces a cent of your thyroid hormones, while thyroxine-large molecule, which cannot escape from the
    binding albumin carries the other 5 per cent.
    body through the urine, thus preserving iodine.
    Exactly why proteins carry thyroid hormones is
    not clear. Thyroid hormones that are bound to
    more thyroid hormone to bind to these proteins, keeping the unbound thyroid hormone constant and normal. Yet the results of a total T4 blood test will be elevated. Conversely, testosterone, the male hormone, causes a decrease in the thyroxine-binding proteins. If your testosterone level rises, your thyroid makes less thyroxine and a measurement of total T4 shows a decrease (while the unbound T4 again remains normal).
    Understanding the Function
    of Thyroid Hormones
    Your thyroid hormones affect just about every cell and organ in your body.
    They perform general functions that increase the efficiency of each organ’s specific functions, whatever they are. This section tells you all about those functions and explains what too much or too little of the thyroid hormones can do to a healthy person.
    Many of these changes are caused by other factors besides too little or too much thyroid hormone. For example, an infection can raise your body temperature just as having too much thyroid hormone does.