When trying to determine if fatigue, weight gain, dry skin, and other conditions are caused by a thyroid condition, other health disorders, or simply normal life variations, it all comes down to blood tests. These tests help doctors determine if you have appropriate amounts of thyroid hormone in your blood.
A normal and healthy amount of thyroid hormone is known as euthyroidism. If the thyroid hormone levels are insufficient to be euthyroid, we call this hypothyroidism (described in Chapter 3). If the thyroid hormone levels are too high, the condition is termed thyrotoxicosis (described in Chapter 4). Sometimes thyrotoxicosis is present because the thyroid is overactive, making too much thyroid hormone, a situation known as hyperthyroidism. However, thyrotoxicosis can also be present without hyper-thyroidism, such as when someone takes too high a dose of thyroid hormone tablets. In this case the thyroid itself is not overactive (thus not hyperthyroid); however, the levels of thyroid hormone in the blood are too high.
Often, when I'm (Ken) lecturing to large audiences of physicians, usually late in the evening, I ask, "Who is tired? Who is a bit overweight? Who suffers from constipation? Who has dry skin?" Each question results in a large number of raised hands. Then I ask, "Is anyone here hypothyroid and not taking thyroid hormone?" No one raises his or her hand. Yet many of these symptoms are the same as those of hypothyroidism. The symptoms of hypothyroidism completely overlap with those of normal life variations or those of unrelated health disorders. The proper laboratory test acts as an "honest broker," providing objective evidence for making the correct diagnosis. Know, however, that such tests are not infallible.
This chapter will take you through all of the laboratory tests that can be useful in providing diagnostic answers with respect to your thyroid gland. We'll first visit the tests that measure the levels of thyroid hormones, including measuring free T4. Then we'll discuss how to assess the pituitary gland's responses to these thyroid hormone levels through measuring thyroid stimulating hormone. This will be followed by a review of tests that evaluate the function of the thyroid gland itself. Finally, we'll see how thyroid disease affects laboratory tests that assess other parts of the body. After this, we'll fit all of these different tests into a framework based on the different types of thyroid disease.
Throughout this chapter (and in much of this book), we use the term protein. Proteins are one of the major types of substances that make up the body. Proteins are made of amino acids that are connected together. Nearly all hormones, enzymes, and body structures are made of different types of proteins. Some of these proteins are dissolved in the bloodstream and some form solid structures in the body. In addition, through the actions of protein enzymes, the body's cells can produce a wide range of nonprotein chemicals to perform a variety of functions.
Three other major types of substances make up the body. Carbohydrates, another name for sugars, can be dissolved in the blood and used as fuel for the body's cells, or they can be attached to some proteins to become part of their structure. Fats, also known as lipids, can be found in the blood as free fatty acids, attached to proteins (sometimes known as lipoproteins), or in solid lumps as body fat. Lastly, minerals can be found in the bones of the body, forming the fourth type of body substance.
Besides forming proteins, some amino acids are used to form hormones, such as the amino acid tyrosine, which forms thyroid hormones when combined with iodine. The thyroid hormones, thyroxine and triiodothyronine, are members of the steroid hormone superfamily, which also includes cortisol, estrogen, progesterone, androgen, aldosterone, and vitamin D. All of these different hormones stick to special proteins in the nucleus (center part) of the cell called nuclear receptor proteins, which then stick to particular parts of different genes to do their own particular jobs. (See Chapter 1.) Proper use of laboratory tests ensures that hormones are doing their jobs.
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