Measuring Thyroid Antibodies

The body has an immune system that is important to help it fight infectious diseases. There are several parts of the immune system, but the portions of the body responsible for this are the white blood cells. Some kinds of white blood cells, neutrophils, engulf bacteria and viruses. Other white blood cells, lymphocytes, make and release special proteins called antibodies that are tailor-made to stick to specific types of invading bacteria or viruses. Sometimes, some of the lymphocytes get mixed up and make antibodies that target parts of your own body. When this causes an illness, it is called an autoimmune (self-immune) disease. If the antibodies target the joints, this causes rheumatoid arthritis. If the antibodies target the muscles, this causes myasthenia gravis. Antibodies that target the thyroid can cause thyroid disease.

There are three major kinds of antithyroid antibodies. Antithyroid peroxidase (TPO) antibodies used to be called anti-microsomal antibodies. TPO antibodies attack the portion of the thyroid cell responsible for making thyroid hormones. The thyroid dis ease that is associated with these antibodies is Hashimoto's thyroiditis (also known as Hashimoto's disease). Hashimoto's thyroiditis is classically diagnosed by examining the thyroid gland or a piece of the thyroid gland (from a biopsy) under a microscope. Since more than 95 percent of people with Hashimoto's thyroiditis have TPO antibodies in their blood, the presence of these antibodies is usually sufficient to make this diagnosis without doing a thyroid biopsy. (Hashimoto's disease is discussed more in Chapter 5.)

Another antibody associated with Hashimoto's thyroiditis is one that sticks to thy-roglobulin, the antithyroglobulin antibody. This particular antibody is important because its presence, in people with thyroid cancer, interferes with the ability of a laboratory to measure the level of thyroglobulin in the blood.

The third major type of thyroid antibody is called thyroid stimulating immunoglob-ulin (TSI, also known as thyroid stimulating antibody or TSA). Normally, TSH from the pituitary will stick to the TSH receptor (a "docking station" for TSH) to turn on each thyroid cell to start to suck up iodine and make thyroid hormone. However, in this case, TSI will stick to the TSH receptors and do all of the thyroid cell stimulation that TSH would have done. Unlike TSH, however, TSI is not turned off when thyroid hormone levels become too high. The continued stimulation of the TSH receptor by TSI causes thyrotoxicosis. This is known as Graves' disease, discussed more in Chapter 6. TSI will also stick to parts of the eye muscles, causing eye irritation and swelling of these muscles sufficient to make the eyeballs protrude from the eye socket, another feature of Graves' disease, called orbitopathy or exophthalmopathy. In patient circles it is known as thyroid eye disease (TED) or Graves' ophthalmopathy (GO), discussed more in Chapter 23.

Graves' disease antibodies are usually a mixture of TPO antibodies and thyroglob-ulin antibodies that help destroy the thyroid gland, and TSI antibodies that stimulate the thyroid gland. This is why all three of these antibodies can be measured in the blood of people with Graves' disease, while only TPO antibodies and thyroglobulin antibodies are seen in people with Hashimoto's thyroiditis.

Different labs report results on these antibodies using different units with different normal ranges. Your doctor should pay close attention to the details of the lab report to know if these antibodies are present in high enough amounts to be significant. Again, you can request copies of your lab reports, which will have the normal range for that particular lab listed; then you can compare whether your own levels are higher than normal, meaning that you indeed have antibodies in significant amounts. Copies of your lab reports also help in cases where you seek out a specialist, or a second or even third opinion. Almost one-quarter of women in North America show significant levels of TPO antibodies at some time in their life, accounting for a high prevalence of hypothyroidism. Elevated levels of TPO antibodies are also associated with a higher risk of postpartum thyroiditis (an inflammation of the thyroid gland occurring in moth-

TABLE 2.2. Lab Tests Used to Measure Thyroid Function

Laboratory Tests

Normal Range Common Units (International Units)

How It's Used (Condition)

Free T4

0.9-1.6 ng/dL (12-21 pmol/L)

Measures thyroid hormone available to enter cells (hypothyroidism, thyrotoxicosis)

T3

80-180 ng/dL (1.2-2.8 nmol/L)

Measures total T3 (thyrotoxicosis)

Free T3

2.2-4.0 ng/L (3.4-6.1 pmol/L)

Measures free (unbound) T3 (thyrotoxicosis)

Reverse T3

90-350 pg/mL (140-538 pmol/L)

Measures reverse T3, an inactive degradation product of T4, increased in illness (not used)

TSH

0.6-4.5 |jU/mL (0.6-4.5 mU/L)

Most sensitive measure of thyroid status (hypothyroidism, thyrotoxicosis, thyroid cancer care)

Thyroglobulin (TG)

less than 35 ng/mL (less than 35 |g/L)

Measures thyroglobulin, a unique protein from thyroid cells (thyroiditis, thyroid cancer care)

Thyroxine-binding globulin (TBG)

13-30 |g/mL (13-30 mg/L)

Measures TBG, a protein in blood, made in the liver, that sticks to thyroid hormone (not used)

TPO antibody

0-70 lU/mL

Measures TPO, an autoimmune antibody in thyroid disease (Hashimoto's, Graves' disease, pregnancy)

Thyroglobulin antibody

0-2.2 lU/mL

Measures autoimmune antibody to thyroglobulin (Hashimoto's, thyroid cancer-check TG)

Thyroid-stimulating immunoglobulin (TSI or TSA)

Less than 130% of basal activity

Measures autoimmune antibody to TSH receptor (Graves' disease)

ers after giving birth) and a higher risk of miscarriage as compared to other women, both discussed in Chapter 13.

Table 2.2 shows units used and normal ranges for various thyroid blood tests.

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