Thyroid Hormone Binding Proteins

Several proteins in the blood, produced by the liver, stick to thyroid hormones. They include thyroxine-binding globulin (TBG), thyroxine-binding prealbumin (TBPA), and albumin. In addition, several cholesterol-containing particles in the blood, lipoproteins, are able to bind T4 and T3.

Before good tests were available to measure the free T4, it was useful to measure the TBG level as a way to estimate the portion of the total T4 that was free. Since modern laboratory tests no longer need this estimate, the TBG level is not usually used for diagnosis of thyroid problems. It is important to note that the TBG level is frequently mistaken for a thyroglobulin level (often abbreviated as TG) with the result that the TBG is wrongly obtained in following thyroid cancer patients who need the TG level measured. The only way to ensure that you are getting a TG test (or have had one) and not a TBG test is to request a copy of all of your lab reports. If you suspect this common error was made, you must educate your doctor about this mix-up, and if need be, photocopy a passage of this book to help bring the matter to your doctor's attention.

In some unusual cases, there may be abnormal thyroid binding protein levels, or altered binding proteins, which can change other thyroid function tests. For example, some people make abnormal TBG, no TBG, or too much TBG. This can result in the total T4 levels being low or high, without having much effect upon the free T4 levels and absolutely no effect upon the TSH. Likewise, altered albumin can stick to T4 much more than normal and cause the total T4 to be much higher than normal, without affecting the person or changing the TSH level. This is another reason why it is very important to avoid using the total T4 level, rather than the free T4 level, as the method to evaluate your thyroid hormone.

Table 2.1 summarizes appropriate thyroid blood tests and their expected results for various thyroid conditions.

TABLE 2.1. Thyroid Conditions and Appropriate Thyroid Blood Tests

Thyroid Condition

Free T4

T3 or Free T3

TSH

Thyroglobulin

Normal thyroid function (euthyroidism) in a person with a normal thyroid gland

Normal

Normal

Normal

Normal

Normal thyroid function (euthyroidism)in a person without a thyroid gland

Normal

Normal

Normal

Very low or none

Early or subclinical hypothyroidism

Normal or low

Not used

Slightly high

Not used

Hypothyroidism

Low

Not used

High

Not used

Mild or subclinical thyrotoxicosis

Normal or high

Normal or high

Slightly low but greater than 0.2

Not used

Thyrotoxicosis

High

High

Low (less than 0.2)

Not used

Thyrotoxicosis from thyroid pills

High

High

Low

Low

Thyrotoxicosis from hyperthyroidism (Graves' disease or toxic thyroid nodule)

High

High

Low

High

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