With so many different symptoms and so many different organ systems potentially affected by thyroid system dysfunction, one might think that a diagnosis would be easy. However, in spite of the available blood tests for thyroid/pituitary/liver/adrenal function, the diagnosis is often missed.(1,2) One of the most common mis-conceptions regarding thyroid function is the assumption that and reliance on the requirement that the diagnosis of hypothyroidism depends on an elevated TSH level. Normally, the pituitary gland will secrete Thyroid Stimulating Hormone (hence TSH) in response to a low circulating thyroid hormone level. This is thought to reflect the pituitary's sensing of inadequate thyroid hormone levels in the blood that would be consistent with hypothyroidism. There is no question that an elevated TSH can confirm the diagnosis of hypothyroidism, but it is far too insensitive a measure, in other words the vast majority of patients who have hypothyroidism do not have an elevated TSH level. Some have suggested that perhaps the upper limit of what is considered normal is too high, instead of the normal TSH range being from 1.0-4.5, the range of normal for TSH should be 0.5-1.5. In that way more patients would be considered hypothyroid.

Furthermore, the lab level of TSH tends to vary throughout different times during the day making it less useful to rely on as the average level. MSG (monosodium glutamate) and stress tend to lower the TSH level, for example.

The most commonly used tests of thyroid hormone levels (note that I use the term level rather than function because the two are not always equal) are the T4 (or total T4), T3-uptake, FTI (also called the T7 or Free Thyroxine Index), and total T3 (sometimes called the T3-by-RIA). These tests are also unreliable because they do not reflect the hormone level that is actually available for action. Only the free T4 and free T3 are available to act on the cells. The total T4 and total T3 (as is most commonly measured) is a mixture of protein-bound T4 and T3 (and therefore not available to the cells) and the free T4 and T3. A large percentage of patients have low levels of the free T4 and free T3 even when all the other more commonly used tests are normal. Complicating the problem is the fact that these symptoms may present themselves while all the usual blood tests (TSH, FI, Total T3, etc) appear to be normal. When patients with Free T4 and Free T3 hormone levels below normal with or without an elevated TSH are given appropriate therapy, many report a tremendous improvement in the symptoms classically associated with hypothyroidism. Even when the labwork does not indicate low thyroid levels, many patients appear to fit the profile for low thyroid action. In fact, many of the best thyroidologists use the response to therapy as the major determinant of whether or not the patient was in fact hypothyroid. The diagnosis was confirmed by the response to the proper therapy. Even many of the most prestigious textbooks validate this approach.

http://www.drkaslow.com/html/thyroid.html

What are the lab tests for thyroid function?

The standard tests of thyroid function are serum levels of TSH, total T4, Free T4, T3, T3 uptake, Free T3, the T4 to T3 ratio (also known as "T7"), anti-peroxidase and anti-microsomal antibodies. Most physicians order a T4 as part of a general metabolic panel (if they have any interest in the thyroid at all). Physicians monitoring response to thyroid medication generally watch the TSH as an indicator to direct the dosage of Synthroid (T4) or other hormone replacements. Older tests like protein-bound iodine and basal metabolic rate are rarely used today. Another rare test, the TSH stimulation test challenges the pituitary with an injection of TRH. A lack of response indicates pituitary hypofunction or hypothalamus/pituitary hypofunction. If this test is positive, imaging such as CT and MRI are required along with other neurological tests to look for anatomical lesions of the brain, pituitary, and hypothalamus. Uses for the tests are listed below:

1. A high TSH and low T4 and T3 indicate thyroid gland disease.
2. High T4 to T3 ratio (T7) or a high r-T3 (another rare test) are suggestive of peripheral cellular resistance as these levels indicate a decreased conversion of T4 to T3. Decreased conversion may also be due to selenium deficiency or mercury toxicity. If this ratio is high in conjunction with a high serum or 24-hour urine cortisol, this may indicate cortisol-induced decrease of T4 to T3 conversion.
3. Free (unbound) T4 is perhaps the best early indicator of hypothyroidism.
4. High free T3 is the best early indicator of hyperthyroidism.
5. T3 uptake and total T4 levels rise or fall together with hyper or hypothyroidism. If the levels diverge, the cause is more likely to be due to abnormalities in transport binding proteins.
6. High T4 and low T3 uptake indicate excess thyroid binding protein.
7. Low T4 and high T3 uptake indicate low thyroid binding protein.
8. R-T3 is abnormal in starvation, cirrhosis, insulin-dependent diabetes and conditions of elevated cortisol such as Cushing's disease.
9. FTI is calculated by multiplying T3 uptake by total T4. This is used as a measure of Free T4.
10. The anti-peroxidase and anti-microsomal antibodies indicate auto-immune diseases such as Grave's or Hashimoto's.

Since these conditions are associated with hyperthyroidism, I will not discuss them here other than to point out that following a hyperthyroid episode, these enzymes inhibit the production of thyroxine. They also bind with cells of the adrenals, pancreas and parietal cells of the stomach.

What other tests can be used?

The most important single test for thyroid function is basal temperature. The reason for this is that thyroid hormone increases the conversion of ATP to ADP to power all the metabolic functions of the cell. In that conversion, 50% of the energy is released as heat. That heat is needed to maintain bodily functions by maintaining the proper temperature range for enzymatic activities necessary for life. Body temperature is a good measure of basal metabolic rate and is cheaper and simpler to perform than measuring the rate of conversion of oxygen to carbon dioxide, which is the "gold standard" measure of metabolic activity.

http://www.drwells.net/nutrition/topics/Hypothyroid.htm

Here is a great thyroid website:

http://www.tiredthyroid.com/optimal-labs.html