Thyroid
The thyroid gland is a critical part of the endocrine system, and controls or influences many of the bodies functions and processes. Of particular interest to us, "Neuromuscular disorders are common in hypothyroidism.", and in fact "Proximal muscle weakness in hypothyroidism may
mimic polymyositis". (Smit, Staniland 2003) [1] So if you are hypothyroid, or even think you might be, a full thyroid panel will give you a much better indication if your thyroid may be influencing your muscular disease.
"A TFT panel typically includes thyroid hormones such as (TSH, thyrotropin) and thyroxine (T4), and triiodothyronine (T3) depending on local laboratory policy.
Other tests include thyroid hormone binding ratio, thyroxine binding index or uptake analysis. A complete thyroid panel may be useful in determining the thyroid interaction in the case of complex disorders.
TFTs may be requested if a patient is thought to suffer from hyperthyroidism (overactive thyroid) or hypothyroidism (underactive thyroid), or to monitor the effectiveness of either thyroid-suppression or hormone replacement therapy. It is also requested routinely in conditions linked to thyroid disease, such as atrial fibrillation and anxiety disorder." (Wikipedia)[2]
- T4-to-T3 Conversion and Hypothyroidism page: Read why in-range TSH and T4 levels may be present without adequate T3 levels.
"Thyroid insufficiency. Is TSH the only diagnostic tool?" The explanation is that TSH is grossly in feedback with serum T4 only, not so much with serum T3, while the patient's wellbeing depends on the free T3 that is disposable inside the cells. As hypothyroid patients are usually unable to convert inactive T4 into active T3, owing to a lack of 5' -deiodinase in the liver and kidneys, the administration of T4 can eventually correct the serum TSH level, but rarely provides the patient with the T3 needed to be relieved of his symptoms. (Baiser, Hertoghe et all 2000) [3]
These findings indicate that in hypothyroid patients L-T4-replacement...that is sufficient to maintain a normal serum TSH, is accompanied by a serum free T4 that is higher than that in untreated euthyroid patients or normal individuals and may not result in an appropriately normal serum free T3 concentration. (Woeber 2002) [4]
THYROID TESTS
A pretty excellent sheet of overviewing tests available for thyroid panel, and what is being tested, can be found at the Navy's Bureau of Medicine and Surgery. [5] It also gives a brief explanation of the tests.
Routinely three hormones are often measured namely Thyroxine, tri-iodothyronine and Thyroid Stimulating Hormone (TSH).
THYROXINE (T4)
Thyroxine is found in the blood in two forms i.e. bound to proteins and free of protein binding.The free component is the active form of the hormone and comprises only 0.03% of the circulating total T4. Some laboratories still measure total thyroxine which comprises both the bound and free forms. The trouble with this is that the level of total thyroxine very much depends on the amount bound to proteins and therefore the level of binding proteins in the blood. The major protein binding thyroxine is called Thyroid Binding Protein (TBG). TBG can be low in some patients due to an inherited but seemingly harmless deficiency. In these patients total thyroxine is low but the free and active component is normal. Medicines such as the contraceptive pill and life events such as pregnancy can also alter binding proteins giving spuriously high levels of total thyroxine.
For these reasons most laboratories have now introduced the measurement of free thyroxine and this is what we measure.
TRI-IODOTHYRONINE (T3)
There are two assays available, one measures total T3 and the other free T3. The total T3 comprises of both protein bound and free T3. The free component is the active form and comprises 0.3% of total circulating T3. Gradually laboratories are moving over to free T3 measurements as more reliable free T3 assays become available.T3 is the biologically active thyroid hormone, possessing 5 times the metabolic power of T4. In man some 80% of T3 is produced from T4 by conversion in liver and kidney. Therefore little is produced in the thyroid itself. The conversion of T4 to T3 can depend on a number of situations such as chronic illness or surgical stress which cause a fall in T4 to T3 conversion (called low T3 syndrome). Starvation also alters T4 to T3 conversion with a fall in T3 as the body tries to reduce its metabolism to conserve energy.
What does T3 do?
T3 alters the metabolism of the body. It alters protein manufacture, cellular activity and is essential for growth and well being. Without T3 the patient develops hypothyroidism. Too much and the patient develops an overactive thyroid called thyrotoxicosis.
TSH
TSH is released by the pituitary gland and circulates in the blood stream to the thyroid where it controls release of the thyroid hormones T4 and T3. TSH release is very sensitive to alterations in the blood thyroid hormones, with small decreases augmenting TSH secretion and small increases reducing release. Therefore in hypothyroidism, TSH is raised above normal reference ranges whereas in thyotoxicosis TSH is suppressed into the undetectable range. In thyrotoxicosis the thyroid automatically manufactures too much T4 and T3 without the need for TSH to switch on. As TSH is so sensitive to changes in thyroid hormone levels it is used as the number one test for screening for thyroid disease. If the laboratories notice a raised TSH then automatically T4 is measured. If the laboratories notice a suppressed TSH then T4 and T3 are measured. Why both? Because there are some patients whose thyroid oversecrete only T3 called T3 Toxicosis and both hormones need to be measured to detect this form of thyrotoxicosis; it is usually seen in those who have had previously a thyroidectomy or radioactive iodine for thyrotoxicosis in the past.
TRH Testing
When the level of TSH is borderline low or high sometimes the specialist will request a TRH test. This a hormone released by the brain (hypothalamus) which controls the pituitary release of TSH. By using this simple test the specialist can tell whether a patient has in reality an overactive, normal or underactive thyroid. It is interesting to note that a large study of TRH tests showed a significant number of cases of hypothyroidism being missed by using too high a reference range, and suggested a normal range should not be above 3.5 mIU/l. (Moncayo 2006) [5]
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