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Thyroid Homones

At the present, there are several tests for thyroid hormones as following:
1. Total thyroid hormones :
measurement of T4 and T3 was performed by radioimmunoassay (RIA), enzyme immunoassay(EIA) , immunochemiluminometric asssay (ICMA) and electrochemiluminescence (ECL) The T4, T3 can be error in the event of abnormal thyroid binding protein . Most T4 bind with thyroxine-binding protein which are thyroxine-binding globulin(TBG), thyroxine-binding prealbumin (TBPA) or may bind with transthyretin and albumin. Only 0.3% of T4 are free form. In case of the high number of TBG, total T4 and T3 is higher than normal whereas the low number of TBG results in the lower amount of total T4 and T3.
The cause of the abnormal amount of TBG (lower or higher)
  • Received a dose of se hormones
  • the pregnancy
  • Chronic liver disease
  • Take pills for birth control
  • Active Acromegaly
  • Chronic active hepatitis
  • The systemic illness
  • Acute intermitten porphyria
  • Lower protein in blood
  • AIDS
  • Chronic kidney disease
  • Lung cancer type oat cell
  • Take glucocorticoid
  • Genetics

2. Free thyroid hormone:
in case of the abnormal amount of TBG, the testing of free thyroid hormone is more accurate in measuring hormones than total form.

3. The measurement of TSH :
This is the test to diagnose the long lasting abnormal of thyroid gland. The level of TSH is high in primary hypothyroidism. Later, when the level of TSH is more sensitive, it is used in the diagnosis for hyperthyroidism.

The mechanism of thyroid hormone releasing
The function of the thyroid gland from creation through hormone secretion is controlled by hormones TRH and TSH. The Hypothalamus secretes TRH to stimulate the anterior pituitary for TSH secretion. After that TSH stimulates thyroid gland to secrete T4 and T3. There is negative feedback of T4 and T3 to inhibit hormone secretion from anterior pituitary and hypothalamus.

Transportation and metabolism of thyroid hormone
Most of T4 and T3 in the body are extra-thyroidal. Its form is bound form by binding with 2 important specific binding proteins which are TBG and TBPA. TBG is glycoprotein, its molecular weight is 50,000 Kilo Dalton. TBG affinity with T4 and T3 is 1000 times higher than TBPA. TBG can bind with plasma up to20 ug/dl. In the case of normal condition, TBG has non-covalent bond with T4 and T3 in plasma.

Total hormone
free hormone half-life in
blood (days)
% of total ng/dl molarity
T4 = 8 0.003 2.24 3.0 x 10-11 6.5
T3 = 0.15 0.3 0.4 0.6x10-11 1.5
The advantage of TBG measurement is to know the function of thyroid hormone. Due to the measurement of T4 and T3 is quantified in plasma rather than free releasing hormones. TBG is produced in the liver. The production of TBG will be increased in the condition of high estrogen such as in pregnancy or birth control by pills. In the case of taking androgen or glucocorticoids or some liver diseases, the production of TBG will be decreased. These conditions will change the level of T4 and T3 without changing the level of free releasing hormones. Some organic compound such as phenotoin and salicylates will compete with T4 and T3 in binding with TBG. Decrease of the level of total hormone without changing of free hormone level should be considered carefully for all diagnosis.

T3 is an active form because T3 has the 10 times higher affinity with thyroid receptors than T4. About 80% of T4 will be changed to T3 form or release T3 (rT3). In blood stream rT3 is weak antagonist. It will be produced more in the condition of chronic diseases, the lack of carbohydrate and in fetus.

There are 3 groups of thyroid gland disorder:
  1. Hypothyroidism: thyroid gland disorder with very low thyroid hormones, common diseases include primary hypothyroidism , secondary hypothyroidism.
  2. Euthyroid: over function of thyroid gland, gland is bigger and get goiter, eventually. This disorder is often found in young women or women during pregnancy. However, the level of hormone is normal.
  3. Hyperthyroidism or thyrotoxicosis: hyperthyroid due to very high level of thyroid hormone. Common diseases include Graves' disease , multitoxic nodular goiter(Plummer's disease).