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Tests for Thyroid Function


Direct Tests


Thyroid-stimulating hormone (TSH) level: Thyrotrophin or TSH is the hormone produced by the pituitary gland to control the levels of thyroid hormones in the bloodstream. TSH levels are measured to confirm the diagnosis of hypothyroidism or hyperthyroidism. TSH levels rise, as the thyroid becomes more underactive, to stimulate the thyroid gland to produce more thyroid hormones. TSH levels fall as the thyroid becomes overactive, so as not to stimulate excess production of thyroid hormones.


Total serum thyroxine (T4) level: TT4 is a blood test measuring the total amount of thyroxine per unit of blood. This includes free thyroxine and thyroxine bound to carrier proteins for transport around the body.


Total serum triiodothyronine (T3) level: TT3 is a blood test measuring the total amount of triiodothyronine per unit of blood. This includes free triiodothyronine and triiodothyronine bound to carrier proteins for transport around the body.


Free thyroxine (T4) level: fT4 is a blood test measuring the amount of thyroxine present in the water of the blood that is free or unbound to carrier proteins. It is the free T4 fraction in the blood that is available to the cells of the body. Free T4 is then converted within the cells into free T3 to influence metabolic processes.


Free triiodothyronine (T3) level: fT3 is a blood test measuring the amount of triiodothyronine present in the blood that is free or unbound to carrier proteins. Free T3 is available to the cells of the body and can be used immediately in metabolic actions.


Thyroxine-binding globulin: TBG is the main protein that carries thyroxine in the bloodstream. Measuring TBG levels may help in diagnosing abnormal thyroid function. Low levels of total serum thyroxine may be due to abnormally low thyroxine binding globulin. Low levels of free T4 may be due to abnormally high levels of thyroxine binding globulin.


Thyroglobulin: is a protein to which T4 and T3 are attached when they are stored in the cells of the thyroid gland. When the body requires more thyroid hormones, the T4/T3/thyroglobulin compound is broken down and free T4 and free T3 are released into the bloodstream. Detection of thyroglobulin may indicate that active thyroid tissue is still present even after a total thyroidectomy or ablative therapy with radioactive iodine, as a sensitive marker for remaining thyroid cancer cells.


Reverse T3: rT3 is a biologically inactive form of the T3 hormone, triiodothyronine. In situations where the body is under extreme stress due to trauma, severe illness, starvation, or operative procedures the production of reverse T3 may increase. This will cause the levels of free T3 (active hormone) to decrease, resulting in a slower metabolic rate to conserve energy.


Thyrotrophin-releasing hormone: TRH is secreted by the hypothalamus to stimulate the pituitary gland to produce TSH. Occasionally, a TRH test is required to determine whether the pituitary gland is functioning properly. This is done to clarify the diagnosis of primary or secondary hypothyroidism when standard blood tests appear confusing or normal in the light of serious symptoms. An injection of synthetic TRH is given to see how the pituitary gland responds. An exaggerated response can indicate primary hypothyroidism, while a blunted response may indicate a problem with the pituitary gland.


Indirect Tests


Iodine levels: are usually tested from a urinary sample. Hair analysis can also be used to determine iodine levels. Iodine supplementation may be required for those with severe iodine deficiency. However, over zealous consumption of iodine can also induce thyroid abnormalities or aggravate a pre-existing thyroid condition.


Radioactive-iodine uptake: RAIU is a radioactive isotope scan used to view the tissue of the thyroid gland and to assess thyroid function. A trace amount of radioactive iodine is given orally or by injection to see how the thyroid gland absorbs and utilizes iodine. This test can be used to investigate thyroid nodules, cysts and goitres. It is also used to detect metastasised thyroid cancer.


X-rays: may be taken to assess the position of the thyroid gland and whether a goitre is pressing on other vital structures in the neck area.


Calcitonin: is a hormone produced by non-thyroid cells called C cells that reside in the tissue of the thyroid gland. Calcitonin influences calcium levels in the blood and bones. In medullary thyroid cancer the C cells overproduce calcitonin. The measurement of calcitonin levels has been useful in determining the effectiveness of treatment in certain thyroid cancers.


Parathyroid hormone: is a hormone produced by the four tiny parathyroid glands that are tightly attached to the tissue of the thyroid gland. The parathyroid glands have a totally separate function from the thyroid gland. They regulate the levels of calcium, along with calcitonin, in the blood and bones. The parathyroid glands can be bruised or damaged during surgical procedures, and therefore parathyroid hormone and calcium levels will need to be assessed.


Reflex time: the speed of tendon reflexes can be tested by your doctor to determine the reaction of the tendons to stimulation, and how long they take to relax after contraction. A slow response and recovery time may indicate poor thyroid function.


Cholesterol levels: can be tested when assessing thyroid function. The levels of cholesterol in the blood are influenced by the amount of thyroid hormones in circulation. As the thyroid gland becomes increasingly underactive cholesterol levels will usually rise. When thyroid hormones are brought back into balance, cholesterol levels generally return to normal.


Erythrocyte sedimentation rate: an ESR is a non-specific test to see whether a person is ill. Elevated sedimentation rates indicate the presence of inflammation or infection. 


Prolactin: pronounced hypothyroidism can cause an increase in the level of prolactin, a hormone responsible for breast milk production. Too much prolactin can cause breast tenderness and milk discharge even when you are not lactating. Excess prolactin can also block oestrogen production and adversely affect periods, ovulation and the ability to sustain a pregnancy.


Sex hormone binding globulin: SHBG is a carrier protein for various sex hormones in the bloodstream. SHBG has been shown to be a sensitive marker for thyroid hormone action. As the thyroid gland becomes underactive SHBG levels also drop. Low levels of this globulin can be another helpful indicator of thyroid hormone insufficiency.


Muscle enzymes: a blood test measuring muscle enzymes may be abnormal when a person’s hypothyroidism is particularly bad. Elevated levels of muscle enzymes are the result of damaged muscles and may indicate a disease process. Muscle enzyme levels usually normalize when thyroid hormone levels are optimised.


Liver enzymes: a blood test assessing liver function may show abnormalities when the thyroid gland is underactive or overactive. Elevated liver enzymes may result from abnormal thyroid function and generally return to normal when treatment protocols are established and thyroid hormones are optimised within normal reference ranges.


Tests for Causes


Thyroid-inhibiting antibodies: a blood test can be performed to measure the levels of thyroid autoantibodies present in the bloodstream. Thyroid peroxidase (TPO) and thyroglobulin antibodies (TG) are produced by white blood cells to inhibit and destroy functioning thyroid cells. TPO antibodies are sometimes called microsomal antibodies. The presence of these antibodies is commonly associated with Hashimoto's Thyroiditis, but is also seen in the early onset of Graves' disease. There are also TSH receptor inhibitory antibodies (TSHRI) that bind to the TSH receptor and block its action. High levels, of any one of these antibodies, indicates that an autoimmune disease process is taking place.


Thyroid-stimulating antibodies: TSA, also known as thyrotropin-receptor stimulating antibodies, act on the TSH-receptor sites on the surface of thyroid cells, over stimulating the secretion of thyroid hormones. TSA are commonly associated with hyperthyroidism and are the main cause of autoimmune Graves’ disease. These antibodies are occasionally found in patients suffering with Hashimoto’s thyroiditis, and may cause a transient episode of hyperthyroidism.


Radioisotope thyroid scan: a trace amount of radioactive iodine is injected into a vein and is quickly absorbed and utilized by the thyroid gland. This test can be used to further investigate the cause of thyroid malfunction by revealing active nodules, cysts and inactive areas of the thyroid tissue. 


Ultrasound scan: can be used to assess the structure of the thyroid gland and can reveal the size and position of a goitre. Thyroid nodules can be viewed, determining whether they are solid lumps or fluid filled cysts.


Biopsy: also referred to as a fine needle aspiration biopsy (FNAB). This term describes the technique used to remove a small piece of thyroid tissue for examination under a microscope. A fine needle is used to suck out a small sample of thyroid tissue, which can be carefully assessed under a microscope to determine what is wrong with the thyroid gland. A biopsy can confirm the diagnosis of an autoimmune condition, and be used to detect cancerous tissue or growths. Recently, ultrasound guided FNABs have been used which is an extremely accurate technique of assessing thyroid gland tissue. A biopsy using a larger needle is sometimes required for assessing suspicious nodules that produce a negative FNAB result.


© 2004 by Robyn Koumourou

Thyroid Disorders | Thyroid Matters

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