Thyroid Metabolism and Function Tests (PDF)

THYROID METABOLISM AND THYROID FUNCTION TESTS Clinical Biochemistry HML 3033 LO 03 Thyroid Gland is An endocrine gland located in front of and on each side of the larynx. It secretes two non- steroid hormones Triiodothyronine (T3) & Thyroxine (T4) that control met...

THYROID METABOLISM AND THYROID FUNCTION TESTS Clinical Biochemistry HML 3033 LO 03 Thyroid Gland is An endocrine gland located in front of and on each side of the larynx. It secretes two non- steroid hormones Triiodothyronine (T3) & Thyroxine (T4) that control metabolism and growth. Structure and Secretions The thyroid gland is a brownish-red organ having two lobes connected by an isthmus; it normally weighs about 28 g. It consists of cuboidal epithelial cells arranged to form small sacs known as vesicles or follicles. The vesicles are supported by connective tissue that forms a framework for the entire gland.  Thyroid Hormone Affects Many Organs and General Health Eyes Lungs Brain Heart Kidney Liver Skin GI Tract Uterus Structure and Secretions (cont.) In the normal thyroid gland, the vesicles are usually filled with a colloid substance containing the protein thyroglobulin combined with the two thyroid hormones T3 and T4 Thyroxine (T4) also called tetraiodothyronine Triiodothyronine (T3 ). The T4 and T3 hormones are composed of the amino acid tyrosine.  I I 3 2 3 2 NH2 O 4 1 O4 1 CH2 C COO H H H 5 6 5 6 I T3 contains three iodine atoms and T4 contains four iodine atoms. I I 3 2 3 2 NH2 O 4 1 O4 1 CH2 C COO H H H 5 6 5 6 I I T3 and Reverse T3 (no biological activity) Thyroid hormones are unique in that they contain 59-65% of the Iodine trace element iodine (I). Iodine is integral to the thyroid hormones’ composition, hence the Thyroid Hormone name triIODOthyroinine (T3) and tetraIODOthyronine (T4). That is why too much or too little iodine result in deleterious effects. The thyroid gland concentrates and traps iodide and produces and stores thyroid hormones. Thyroglobulin is especially rich in iodine. The thyroid gland constitutes about 0.5 percent of the total human body weight. It holds about 25 percent of the total iodine in the body, which is obtained from food and water in the diet. Iodine usually circulates in the blood as an inorganic iodide and is concentrated in the thyroid to as much as 500 times the iodide level of the blood. The role of thyroid hormones in regulating bodily functions Thyroid hormones regulate and speed up many metabolic functions in the body. a) They are essential for normal growth. b) Mental development. c) Sexual maturation. They also influence cardiac output and the heart rate. They do this by making these tissues more sensitive to catecholamines (adrenaline and Overactive Thyroid Causes nor-adrenaline) Abnormal Heart Rhythm Regulation of thyroid hormones  Regulation of thyroid hormones The amount of Thyroid hormones secreted by the thyroid is controlled by the thyroid-stimulating hormone (TSH) of the pituitary gland which, in turn, is regulated by a thyroid- releasing hormone (TRH), which is secreted by the hypothalamus. T3, T4 (-) Hypothalamus TRH (+) Pituitary T3, T4 (-) TSH (+) Thyroid Thyroid Stimulating Hormone (TSH) Hypothalamus regulates thyrotroph cells (in the pituitary gland) Thyrotroph cells produce TSH TSH stimulates the synthesis & secretion of T3 and T4 Thyroid Releasing Hormone is a tripeptide secreted by the hypothalamus gland. In health, T4 and T3 control the output of TSH by the pituitary. They do this by having a negative feedback effect on the Pituitary and the Hypothalmus. Negative Feedback Systems Most of the T3 in the plasma is derived from T4 by the removal of an iodine atom in the tissues. Only a small amount of T3 in plasma is synthesized in the thyroid. T3 is much more chemically active than T4. (T4 has no biological activity) The biological effects of the thyroid hormones include… Increased synthesis of cellular protein. Increased rate of mitochondrial respiration (increase oxygen consumption). Growth: essential for normal growth of skeletal system. The Nervous System: Enhances wakefulness and alertness Enhances memory and learning capacity Required for normal emotional tone The Cardiovascular System: Increase heart rate Increase force of cardiac contractions Increase Cardiac output  THYROID HORMONES IN THE BLOOD Approximately 99.98% of T4 is bound to 3 serum proteins: Thyroid binding globulin (TBG) ~75%; Thyroid binding prealbumin (TBPA or transthyretin) 15-20%; Albumin ~5-10% Only ~0.02% of the total T4 in blood is unbound or free. Only ~0.4% of total T3 in blood is free.  REVERSE T3 ( 3,3,5 tri-iodothyronine) I I 3 2 3 2 NH2 OH O CH2 CH COOH 4 1 4 1 5 6 5 6 T3 I Unlike T3, does not stimulate thyroid hormone receptors. However, it binds to these receptors, thereby blocking the action of T3. The further metabolism of T4 to T3 and reverse T3 is by deiodination and by deamination (to form propionic acids). Iodine, which is released in this process, is mostly taken up by the thyroid gland to be re-used. The deiodinated T4 and T3 metabolites remaining in the plasma are conjugated in the liver and excreted in the bile. T4 is converted to T3 in the liver and many other tissues by the action of T4 monodeiodinases. Some T4 and T3 is conjugated with glucuronide and sulfate in the liver, excreted in the bile, and partially hydrolyzed in the intestine. Some T4 and T3 formed in the intestine may be reabsorbed. -About 10% of the daily production of T4 and T3 is excreted in the bile. When these conjugates reach the intestine, they are hydrolyzed to give unconjugated T4 and T3. -This T4 and T3 is reabsorbed from the intestine and is used again in the synthesis of more T4 and T3. Small amounts of unmetabolised T4 and T3 are excreted in the urine. Tests required to assess thyroid function Thyroid tests are required for two purposes. 1. Grouping people into one of three classes. Euthyroid Normal healthy levels of thyroid hormones. Hyperthyroid High levels of thyroid hormones. Hypothyroid Low levels of thyroid hormones. 2. Monitoring the effects of treatment.  Thyroid Diseases Hyperthyroidism Hypothyroidism Thyrotoxicosis – excess thyroid hormones from any cause with or without symptoms. Hyperthyroidism – excess thyroid hormones from overactivity of the thyroid gland.  Reference Ranges for Thyroxine Hormones Thyroid tests Total T4 50 - 150 nmol/L Free T4 9.1 - 23.9 pmol/L Free T3 4.7 - 8.2 pmol/L TSH 0.3 - 5.0 m IU/L The tests used to assess thyroid function are: 1. Total Thyroxine (T4). 2. Total Tri-iodthyronine (T3) 3. Free thyroid hormones Free Thyroxine (FT4) Free T3 (FT3) Free Thyroxine Index (FTI) 4. Tests of the “Hypothalmic - Pituitary - thyroid” function. 5. Thyroid Autoantibodies. 6. In vivo radioactive uptake tests. 7. Miscellaneous tests.  Total Thyroxine (T4) Over 99.97 % of the T4 in the plasma is bound to: a- TBG (Thyroxine Binding Globulin), b- Thyroxine-Binding Pre-albumin (TBPA) c- Albumin. Most T4 is carried by TBG. Because of this, a change in the TBG levels in the plasma will cause a corresponding change in the Plasma Total T4. For example…….. A fall in plasma TBG will cause a fall in T4. A Rise in TBG will cause a rise in plasma T4. although in each case, the Free T4 remains unchanged. Apparent Hyperthyroidism: Situations where we see an increase in plasma Thyroxine Binding Globulin (TBG), will also display an apparent increase in Total T4. Examples are …… 1. Pregnancy: due to high oestrogen levels which increase many other proteins. 2. Oestrogen therapy: for example taking oral contraceptives. 3. Hereditary TBG excess Very rare cause. Apparent Hypothyroidism: Situations where we see a apparent or real decrease in TBG (Thyroxine Binding Globulin) levels may also display an apparent decrease in Total T4. Apparent Hypothyroidism (cont.) Examples are……. People with illness causing Nephrotic Syndrome. protein loss. Chronic liver disease. People treated with drugs Salicylates (Pain killers, which displace T4 on the TBG. e.g. aspirin) Danazol. People treated with drugs androgens, which cause an increase rate of corticosteroids metabolism at the tissues. anticonvulsants People receiving drugs which Lithium (Treatment for inhibit the release of thyroid depression) Phenylbutazone hormones from the thyroid. Inherited TBG deficiency Very rare condition. In all of the above examples of apparent Hyperthyroidism and apparent Hypothyroidism ……. “the Free T4 plasma levels remain unchanged” With genuine Hyperthyroidism, plasma T4 is usually increased to more than 150 nmol/L although a few patients with clinical Thyrotoxicosis have normal T4 levels. These patients usually have high levels of plasma T3. With genuine Hypothyroidism, plasma T4 levels are usually low, with values less than 70 nmol/L. Plasma total T4 levels are a good test for hypothyroidism, but is less sensitive than TSH levels.  Total Tri-iodothyronine (T3) 99.7 % of the total T3 is carried in the plasma by the same proteins which carry T4, that is TBG, TBPA and Albumin. This means that only 0.3 % of the T3 is Free T3 which is available for biological activity. Total T3 concentrations also change with alterations in binding proteins, although to a lesser degree than T4. The main value of Total T3 is in the diagnosis of T3- toxicosis. In most cases of Hyperthyroidism, both T4 and T3 are raised. But in a small number of clinically hyperthyroid patients, Total T4 and FT4 are both normal whereas Total T3 and Free T3 are raised. This condition is known as T3 Toxicosis and the measurement of T3 is required for the diagnosis. Low Total T3 is seen in people in the absence of thyroid disease. This includes……….. Old and severely ill patients. After M.I. (myocardial infarction) and after surgery. In these cases there is impaired conversion of T4 to T3 in the peripheral tissues. There is increased conversion of T4 to Reverse T3 instead and this molecule has no activity.  Free Thyroid Hormones These include FT4 and FT3. FT4 0.3 % of the Total T4 exists as FT4. FT4 is a more reliable test of thyroid status than plasma total T4. The Free T4 is not effected by changes in the plasma binding proteins such as occur in pregnancy and in women taking oestrogen-containing oral contraceptives. Free Tri-iodothyronine (FT3) FT3 is not effected by changes in binding proteins and because of this may be used to diagnose T3-toxicosis in patients who may also have changes in their protein levels. In these cases, FT3 will be a more reliable test than Total T3. The Free Thyroxine Index The Free Thyroxine Index (FTI) provides a measure of plasma FT4, without measuring FT4 directly. We can calculate the FTI by determining …… Total T4. UTBG (Unsaturated Thyroid Binding Globulin) Total T4 and expressing the result as Free T4 = K. FTI UTBG The ratio Total T4 is known as the Free Thyroxine Index. UTBG We can obtain a direct measurement of the UTBG (unbound thyroxine binding globulin) by performing a T3 Resin Uptake Test. To perform this test, radioactive T3 is added to a quantity of the patient’s serum. Some binds to the unbound sites on the TBG. The remaining unbound T3 is adsorbed on to an ion- exchange resin and the radioactivity is counted. This provides a measure of plasma UTBG. The T3 resin uptake is inversly related to free sites on the protein Tests of Hypothalamic-Pituitary-Thyroid function TSH is commonly used as a test of the Hypothalamic- Pituitary-Thyroid axis. In Hypothyroidism, TSH is very valuable. This is because (Total T4) and (Total T3) are usually normal in the early stages of the disease. However, plasma TSH is always almost increased due to the removal of the feedback inhibition. Later as the disease progresses, (Total T4) and (Total T3) fall, but TSH remains raised. More rarely, Hypothyroidism is secondary to hypofunction of the pituitary. In these patients, TSH is low. Therefore it is easy to distinguish between Primary Hypothyroidism and Secondary Hypothyroidism. Primary Hypothyroidism will display high levels of TSH Secondary Hypothyroidism will display low levels of TSH. The TRH Test This test measures the response of the Pituitary to TRH (Thyroid Releasing Hormone). It tests that all parts of the Hypothalamic-Pituitary-Thyroid Axis is functioning properly. Procedure for TRH Test: To perform the test, a) a basal sample of blood is taken. b) Then 200 mg of synthetic TRH is injected intravenously. c) Another sample of blood is taken 20 minutes later. d) The TSH is measured on both the Basal and the 20 minute samples. Normal individuals will show a rise of TSH to more than 2 mU/L above the basal level after 20 minutes. Failure to respond may occur in Patients with non-toxic multinodular goitre. Some patients with opthalmic Graves disease. Patients treated for hyperthyroidism and regained the euthyroid (having normal thyroid gland function ) within the previous months. A few patients with Chronic Renal Failure or Cushings disease. In Primary Hypothyroidism basal levels of TSH are high and go even higher with the injection of TRH. The levels also take longer than normal to return to basal levels. The main value of the TRH test is in the diagnosis of HYPERTHYROIDISM. This diagnosis can be excluded if there is an increase in plasma TSH in response to TRH injection. Thyroid Auto-antibodies; Several types of antibody to thyroid tissue have been detected in the serum of patients with thyroid disease. These antibodies help demonstrate the presence of auto- immune disease. Examples of Thyroid Auto-antibodies are………. 1. Complement-fixing antibodies Present in over 80% of patients specific for thyroid tissue withHashimoto’s disease. 2. Thyroglobulin antibodies. May be detected in most cases of early hypothyroidism. Detected also in about 80% of hyperthyroid patients.  Thyroid Auto-antibodies ( cont.) 3. Colloid antigen antibodies. Detected in all forms of auto- immune thyroiditis. Helps in the diagnosis of de Quervains thyroiditis (also called subacute thyroiditis). 4. Thyroid-stimulating IgG antibodies against the TSH immunoglobulins. receptors in the Thyroid are (TSI antibodies) found in people with Graves’ disease. These tissues are stimulated to produce high levels of T4. 5. Thyroid Growth These antibodies stimulate Immunoglobulins. thyroid growth, but not hormone (TGI) production. Graves’ disease (anti-thyroid stimulating hormone receptor) In Graves’ disease, the antibodies do not destroy the thyroid but act as if they are TSH (i.e., they bind and activate the TSH receptor) (agonist)  In vivo radioactive uptake tests These tests measure the… uptake of an oral dose of radioactive iodine (132I) by the thyroid gland. uptake of an injected dose of technetium 99m (99mTc) by the thyroid gland. Both these isotopes have short half-lives, for example….. 132I half-live = 140 mins. 99mTc half-live = 6 Hours. The uptake of radioactivity is measured after 20 mins (with 99mTc). In hyperthyroidism the rate of uptake of 132I by the thyroid is increased in over 90% of cases. In hypothyroidism the rate of uptake of 132I by the thyroid is reduced and much slower than a normal person’s uptake. Disadvantages of in vivo radioactive uptake tests include……. Interpretation is difficult. Cannot be used on pregnant women. The in vivo radioactive uptake tests still have a limited place in the management and diagnosis of thyroid disease,but have been mainly replaced by the chemical tests already mentioned. The uses of these tests are…… To monitor thyroid function in patients on anti-thyroid drugs. To determine whether a thyroid nodule is ‘hot’ or ‘cold’. Hot, or hyperfunctioning, nodules are typically benign. Cold, or hypofunctioning, nodules may be solid or cystic. Some may prove to be malignant. Isotope Scan  Miscellaneous Tests There are several tests that may be effected by increased or decreased activity of the thyroid gland and these include……. Basal Metabolic Rate (BMR). Glucose Tolerance Test. Plasma calcium. Plasma LDL-cholesterol. The Basal Metabolic Rate: is raised in hyperthyroidism and reduced in hypothyroidism but is now of historical interest only and is rarely performed. Glucose Tolerance Tests: are sometimes abnormal in hyperthyroid disease and these may show a diabetic-like response to an oral glucose load of 75 kg. Plasma Calcium: is sometimes raised in hyperthyroidism along with Alk Phos. Plasma LDL-cholesterol: is often markedly increased in hypothyroidism. It is also sometimes decreased in patients with hyperthyroidism.  Thyroid Cases Congenital Hypothyroid: Congenital hypothyroidism is a relatively common disorder which is screened for at birth. Thyroid hormone is necessary for brain development in the first two years of life. If it is not present in adequate amounts during this period of time, permanent brain damage will occur. As thyroid hormone is necessary for linear growth, severe growth retardation occurs if it is absent during childhood. If treatment is started while the epiphyses (rounded end of a long bone) are open and undamaged, growth will resume. In addition to the above problems, these patients may have all the other signs and symptoms of hypothyroidism. Congenital Hypothyroid: Typical History A 44 yr. old man who has always been cared for by his mother is presented. He is severely retarded and cannot communicate. He is only 40” tall, his epiphyses are still open, although severely damaged. He has very thick, dry skin. His tongue is thick, and he has a saddle nose. He still has his primary teeth; the secondary teeth have not erupted.  Thyroid cases Primary hypothyroidism occurs when disease of the thyroid gland prevents it from producing adequate amounts of thyroid hormone. Symptoms may vary from mild to severe and from nonspecific to very specific. In general, all metabolic processes slow down. Patients are often fatigued, and may also have depression, decreased intellectual function. Primary Hypothyroid: Typical History 43 y/o patient complains of fatigue, inability to finish tasks, sleeping more, yet always being tired. She has also noticed a decreased cold tolerance, constipation, and dryness of the skin. She comments that she is always cold, even in the summer. She has a positive family history for thyroid problems.