Amr Mix Thyroid Lecture

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Biochemistry of Thyroid HormoneDr. Amr Amin, PhD BiochemistryDr. Abeer Ahmed ObjectivesBiosynthesis of thyroid hormonesProduction, storage , and release of thyroid hormonesMetabolic effect of thyroid hormonesClinical significance of thyroid H. disorders Thyroid hormone systemThyroglobulin:large glycoprotein molecule -the major component of colloid- (the precursor for all thyroid hormones).Iodide, the essential ion incorporated in Thyroglobulin mol.Mono-iodotyrosine (MIT) and Di-iodotyrosine (DIT).Thyroid peroxidase, the enzyme catalyzing main steps in TH biosynthesis.Thyroxine (T4) and tri-iodothyronine (T3), The two active hormones produced by the thyroid gland.Transport proteins3ThyroglobulinThyroglobulin is the precursor of t4 and T3. It is a large iodinated, glycosylated protein of 660 kDa (5000 aa), Carbohydrate accounts for 810% of the weight of thyroglobulin and iodide for about 0.21%, depending upon the iodine content in the diet.Thyroglobulin is composed of two large subunits. It contains 115 tyrosine residues, each of which is a potential site of iodination.About 70% of the iodide in thyroglobulin exists in the inactive precursors, monoiodotyrosine (MIT) and diiodotyrosine (DIT), while 30% is in the iodothyronyl residues, T4 and T3. (the coupling of two iodinated tyrosine molecules to form either T3 or T4).Tg is synthesized in the basal portion of the cell and moves to the lumen, where it is a storage form of T3 and T4 in the colloid; several weeks supply of these hormones exist in the normal thyroid. Within minutes after stimulation of the thyroid by TSH, colloid reenters the cell and there is a marked increase of phagolysosome activity. Various acid proteases and peptidases hydrolyze the thyroglobulin into its constituent amino acids, including T4 and T3, which are discharged from the basal portion of the cell Thyroglobulin is thus a very large pro-hormone. 4Iodide metabolismIodine is an essential dietary component because of its role in thyroid hormone synthesis.Iodide is actively transported (Na-K ATPase) into the thyroid gland against gradients by a sodium/iodide cotransport system located in the basal membrane of the thyroid follicular cells called an iodide trapThe thyroid is the only tissue that can oxidize iodide I to a higher valence state iodine I+, an obligatory step in I organification and thyroid hormone biosynthesis. This step involves T.peroxidase (and occurs at the luminal surface of the follicular cell).(A number of compounds inhibit I oxidation and therefore its subsequent incorporation into MIT and DIT. The most important of these are the thiourea drugs. They are used as antithyroid drugs because of their ability to inhibit thyroid hormone biosynthesis. Iodide Iodine organification: incorporation of iodine into organic molecule (phenol ring of tyrosine residue) to form monoiodotyrosine (MIT) and diiodotyrosine (DIT) within the thyroglobulin.

About 70% of the iodide in thyroglobulin exists in the inactive precursors, monoiodotyrosine (MIT) and diiodotyrosine (DIT).About 30% is in the iodothyronyl residues, T4 and T3.

Once iodination occurs, the iodine does not readily leave the thyroid. MIT / DIT

I2IIIIIII7

Thyroperoxidase, a tetrameric protein with a molecular mass of 60 kDa, requires hydrogen peroxide as an oxidizing agent. The H2O2 is produced by an NADPH-dependent enzyme resembling cytochrome c reductase. Thyroid peroxidase is a heme-containing peroxidase enzyme that iodinates the tyrosine molecules and conjugates MITs and DITs. Thyroid peroxidase8Thyroxine T4 and Tri-iodo thyronine T3 Actually two related compoundsT4 (thyroxine); has 2 tyrosine molecules + 4 bound iodine atomsT3 (triiodothyronine); has 2 tyrosines + 3 bound iodine atoms.

Both T3 and T4 are resulted from the conjugation of MIT and DIT molecules in colloid.T3 is responsible for most of the effects of thyroid hormones on target cells, T4 acts as a reservoir for T39Release of thyroid hormonesOnce thyroglobulin has been iodinated, it is stored in colloid.Under stimulation of TSH, thyroglobulin is transferred from the lumen of the follicle into the thyroid cell by endocytosisLysosomal proteases within the cell hydrolyze Tg into its constituents, liberating T4 and T3.(~10% T4 undergoes mono-deiodination to T3 before secretion).T3 and T4 diffuse through the basal membrane and enter the bloodstream (100g T4 & 10g T3/day)Liberated MIT and DIT are hydrolyzed through de-Iodinase into tyrosine and Iodide for re-utilization within thyroid cell.10 Synthesis, storage & release of T4 & T3

Blood Transport of thyroid hormonesT3 and T4 are hydrophobic molecules, need carrier proteins for transportation in blood.

More than 99% of T4 is bound on plasma proteins.

During this period, part of T4 is deionidated to T3 because T3 is 10 times more metabolically active.

Both bind to target receptors, but T3 is 10 times more active than T4The Binding Proteins (BPs)Thyroxine-Binding Globulin TBG (major transport protein) (affinity for T4 is 10 times higher than for T3 )Transthyretin TTR (~15%) (called Thyroxine-Binding pre-Albumin TBPA)Albumin ALB (~15%)

Blood Transport of Thyroid HormonesFactors Affecting BindingChanges in BP ConcentrationAbnormal BP affinityBinding InhibitorsPathological BPsTotal T4 conc. 55-145 nmol/l Total T3 conc. 1.4-2.8 nmol/l A peripheral deiodinase in target tissues such as pituitary, kidney, and liver selectively removes I from the 5 position of T4 to make T3In this sense, T4 can be thought of as a pro-hormone, though it does have some intrinsic activity.The de-Iodinization of T4 to yield T3 that occurs in target tissues yields equal amounts of two isomers of T3, the active T3) and the inactive (reverse T3).So, the ratio of T3 to rT3 falls in febrile illness.During starvation, T4 may be converted to reverse T3 (rT3), which is biologically not active.

Both T3 and rT3 are deiodinated to inactive T2 (bis-Iodothyronine).

Regulation of THNegative feedback regulation of TH release Rising TH levels provide negative feedback inhibition on release of TSH

Hypothalamic thyrotropin-releasing hormone (TRH) can overcome the negative feedback during pregnancy or exposure to coldFigure 16.7

HypothalamusAnterior pituitaryThyroid glandThyroidhormonesTSHTRHTarget cellsStimulatesInhibits

Functions of thyroid hormonesStimulate metabolic rate: increasing number and size of mitochondria, increasing enzymes in the metabolic chain, increasing Na+/K+ ATPase activity. Resting metabolic rate may increase 100% with excess thyroid hormones or decrease by 50% with a deficiency

role of TH on carbohydrate metabolismCarbohydrate metabolism: Thyroid hormones stimulate almost all aspects of carbohydrate metabolism, including enhancement of insulin-dependent entry of glucose into cells and increased gluconeogenesis and glycogenolysis to generate free glucose.

Increases heat production and oxygen conumption.

Stimulates glycogenolysis and gluconeogenesis in liver, both directly and thru potentiation of adrenaline effect.

Stimulates intestinal absorption of glucose.

T3 increases the glucose uptake by muscles, Increases muscle glycolysis. Role of TH on Lipid metabolismLipid metabolism: Increased thyroid hormone levels stimulate fat mobilization, leading to increased concentrations of fatty acids in plasma. They also enhance oxidation of fatty acids in many tissues.

Stimulates the release of FFA from adipose tissues .

Stimulates the oxidation of FA.

Reduces the conc of circulating cholesterol.Role of TH on Protein metabolismStimulates synthesis of proteins, mainly enzymes involved in oxidative reactions required for protein syn.

Stimulates protein catabolismTSH TestsTSH test is the best way to initially test thyroid function.

A high TSH level indicates a thyroid gland failure (a problem that is directly affecting the thyroid (primary hypothyroidism).

A low TSH level indicates an overactive thyroid that is producing too much thyroid hormone (hyperthyroidism).

Occasionally, a low TSH may result from an abnormality in the pituitary gland, which prevents synthesis of enough TSH to stimulate the thyroid (secondary hypothyroidism).

In most healthy individuals, a normal TSH value means that the thyroid is functioning normally.

T4 TestsT4 circulates in the blood in two forms: T4 bound to proteins that prevent the T4 from entering the various tissues that need thyroid hormone and free T4, which enter the various target tissues to exert its effects.The free T4 is important test to determine how the thyroid is functioning, and tests to measure this are called the Free T4 (FT4) and the Free T4 Index (FT4I or FTI). Individuals who have hyperthyroidism will have an elevated FT4 or FTI, whereas patients with hypothyroidism will have a low level of FT4 or FTI. Combining the TSH test with the FT4 or FTI accurately determines how the thyroid gland is functioning. The finding of an elevated TSH and low FT4 or FTI indicates primary hypothyroidism due to disease in the thyroid gland. A low TSH and low FT4 or FTI indicates hypothyroidism due to a problem involving the pituitary gland. A low TSH with an elevated FT4 or FTI is found in individuals who have hyperthyroidism.

T3 TestsT3 tests are often useful to diagnosis hyperthyroidism or to determine the severity of the hyperthyroidism.

Patients who are hyperthyroid will have an elevated T3 level. In some individuals with a low TSH, only the T3 is elevated and the FT4 or FTI is normal.

T3 testing rarely is helpful in the hypothyroid patient, since it is the last test to become abnormal. Patients can be severely hypothyroid with a high TSH and low FT4 or FTI, but have a normal T3.

In some situations, such as during pregnancy or while taking birth control pills, high levels of total T4 and T3 can exist. This is because the estrogens increase the level of the binding proteins. In these situations, it is better to ask both for TSH and free T4 for thyroid evaluation.