Thyroxine biological function

Iodine is an essential element in humans and other mammals, which is used for the synthesis of the thyroid hormones triiodothyronine (T3) and thyroxine (T4). These hormones play a prominent role in the metabolism of most cells of the organism and in the process of early growth and development of most organs, especially brain (Anderson et al., 2000). Besides T3 and T4, reverse T3 (rT3), monoiodotyrosine (MIT), and diiodotyrosine (DIT) are also synthesized and distributed in the body of humans and animals, but only T3 and T4 have a biological function. Iodine in the human body mainly comes through dietary and water intake, and inhalation of atmospheric iodine. Due to low concentrations of iodine in the air (10—20ng/m ), food and water intake form the major source of iodine for adults, while for infants it is milk. The concentration of iodine in foodstuffs is directly related to that in the environment where the foods come from. Iodine deficiency disorders are mainly found in places where the concentration of iodine in the soil and drinking water is very low. In the water, foodsmffs, and... 

A protein is a polypeptide of 50 or more amino acids that has biological activity. The primary structure of a protein is the particular sequence of amino acids held together by peptide bonds. For example, a hormone that stimulates the thyroid to release thyroxin is a tripeptide with the amino acid sequence Glu-His-Pro. Although five other amino acid sequences of the same three amino acids are possible, such as His-Pro-Glu or Pro-His-Glu, they do not produce hormonal activity. Thus the biological function of peptides and proteins depends on the specific sequence of the amino acids. 

Transport. A wellknown transport protein is hemoglobin in the erythrocytes (bottom left). It is responsible for the transport of oxygen and carbon dioxide between the lungs and tissues (see p.282). The blood plasma also contains many other proteins with transport functions. Prealbumin (transthyretin middle), for example, transports the thyroid hormones thyroxin and triiodothyronine. Ion channels and other integral membrane proteins (see p.220) facilitate the transport of ions and metabolites across biological membranes. 

B. This patient appears to have a hyperthyroid condition even though the thyroid does not appear to be enlarged. Thyroid function tests would be most helpful to determine if this is the case. The free thyroxine level is a direct measure of the amount of free T, the biologically active T, in the serum. Elevation of the free T indicates a hyperthyroid condition. 

In thyroxine 11.14) and the more biologically active tri-iodothyronine, the iodine atoms in the 3- and 4-positions force the two rings into a conformation in which they are perpendicular to one another. Further work, with analogues, indicates that this arrangement is essential for thyroid function (Dietrich etal., 1977). See Section 2.4 for other stereochemical data on how thyroid hormones fit their receptor. 

Biologic Effect of the Thyroid Hormone. The thyroid gland regulates the entire metabolism of the adult organism. Disturbances of the thyroid function are reflected in the basal metabolic rate (metabolism while resting, cf. Chapt. XXII-1). In hyperthyroidism (thyrotoxicosis) the rate is elevated during hypofunction (clinical manifestation myxedema), it is lowered. This phenomenon is used frequently in judging thyroid function. Administration of thyroxine raises the basal... 

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