Thyroxine iodine content

Figure 72.2 Amiodarone structure in comparison with i-thyroxine. Note the high iodine content (broken circies) and the simiiar structures of the two moiecuies.

Although thyroid slices of hyperthyroid patients take up iodine more quickly than normal thyroid slices, the total iodine content of the thyroid gland (1.8 mg/g of dry weight in normal individuals) is consideraWy reduced in hyperthyroidism (0.26 mg/g of dry weight). The reduction affects both inorganic and organic iodine, but when all iodinated compounds in the thyroid are analyzed, thyroxine seems to be the compound most reduced in hyperthyroidism. 

Essential trace elements with their functions in enzymes and hormones are distributed in different parts of the body. Iodine is special. Some 75% of the total iodine content in the human body (about 14 mg) is found in the thyroid gland, a small organ in the neck. There, it participates in the formation of the important thyroid hormone, thyroxine. This is vital for normal mental and physical development and for regulating the body s metabolism. A recommended daily intake is 100-200 pg. Deficiency of iodine will cause goitre. This has to be treated by additional potassium iodide or iodine casein. In developed countries, iodine deficiencies are now rare because iodine is added to table salt. 

Column methods. Thyroxine can be isolated by ion-exchange column chromatography and its iodine content determined as described for PBI. 

Thyroxin (tetraiodothyronine) is circulated in the blood in temporary union with blood proteins hence, what the chemical test actually measures is the protein-bound iodine content of the blood, which, in turn, serves as an index of relative activity of the thyroid in releasing its hormone into the blood—hence, the approximate basal metabolic rate. 

Larsen PR. 1975. Thyroidal triiodothyronine and thyroxine in Graves desease correlation with presurgical treatment, thyroid status, and iodine content J Clin Endocrinol Metab 41, 1098-1104. 

Thyroid hormone synthesis as pg/rat increased 10-20 times to acute excess iodide (Fig.3) and 2-4 times to chronic excess iodide Thyroid hormone concentrations expressed as FBI or iodothyronines were constant regardless of iodide dose, and the degradation rates of thyroxine were not affected by excess iodide. Indeed, increase of organic iodine formation is far more than the increase of thyroidal organic iodine content To release the excessively organified iodine in the form of iodide from the thyroids is another good example of autoregulation by iodide. 

Its biologic effect consists in the control of the thyroid gland. Thyrotropin stimulates the elaboration of thyroxine, while the iodine content of the gland drops drastically. Circulating thyroxine has a feedback effect on the hypophysis, inhibiting the further elaboration of the stimulating hormone (compare with corticotropin, below). 

Tab. 9.4-18 Iodine and selenium intake of nursing women, and iodine, thyroxine and free triiodothyronine content of the blood...

Escobar del Rey F, Pastor R, Mallol J, Morreale de Escobar G, Effects of maternal iodine deficiency on the L-Thyroxine and 3,5,3 -Triiodo-L-Thyronine contents of rat embryonic tissues before and after onset of fetal thyroid function. Endocrinology 118 1259 (1986)... 

A method of thyroid hormone estimation which consists of extracting iodinated amino acids from serum using n-butanol. The extract is washed with alkali which removes inorganic iodine and mono- and di-iodotyrosines, leaving behind triiodothyronine and thyroxine. The butanol extract can then by evaporated to dryness and analysed for iodide content. 

Distribution of radioiodine in two goitrous subjects (FL and DD) from a severe iodine deficient area in Zaire. 3 I thyroid content, extrathyroidal 31i pool (PB 1311 x distribution volume of thyroxine) and urine (cumulative excretion) are expressed in % of 1311 administered. Adapted from Ermans et al. (2) with the permission of the publisher... 

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