Hormone radioiodinated

Using any of the carrier proteins available in highly purified form, eg, TBG or TBPA, a convenient and accurate quantitative determination of and is possible by displacement of radioiodinated or T. This procedure enables their quick determination at low concentrations even in the presence of coundess other substances that occur in body duids (31). In a similar fashion, intact cell nuclei or solubilized proteins from rat fiver cell nuclei, which display high affinities for thyroid hormones, especially T, have been used to establish relative binding affinities of many thyromimetic compounds (7). 

Increased radioiodine uptake in the thyroid indicates increased hormone production by the thyroid gland. 

Medicinal or dietary iodine interferes with all in vivo tests of radioiodine uptake, producing low uptakes that may not reflect a true decrease in hormone synthesis. 

Three main modalities of therapy should be considered for patients with thyrotoxicosis, namely, medical therapy, surgical thyroidectomy, and radioiodine. The choice between these therapies should be dictated by the clinical nature of the disease, the patient s general health, her desire for pregnancy or need to care for young children, and overall patient preference. Treatment is initially monitored by free thyroxine (T4) values, as suppression of thyroid-stimulating hormone (TSH) may persist for months despite adequate management. 

The onset of effect of these drugs is slow 2-A weeks is required before stored hormons are depleted and clinical signs of improvement are observed. Short-term treatment is indicated to prepare patients for surgery or radioiodine therapy, while long-term treatment is indicated for cases inappropriate for surgery or radioactive modalities or where medical therapy alone is used. Usually the patient is advised to continue treatment for 1-2 years spontaneous remission occurs in about 50% of cases treated in this way. There is no way to predict remission confidently, however hormone levels, TSAb titer, and goitre size are useful guides. 

In 50 women taking levothyroxine either for primary thyroid failure or for hypothyroidism secondary to radioiodine treatment for hyperthyroidism, there was no difference between the two groups in terms of bone density at the hip or spine and no difference from the reference population (31). In addition, there was no correlation between bone density and circulating thyroid hormone concentrations or duration of levothyroxine replacement. These findings are reassuring, although large studies of fracture risk are required, in view of previous evidence of an adverse effect of levothyroxine on bone mineral density, especially in post-menopausal women (32). 

Lithium blocks the release of iodine and thyroid hormones from the thyroid and has been used to treat hyperthyroidism, as an adjunct to radioiodine therapy (602-605) and in metastatic thyroid carcinoma (606). However, it can also cause hyperthyroidism. Lithium enhanced the efficacy of radioiodine in 23 patients (607), but was ineffective in a larger comparison of lithium (n = 175) or radioiodine alone (n = 175) (608). In 24 patients with Graves disease, lithium attenuated or prevented increases in thyroid hormone concentration after methimazole withdrawal and radioiodine treatment (602,609). 

Bogazzi F, Bartalena L, Campomori A, Brogioni S, Traino C, De Martino F, Rossi G, Lippi F, Pinchera A, Martino E. Treatment with lithium prevents serum thyroid hormone increase after thionamide withdrawal and radioiodine therapy in patients with Graves disease. J Clin Endocrinol Metab 2002 87(10) 4490-5. 

In rats equilibrated with radioiodine-labelled T4 or T3 roughly half of the radioactivity appears as I- in the urine and the other half as free iodothyronines in the feces [12]. Treatment of the rats with 6-propyl-2-thiouracil (PTU) results in a marked decrease in urinary radioactivity and a reciprocal increase in fecal clearance [12]. Also, in humans, PTU has been shown to inhibit peripheral iodothyronine deiodination besides its well-known effect on thyroid hormone biosynthesis [13]. Compared with the rat, deiodination is an even more important pathway for the clearance of thyroid hormone in man as evidenced by the greater proportion undergoing urinary clearance [2]. Furthermore, estimation of iodothyronine turnover kinetics in humans has demonstrated that a major fraction of T4 disposal is accounted for by plasma production rates of T3 and rT3 [2,3],... 

Astwood EB, Bissell A (1944) Effect of thiouracil on the iodine content of the thyroid gland. Endocrinol 34 282-296 Ching M (1981) Dose-related effect of growth hormone on thyroidal radioiodine uptake. Horn Res 14 234—42 Gutshall DM, Pilcher GD, Langley AE (1989) Mechanism of the serum thyroid hormone lowering effect of perfluoro-n-decanoic acid (PFDA) in rats. J Toxicol Environ Health 28 53-65... 

Lithium blocks the release of iodine and thyroid hormones from the thyroid and has been used to treat hyperthyroidism, as an adjunct to radioiodine therapy... 

The Talc-Resin-Trichloroacetic Acid Test for Screening Radioiodinated Polypeptide Hormones... 

Liothyronine Tabs. Liothyronine is the most rapidly effective thyroid hormone, a single dose giving maximum effect within 24 h and passing off over 24—48 h. It is not used in routine treatment of hypothyroidism because its rapid onset of effect can induce heart failure. Its main uses are in myxoedema coma and psychosis, both rare conditions. A specialised use is during the withdrawal of levothyroxine replacement (to permit diagnostic radioiodine scanning) in patients with thyroid carcinoma. 

Iodine radioiodine which destroys the cells making thyroid hormone iodide, an excess of which reduces the production of thyroid hormone temporarily by an unknown mechanism (it is also necessary for the formation of hormone, and both excess and deficiency can cause goitre). 

Quick relief can be obtained with a p-adrenoceptor blocking drug (judge dose by heart rate) though these do not block all the metabolic effects of the hormone, e.g. on the myocardium, and the basal metabolic rate is unchanged. For this reason they should not be used as sole therapy except in mild thyrotoxicosis in preparation for radioiodine treatment, and should be continued in these patients until the radioiodine has taken effect. They do not alter the course of the disease, nor biochemical tests of thyroid function. Any effect on thyroid hormonal action on peripheral tissues is clinically unimportant. It is desirable to choose a drug that is nonselective for pj and p2 receptors and lacks partial agonist effect (e.g. propranolol 20-80 mg 6-8-hourly, or timolol 5 mg once daily). Usual contraindications to P-blockade (see p. 478) should be observed, especially asthma. 

In hyperthyroidism the beneficial effects of a single dose may be felt in one month, and patients should be reviewed at 6 weeks to monitor for onset of hypothyroidism. The maximal effect of radioiodine may take 3 months. P-adrenoceptor blockade and, in severe cases, an antithyroid drug (but see footnote 1) will be needed to render the patient comfortable whilst waiting this is more likely when radioiodine is used for treatment of patients with relapsing thyrotoxicosis. Very rarely radiation thyroiditis causes excessive release of hormone and thyroid storm. Repeated doses are sometimes needed. 

Radioiodine uptake can be used to test thyroid function, though technetium would be more usual. Scanning may be used for the identification of solitary nodules, and in the differential diagnosis of Graves disease from the less common thyroiditides (e.g. de Quervain s thyroiditis). In the latter, excessive thyroid hormone release caused by follicular cell damage can cause clinical and biochemical features of hyperthyroidism, but uptake is reduced. 

Ross DS, Nussbaum SR. Reciprocal changes in parathyroid hormone and thyroid function after radioiodine treatment of hyperthyroidism. J Clin Endocrinol Metab 1989 68 1216-9. 

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