Interpretation of serum tri-iodothyronine levels measured by the sterling technic

Screening thyrometabolic disorders using total serum thyroxine level

The results of in-vitro thyroid function tests and clinical details of 1,517 patients are reviewed, and the total serum thyroxine levels and the free thyroxine indices are compared in terms of their diagnostic value. Only 3-7% of the patients investigated had a total serum thyroxine level (T4) that did not correlate with the free thyroxine index. We believe that the evidence is strong enough to suggest that estimation of the T4 is an adequate screening test for thyrometabolic disease, and that performing the extra test (T3 resin uptake) in order to estimate the free thyroxine index (FTI) cannot be justified in terms of cost-benefit. We suggest that the FTI is indicated only in those cases in which the T4 is outside the 80% limit range. A simple scheme for the investigation of thyrometabolic disorders is presented.

Radioimmunoassay and the hormones of thyroid function

Radioimmunoassay (RIA) has provided the tools for wide-reaching investigations that have changed and continue to change many important concepts of thyroid physiology and pathosphysiology. The RIA vor human thyrotropin (TSH) was developed in 1965; development of the RIA for triiodothyronine (T3), thyroxine (T4), thyroxine-binding globulin (TBG), and, recently, thyrothropin-releasing hormone (TRH) and thyroglobulin (Tg) followed. The capacity to measure nanogram and picogram concentrations with relative ease and speed has permitted the demonstration of dynamic relationships of the intrathyroidal and circulating thyroid hormones to each other and to the pituitary and hypothalamic regulating hormones. Evidence for the presence of cross-influences between TRH and other hypothalamic regulating hormones on the secretion of pituitary hormones has accumulated. The impact of the new information on clinical practice is now becoming evident. There is new appreciation of the value of assaying serum T3 and TSH concentrations in the clinical management of patients with disturbed function of the thyroid, pituitary, or hypothalamus. The necessary components for RIA performance can be purchansed separately or in kit form from commercial sources. With appropriate quality-control procedures, precise, sensitive, and reliable data can be generated. Awareness of the specific technical problems relating to the RIA of these hormones is absolutely necessary to assure reliable results. The availability of kits or their components permits the performance of these studies in the community hospital and in reliable commercial-service laboratories.

Solitary autonomously functioning thyroid lesions. Diagnosis, clinical features and pathogenetic considerations

One hundred sixty-four patients with autonomously functioning thyroid lesions (AFTL), 140 with nontoxic and 24 with toxic lesions, were studied over 12 years. To avoid pitfalls in diagnosis of nontoxic AFTL, a suppression procedure should precede the repeat scan after the administration of thyroid stimulating hormone. Women outnumbered men by 13 to 1 for nontoxic AFTL and 3.8 to 1 for toxic AFTL. Larger and toxic lesions were found in older patients, suggesting their derivation from smaller nontoxic lesions. Serum triiodothyronine (T3) and thyroxine concentrations exhibited parallel deviations from average normal levels for most of 29 patients with hot AFTL. T3 toxicosis was not seen. Ultrasound was more sensitive than scanning in detecting cystic degeneration. Observation was advised for patients with nontoxic AFTL unless toxicity appeared imminent. Of 51 such patients, followed from 1 to 12 years, none became hyperthyroid, but 10 AFTL increased from slightly to fourfold in size or showed evidence of degeneration. Of 24 patients with toxic AFTL, 20 were treated with radioiodine and the remaining 4 surgically; surgery is the preferred treatment for patients under 40 years of age. Three of 29 patients who were treated surgically had thyroid cancer. Since AFTL have histologic and pathophysiologic features characteristic of benign endocrine neoplasms, the association with cancer may be more than coincidental.

Thyroid function after subtotal thyroidectomy for hyperthyroidism

Among 76 patients who had had a subtotal thyroidectomy for hyperthyroidism from one to seven years previously recurrent hyperthyroidism was found in three and hypothyroidism in 13. The remaining 60 subjects were clinically euthyroid but a raised level of serum thyroid-stimulating hormone (TSH; greater than 5-0 mu U/ml) was found in 39. Analysis of the data showed that their serum thyroxine was significantly lower than in the subjects with a normal TSH. The serum triiodothyronine (T-3) was similar in both groups. It is concluded that subjects with a raised TSH remain clinically euthyroid by maintaining a normal serum T-3 concentration. There was no evidence of any long-term progressive deterioration of thyroid function after subtotal thyroidectomy.

Simultaneous measurement of thyroxine and triiodothyronine peripheral turnover kinetics in man

Serum triiodothyronine (T(3)) kinetics in man have been difficult to define presumably due to the interference of iodoproteins generated during the peripheral metabolism of T(3). The use, in the present study, of an anion-column chromatographic method for separation of serum T(3) as well as thyroxine (T(4)) from these iodoproteins has overcome this technical handicap. Simultaneous measurement of serum (125)I-T(3) and (131)I-T(4) kinetics were performed in 31 subjects from the clinical categories of euthyroid, primary hypothyroid, thyrotoxic and posttreatment hypothyroid Graves' disease, factitial thyrotoxic, and idiopathically high and low thyroxinebinding globulin states. The normal mean T(3) fractional turnover rate (kT(3)) was 0.68 (half-life = 1.0 days), increased in toxic Graves' disease patients to 1.10 (half-life = 0.63 days), and decreased in primary hypothyroid patients to 0.50 (half-life = 1.38 days). The mean T(3) equilibration time averaged 22 hr except in hypothyroid and high thyroxine-binding globulin (TBG) patients where the equilibration period was delayed by 10 hr. The mean T(3) distribution space in normal subjects was 38.4 liters. This was reduced in subjects with high TBG levels (26 liters) and increased in patients with low TBG and in all hyperthyroid states (53-55 liters). The normal serum T(3) concentration was estimated by radioimmunoassay to be 0.106 mug/100 ml. Combined with the mean T(3) clearance value of 26.1 liters/day, the calculated T(3) production rate was 27.6 mug/day. The mean T(3) production rate increased to 201 mug/day in thyrotoxic Graves' disease patients and was reduced to 7.6 mug/day in primary hypothyroid subjects. T(3) production rate was normal in subjects with altered TBG states. The ratio of T(3) to T(4) production rate in normal subjects was 0.31 and was unchanged in patients with altered TBG values. This ratio was increased in all Graves' disease patients with the highest value being 0.81 in the posttreatment hypothyroid Graves' disease group. This apparent preferential production of T(3) may have been responsible for the retention of rapid turnover kinetics for T(3) and T(4) observed in treated Graves' disease patients. The finding that factitial thyrotoxic patients also displayed similar rapid T(3) and T(4) turnover kinetics indicates that these alterations are not a unique feature of Graves' disease per se. When comparing the peripheral turnover values for T(3) and T(4) in man, it is apparent that alterations in metabolic status and serum TBG concentration influence both hormones in a parallel manner; however, changes in metabolic status seem to have a greater influence on T(3) kinetics while alterations in TBG concentrations have a greater effect on T(4). These observations probably relate to the differences in TBG binding affinity and peripheral tissue distribution of these two hormones.

Triiodothyronine radioimmunoassay

Highly specific antisera to triiodothyronine (T(3)) were prepared by immunization of rabbits with T(3)-bovine serum albumin conjugates. Antisera with T(3) binding capacity of up to 600 ng/ml were obtained. The ability of various thyronine derivatives to inhibit the binding of T(3-) (125)I to anti-T(3) serum was found to vary considerably. l-T(3), d-T(3) and several triiodoanalogues were potent inhibitors of the reaction. Little inhibition of T(3-) (125)I binding was produced by l-thyroxine (T(4)) or other tetraiodo- analogues, thyronine or iodotyrosines. Chromatography of several T(4) preparations indicated that most of their very slight activity could be ascribed to contamination with T(3). Successful assay of T(3) in serum was accomplished by the addition of diphenylhydantoin to the assay system. Under these circumstances, recovery of T(3) added to serum was excellent, and addition of T(4) was without significant effect. Serum T(3) concentrations in normal subjects averaged 145 +/-25 ng/100 ml (sd). Increased concentrations (429 +/-146 ng/100 ml) were observed in hyperthyroid patients whereas those with hypothyroidism had serum T(3) levels of 99 +/-24 ng/100 ml. Elevated T(3) concentrations were found also in hypothyroid patients receiving 25 mug or more of T(3) daily and in those receiving 300 mug of T(4) daily. Serial measurements of T(3) concentrations in subjects after oral T(3) administration revealed peak T(3) concentrations 2-4 hr after T(3) administration. Intramuscular administration of thyrotropin (TSH) resulted in earlier and more pronounced increases in serum T(3) than in serum T(4) concentrations. Triiodothyronine (T(3))(1) was recognized to be a biologically active secretory product of the thyroid gland over a decade ago (1). Recent studies have indicated that it is formed extrathyroidally as well (2, 3). Nevertheless, relatively little information concerning the role of T(3) secretion in different thyroid disorders has been accumulated until very recently. Methods for the measurement of T(3) which require its extraction from plasma, and often its separation from thyroxine as well, have been described by several investigators (4-11). These methods have proven useful, but they are relatively complicated, the number of samples that can be assayed is limited, and they may be affected by in vitro deiodination of thyroxine. More recently the radioimmunoassay technique has been applied to the measurement of T(3). Several preliminary reports have appeared describing the preparation of antibody to triiodothyronine by immunization of animals with T(3)-protein conjugates and its use for the measurement of T(3) in serum (12-15). The present report describes the development of a radioimmunoassay for the measurement of T(3), studies of the specificity of the anti-T(3) serum, and some initial studies which indicate that the method is applicable to the measurement of T(3) in unextracted serum.