The effects of an acute load of thyroxine on the transport and peripheral metabolism of triiodothyronine in man

Serum concentration of unsaturated thyroxine-binding globulin in hyper- and hypothyroidism

The concentration of serum thyroxine (T4)-binding globulin (TBG) not binding T4 (unsaturated TBG, u-TBG) was determined in hyper- and hypothyroidism. u-TBG was expressed as the product of TBG concentration and the ratio of free TBG capacity to maximal TBG capacity as determined by reverse-flow electrophoresis. u-TBG concentration in normal sera (n = 40) was 15.5 +/- 2.3 mg/l (mean +/- SD), or 257 +/- 38 nmol/l for a molecular weight of TBG of 60 000 daltons. u-TBG levels were significantly lower in hyperthyroidism (7.1 +/- 2.3 mg/l, n = 16, P less than 0.001) and higher in hypothyroidism (21.7 +/- 5.0 mg/l, n = 22, P less than 0.001). Based on partial correlation analysis, u-TBG was inversely correlated to serum T4 (r = -0.586, P less than 0.001), but not correlated to triiodothyronine (T3) (r=-0.180, NS). There was a reciprocal correlation between u-TBG concentration and the T3 uptake value (r = 0.748, P less than 0.001). There was also a reciprocal correlation of u-TBG with both % free T4 (r = 0.425, P less than 0.001) and % free T3 (r = 0.377, P less than 0.001), when the data were subjected to partial correlation analysis. These results provide the values for u-TBG concentration in hyper- and hypothyroidism, and support the concept that the free fractions of serum thyroid hormones may be determined by the number of binding sites of the TBG molecule that are not saturated with T4 in hyper-and hypothyroidism.

Twenty-four hour variations of thyroid hormones and thyrotrophin concentrations in hypothyroid infants treated with L-thyroxine

The 24 hour plasma profile of T4, FT4, T3 and TSH was measured in a group of infants treated for congenital hypothyroidism with an oral aqueous preparation of L-T4 (5-7.5 micrograms/kg) given as a single dose at 0800 h. Mean basal plasma T4 was 8.4 +/- 2.0 micrograms/dl (mean +/- SD) and remained constant during the study while mean serum FT4 increased significantly from 1.64 +/- 0.50 ng/dl to a maximum of 2.08 +/- 0.63 ng/dl 4 h after medication. Plasma T3 decreased significantly from 241 +/- 48 ng/dl to 202 +/- 36 ng/dl 6 h after drug administration. Finally, plasma TSH decreased significantly from a mean of 24.2 +/- 18.8 microU/ml to 14.8 +/- 10.5 microU/ml 6 h later. Therefore during treatment of congenital hypothyroidism with L-T4 no specific schedule for T4 sampling is required. However, TSH determination 4 to 8 h after medication could slightly underestimate mean levels. In addition, any attempt to monitor treatment by FT4 determination should take into consideration variations of plasma values in the hours following L-T4 absorption.

Reciprocal effects of an acute load of thyroxine or triiodothyronine on their peripheral metabolism and deiodination in the cold-acclimated rat

Wistar rats acclimated to cold received iv 25 micrograms T 4/100 g bw, 21 micrograms T 3/100 g bw or 2.5 micrograms T 3/100 g bw, to observe the changes induced in the peripheral metabolism of 125I-T4 and 125I-T3 relative to cold-adapted untreated animals. Each animal was injected with a tracer dose of either labelled hormone in addition to the T4 or T3 load, and placed in metabolic cages for 24 hour collection of urine and feces. Sequential heparinized blood samples were obtained by cardiac puncture. A T4 load decreased the deiodination of 125I-T4 and 125I-T3 (p less than 0.01) as revelated by urinary 125I. A T3 load, in the two dosages employed, decreased the deiodination of 125I-T3 (p less than 0.01) but had no effect on deiodination of 125I-T4. Similarly, a T4 load increased the fecal excretion of both radioactive iodothyronines (p less than 0.01) whereas a T3 load failed to alter the excretion of 125I-T4. In cold-adapted animals plasma TSH was elevated (p less than 0.05) and plasma T4 was low (p less than 0.001) as compared to rats housed at 22 C. It is concluded that the relative contribution of T4 and T3 to the metabolic state in the rat is not significantly altered by cold exposure.

Influx of thyroid hormones into rat liver in vivo. Differential availability of thyroxine and triiodothyronine bound by plasma proteins

The transport of [(125)I]thyroxine (T(4)) and [(125)I]triiodothyronine (T(3)) into liver was investigated with a tissue sampling-portal vein injection technique in the anesthetized rat. The method allows the investigation of the effects of plasma proteins in human serum on the unidirectional influx of T(4) or T(3) into liver cells. The percent extraction of unidirectional clearance of T(3) and T(4) was 77+/-2% and 43+/-2%, respectively, after portal injection of a bolus of Ringer's solution. Cell membrane transport of T(4) or T(3) was nonsaturable because 50-muM concentrations of unlabeled hormone had no effect on transport. The addition of bovine albumin in concentrations of 1, 5, or 10 g/100 ml bound >98% of T(4) or T(3) in vitro, but had no significant effect on T(3) or T(4) transport in vivo. Conversely, 10% rabbit antisera specific for T(3) or T(4), completely abolished the intracellular distribution of thyroid hormone into liver. In the presence of rat serum, which contains albumin and thyroid hormone binding pre-albumin (TBPA), 18 and 81% of total plasma T(4) and T(3), respectively, were available for transport in vivo. The fraction of hormone available for transport in the presence of normal human serum, which contains albumin, TBPA, and thyroid hormone binding globulin (TBG) was 11% for T(4) and 72% for T(3). The fraction of hormone transported into liver after injection of serum obtained from pregnant or birth control pilltreated volunteers was 4% for T(4) (but this was not significantly different from zero) and 54% for T(3). THESE DATA SUGGEST: (a) The mechanism by which T(4) and T(3) traverse the liver cell membrane is probably free diffusion. (b) Albumin-bound T(4) or T(3) is freely cleared by liver, approximately 50% of TBG-bound T(3) is transported, but little, if any, of TBPA-bound T(4) or TBG-bound T(4) is cleared by liver cells. (c) Although the albumin-bound fraction of T(4) greatly exceeds the free (dialyzable) moiety, the two fractions are both inversely related to the existing TBA or TBG level; therefore, in vitro measurements of free T(4) would be expected to accurately reflect what is available for transport in vivo. Conversely, TBG-bound T(3) is readily transported in vivo; therefore, it is proposed that in vitro measurements of free T(3) do not reliably predict the fraction of T(3) available for transport into liver in vivo.

Changes in the T4/T3 molar ratio following thyrotropin releasing hormone injection in cattle

The injection of thyrotropin releasing hormone into cattle resulted in a rapid decrease in the T4/T3 molar ratio. 2 breeds of cattle, Shorthorn and Africander Cross were studied. The decrease in the T4/T3 molar ratio was significantly greater in the Shorthorn breed. It is concluded that acute stimulation of the thyroid gland with TRH results in enhanced release of both T3 and T4 and that T3 is discharged more rapidly than T4.

The influence of thyroxine on the serum free triiodothyronine concentration

By means of measurement of the serum free triiodothyronine fraction (%FT3) by equilibrium dialysis, the influence of varying serum concentrations of thyroxine (T4) on %FT3 was studied in conditions associated with lack or excess of serum T4 and in serum enrichment with T4 in vitro. The enrichment of pooled serum with T4 caused significant increases of %FT3 with T4 was added to elevate its T4 content between 10 and 25 mug/dl. In 4 subjects with primary hypothyroidism treated with T3 (25 mug once daily for two weeks, twice daily thereafter), progressive increases of serum T3 were observed while their T4, free T4 and T3 fractions remained unchanged. Consequently, the serum free T3 and thyrotropin concentrations became normal after the serum T3 concentrations became hypernormal. When comparisons of the serum thyroid hormone levels were made between 6 subjects with T3 toxicosis and 8 with a usual variety of hyperthyroidism matched for serum T3 concentration, the former sub-group of hyperthyroidism showed significantly lower serum free T3 concentration (0-51 +/- 0.22 ng/dl versus 0.79 +/- 0-21 ng/dl, p less than 0.05). The amount of T4 in serum is considered to affect its free T3 concentration by virtue of sharing serum-binding proteins.

Estimation of thyroxine and triiodothyronine distribution and of the conversion rate of thyroxine to triiodothyronine in man

Studies on peripheral metabolism of simultaneously administered 125-I-labeled L-thyroxine ([125-I]T4) and 131-I labeled L-trilodothyronine ([131-I]T3) were performed in five normal subjects, in four patients with untreated hypothyroidism, and in 3 hypothyroid patients made euthyroid by the administration of T4. The fractional turnover rate (lambda 03) of thyroid hormones irreversibly leaving the site of degradation and the volumes of pool 1 (serum V1) of pool (interstitial fluid, V2), and of pool 3 (all tissues, V3)were obtained by using a three-compartment analysis. In addition to the turnover studies, the ratios for the in vivo T4 to T3 conversion were determined by paper chromatographic study in sera obtained 4, 7, and 10 daysafter the injection. The rate (K12) of the extrathyroidal conversion of T4 to T3 was also estimated by the compartment analysis. The T3 distribution volume (V3) of pool 3, in which T3 is utilized and degraded, was about 60% of totaldistribution volume (V=V1+V2+V3) in normal subjects, whereas only about 25% of the extrathyroidal T4 pool was in the intracellular compartment, indicating that T3 is predominantly an intracellular hormone..

Suppression of pituitary TSH secretion in the patient with a hyperfunctioning thyroid nodule

10 patients with a single hyperfunctioning thyroid nodule each were studied for pituitary thyrotropin (TSH) suppression. They were judged to be euthyroid on clinical grounds. The total thyroxine (T(4)D), free thyroxine (FT(4)), total triiodothyronine (T(3)D), and free triiodothyronine (FT(3)) were normal in most of the patients. Incorporation of (131)I into the hyperfunctioning thyroid nodules was not suppressed by the administration of physiological doses of T(3). Basal serum TSH concentrations were undetectable (<0.5 - 1.0 muU/ml) in all patients. The metabolic clearance of TSH in one patient before and after excision of the thyroid nodule was unchanged (40 vs. 42 ml/min) whereas the calculated production rate was undetectable before the operation (<29 mU/day) and normal after (103 mU/day). These data, in one patient, suggest that the undetectable concentration of TSH in these patients is a result of suppressed TSH secretion rather than accelerated TSH clearance. In eight patients, basal serum TSH concentrations failed to increase after the intravenous administration of 200 mug of thyrotropin-releasing hormone (TRH); minimal increases in serum TSH concentrations were observed in two patients. The suppression of TSH was evident despite "normal" concentrations of circulating thyroid hormones. The observation that normal serum concentrations of T(4)D, FT(4), T(3)D, and FT(3) may be associated with undetectable basal serum TSH concentrations and suppressed TSH response to TRH was also found in four hypothyroid patients given increasing doses of L-thyroxine and sequential TRH stimulation tests.

Serum thyroxine levels in patients receiving L-thyroxine suppression or replacement therapy

Serum thyroxine (T(4)) and protein-bound iodine (PBI) levels were measured in 140 adult patients receiving L-thyroxine, 0.2 or 0.3 mg. daily for at least six months, as suppression or replacement therapy. All were judged to be clinically euthyroid at the time of the measurements. The mean PBI was 8.3 mug./100 ml. and the mean T(4) was 10.8 mug./100 ml. The range of values for the PBI was 3.5 to 13.2 and for the T(4) was 3.1 to 17.0. Because of the wide range of values, it is considered that the judgement of whether a patient is euthyroid on L-thyroxine therapy should be based primarily on clinical evaluation and only secondarily on serum T(4) or PBI levels.

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.

Serum triiodothyronine: measurements in human serum by radioimmunoassay with corroboration by gas-liquid chromatography

Serum triiodothyronine (T(3)) has been measured by radioimmunoassay and corroborated by analysis of the identical samples with a previously described gas-liquid chromatographic technique. Special features of the radioimmunoassay procedure which permit determinations in unextracted serum include the use of a T(3)-free serum preparation for the construction of the standard curve and of tetrachlorothyronine to inhibit binding of T(3) to thyroxine-binding globulin.T(3) values by radioimmunoassay were 138 +/-23 ng/100 ml (mean +/-SD) in 82 normal subjects, 62 +/-9 ng/100 ml in 45 hypothyroid patients, and 494 +/-265 ng/100 ml in 60 patients with toxic diffuse goiter. In the hypothyroid group, the range was similar in patients with both primary and secondary hypothyroidism. There was no overlap between the three thyroidal states. Elevated T(3) levels were seen in 40 cases that appeared clinically hyperthyroid but had normal serum thyroxine (T(3)) determinations, a syndrome we have called T(3) toxicosis. Values obtained with radioimmunoassay agreed closely with those we had previously found by gas-liquid chromatography which were 68 +/-2 ng/100 ml in hypothyroidism, 137 +/-23 ng/100 ml in normal subjects, and 510 +/-131 ng/100 ml in untreated toxic diffuse goiter. Since T(3) is very potent and its level varies in different clinical states, accurate T(3) measurements are required to assess a patient's thyroid status properly. The radioimmunoassay for T(3) appears to be sufficiently sensitive, precise, and simple to permit its routine clinical application for this purpose.

Alterations in thyroid hormone economy during acute infection with Diplococcus pneumoniae in the rhesus monkey

In order to study the alterations in thyroid hormone economy that accompany an acute bacterial infection, rhesus monkeys were inoculated i.v. with a virulent Diplococcus pneumoniae culture containing approximately 10(8) organisms per dose. This was found to produce a well-defined febrile illness followed in most instances by spontaneous recovery, thereby permitting sequential observations to be made during progression from the healthy state through acute infection into convalescence. During the acute febrile period of the infection, the clearance of both exogenously labeled L-thyroxine (T(4)) and 3,3',5-triiodo-L-thyronine (T(3)) from their peripheral pools was accelerated. This alteration was often evident by 8 hr after inoculation with the virulent culture and could not be ascribed to a decrease in extracellular binding. Despite the accelerated hormonal clearance, the concentrations of both endogenously labeled thyroid hormone and stable T(4) in the sera of the surviving monkeys remained essentially unchanged or increased, indicating that hormonal secretion must have increased during this period. During the convalescent period, hormonal clearance was similar to preinfection control values. Leukocytes isolated from blood obtained 6 hr after inoculation with the virulent culture displayed enhanced T(4)-deiodinative activity.

The distribution kinetics of triiodothyronine: studies of euthyroid subjects with decreased plasma thyroxine-binding globulin and patients with Graves' disease

The kinetics of distribution of 3,3',5-triiodo-L-thyronine (T(3)) have been studied employing both a single-injection and a continuous infusion of T(3-) (131)I. External monitoring of radioactivity in the liver during the infusion permitted estimation of the hepatic distribution volume (V(H)) and the one-way hepatic clearance (C(H)) of the hormone. Among 10 euthyroid control subjects, V(H) averaged 2.07 liters +/-0.50 (SD), and the mean value for C(H), 231 ml of plasma per min (+/-64). In three euthyroid men whose plasma showed decreased binding capacity by thyroxine-binding globulin (TBG) abnormally high V(H) and C(H) values were found, the increase in C(H) being proportional to the decrease in binding activity by plasma proteins. Among all 13 subjects, there was a high correlation (+ 0.86) between C(H) and the proportion of free hormone in plasma, measured in vitro. In four patients with hyperthyroid Graves' disease V(H) ranged from 3.2 to 4.2 liters and C(H) was elevated in every case, averaging 989 ml/min. The increase in C(H) in this group was out of proportion to the elevation of free hormone fraction in plasma. Seven patients who were either euthyroid or hypothyroid after treatment of Graves' disease showed a slight but significant increase in C(H) compared with the euthyroid controls without Graves' disease. The percentage of free hormone in the plasma of the treated group was normal or low and therefore could not explain the persistent elevation in unidirectional hepatic clearance observed. The rate of accumulation of labeled T(3) in the tissues of the thigh during the interval from 10 to 60 min of the sustaining infusion of tracer was slow compared to the rate of equilibration in the liver and did not differ significantly among the various groups studied. These latter findings suggest that in slowly equilibrating tissues such as the thigh the kinetics of T(3) distribution are relatively insensitive to alterations in hormone-binding activity by plasma proteins.

Conversion of thyroxine (T4) to triiodothyronine (T3) in athyreotic human subjects

Studies of the possibility that thyroxine (T4) is converted to 3.5,3'-triiodo-L-thyronine (T3) in the extrathyroidal tissues in man have been conducted in 13 patients, all but two of whom were athyreotic or hypothyroid, and all of whom were receiving at least physiological replacement doses of synthetic sodium-L-thyroxine.T3 was found in the sera of all patients, in concentrations ranging between 243 and 680 ng/100 ml (normal range 170-270 ng/100 ml). These concentrations were far in excess of those which would have been expected on the basis of the T3 contamination of the administered T4, as measured by the same technique employed in the analysis of serum. When oral medication was enriched with (125)I-labeled T4 for 8 or more days, labeled T3 and tetraiodothyroacetic acid (Tetrac or TA(4)) were found in the serum to the extent of approximately 2-5% of total radioactivity, as assessed by unidimensional paper chromatography. The same results were obtained with a specially purified lot of radioactive T4 containing less than 0.1% T3 as a contaminant. The identities of the (125)I-labeled T3 and TA(4) were verified by two-dimensional chromatography as well as by specific patterns of binding in serum. The labeled T3 isolated was bound by albumin and by T4-binding globulin (TBG), but not by T4-binding prealbumin (TBPA): in contrast the labeled TA(4) was bound by albumin and TBPA, but not by TBG. To exclude the possibility that the conversion of T4 to T3 was a peculiarity of the oral route of administration, the sera of two additional patients were obtained 48 hr after 7-day courses of daily intravenous injections of a mixture of stable and (125)I-labeled T4. Both stable and labeled T3 were likewise found in these sera. In contrast to earlier experiments in humans in which (131)I-labeled T3 was not definitively demonstrated in serum after a single intravenous injection of (131)I-labeled T4, the present findings are taken to provide conclusive evidence of the extrathyroidal conversion of T4 to T3 in man. These results raise once again the question of the extent to which the metabolic effect of T4 is mediated through the peripheral generation of T3.

The peripheral metabolism of triiodothyronine in normal subjects and in patients with hyperthyroidism

In order to assess the contribution of 3.3',5-triiodo-L-thyronine (T(3)) to overall thyroid hormone economy, conjoint measurements of the kinetics of peripheral T(3) metabolism and the total concentration of T(3) in serum were made in a group of normal subjects and in a group of patients with hyperthyroid Graves' disease. As judged from the disappearance of trichloroacetic acid-precipitable (131)I from serum after a single intravenous dose of labeled T(3), the following mean values were obtained in the normal subjects: volume of distribution, 43 liters or 0.62 liter/kg; fractional turnover rate. 52% per 24 hr: clearance rate, 22.3 liters/24 hr: and absolute disposal rate, 60 mug/24 hr. In the patients with untreated hyperthyroidism, values for all these functions were greatly increased. After treatment, the volume of T(3) distribution returned to normal but the fractional turnover rate remained abnormally rapid.