Jack H. Oppenheimer

Replacement dose, metabolism, and bioavailability of levothyroxine in the treatment of hypothyroidism. Role of triiodothyronine in pituitary feedback in humans.

A change in the formulation of the levothyroxine preparation Synthroid (Flint) in 1982 prompted us to reevaluate the replacement dose of this drug in 19 patients with hypothyroidism. The dose was titrated monthly until thyrotropin levels became normal. The mean replacement dose (+/- SD) was 112 +/- 19 micrograms per day, significantly less (P less than 0.001) than the dose of an earlier formulation--169 +/- 66 micrograms per day--used in a similar study (Stock JM, et al. N Engl J Med 1974; 290:529-33). The fractional gastrointestinal absorption of a tablet of the current formulation is 81 percent, considerably higher than the earlier estimate of 48 percent. Using high-performance liquid chromatographic analysis, we found that the current tablet contains the amount of thyroxine stated by the manufacturer. By measuring the bioavailability of the earlier type of tablet in five patients, we inferred that the strength of the previous tablet had been overestimated. In the present study, the thyrotropin levels of patients on replacement therapy returned to normal when serum triiodothyronine concentrations were not significantly different from those of controls (122 vs. 115 ng per deciliter [1.87 vs. 1.77 nmol per liter]), but when serum thyroxine levels were significantly above those of controls (11.3 vs. 8.7 micrograms per deciliter [145 vs. 112 nmol per liter], P less than 0.001). These findings suggest the possibility that in humans, serum triiodothyronine may play a more important part than serum thyroxine in regulating the serum thyrotropin concentration.

Functional relationship of thyroid hormone-induced lipogenesis, lipolysis, and thermogenesis in the rat.

Metabolic balance studies were carried out to determine the interrelationships of thyroid hormone-induced lipogenesis, lipolysis, and energy balance in the free-living rat. Intraperitoneal doses of 15 micrograms triiodothyronine (T3)/100 g body wt per d caused an increase in caloric intake from 26.5 +/- 1.7 (mean +/- SEM) kcal/100 g per d to 38.1 +/- 1.5 kcal/100 g per d. Food intake, however, rose only after 4-6 d of treatment and was maximal by the 8th day. In contrast, total body basal oxygen consumption rose by 24 h and reached a maximum by 4 d. Since total urinary nitrogen excretion and hepatic phosphoenolpyruvate carboxykinase mRNA did not rise, gluconeogenesis from protein sources did not supply the needed substrate for the early increase in calorigenesis. Total body fat stores fell approximately 50% by the 6th day of treatment and could account for the entire increase in caloric expenditure during the initial period of T3 treatment. Total body lipogenesis increased within 1 d and reached a plateau 4-5 d after the start of T3 treatment. 15-19% of the increased caloric intake was channeled through lipogenesis, assuming glucose to be the sole substrate for lipogenesis. The metabolic cost of the increased lipogenesis, however, accounted for only 3-4% of the T3-induced increase in calorigenesis. These results suggest that fatty acids derived from adipose tissue are the primary source of substrate for thyroid hormone-induced calorigenesis and that the early increase in lipogenesis serves simply to maintain fat stores. Since the mRNAs coding for lipogenic enzymes rise many hours before oxygen consumption and lipolysis, these results suggest that T3 acts at least in part by an early coordinate induction of the genes responsible for these processes.

Propylthiouracil inhibits the conversion of l-thyroxine to l-triiodothyronine: An explanation of the antithyroxine effect of propylthiouracil and evidence supporting the concept that triiodothyronine is the active thyroid hormone

6-n-propylthiouracil (PTU) administered to male Sprague-Dawley rats maintained on 2 and 5 mug L-thyroxine (T(4))/100 g body weight resulted in a marked reduction in the rate of conversion of L-thyroxine to L-triiodothyronine (T(3)). These effects could not be ascribed to induced hypothyroidism since the group maintained on 5 mug T(4)/day had normal levels of liver mitochondrial alpha glycerophosphate dehydrogenase. In confirmation of previous studies, PTU also reduced the fractional rate of deiodination of T(3). These observations provide a possible explanation of the many published observations indicating that PTU antagonizes the tissue effects of T(4) but not of T(3). The data suggest that monodeiodination of T(4) but not of T(3) is essential before hormonal effects can be manifested at the cellular level.

Determination of iodothyronine absorption and conversion of L-thyroxine (T 4 ) to L-triiodothyronine (T 3 ) using turnover rate techniques.

The absorption of L-thyroxine (T(4)) and L-triiodothyronine (T(3)) and the fractional rate of conversion of T(4) to T(3) were determined from the turnover rates of T(4) and T(3) in seven patients without endogenous thyroid function during separate treatment periods with these iodothyronines. Serum T(3) concentration was measured by a radioimmunoassay procedure in which the iodothyronines are separated from the plasma proteins before incubation with anti-T(3) antibody. Metabolic clearance rates were calculated by an integral (noncompartmental) approach since the use of single compartment kinetics led to a 40% overestimation of the metabolic clearance rate of T(3). Based on the amount of hormone ingested and the observed hormonal turnover rates, the absorption of T(4) and T(3) (iodothyronine turnover/iodothyronine ingested) in man could be estimated. Absorption of T(3) was complete in three subjects but decreased to 43% in a fourth who was suffering from mild congestive heart failure. Mean T(4) absorption was 48.0+/-2.6% (SEM) for seven subjects. The mean fractional rate of T(4) to T(3) conversion determined during T(4) replacement therapy (T(3) turnover/T(4) turnover) was 42.6% (range 30.7-50.8%). Thus, approximately one-half of the T(4) which was deiodinated was converted to T(3) suggesting that monodeiodination is an obligatory step in the peripheral metabolism of T(4). Calculations based on these results together with other available data suggest that under normal physiologic circumstances the major portion of the T(3) pool is derived from monodeiodination of T(4).

A New Radioimmunoassay for Plasma l-Triiodothyronine: Measurements in Thyroid Disease and in Patients Maintained on Hormonal Replacement

A new procedure for the radioimmunoassay of l-triiodothyronine (T(3)) in human plasma is described in which the iodothyronines are separated from the plasma proteins before incubation with a specific antiserum to T(3). The antibody bound and free T(3) are separated with dextran-coated charcoal. In this system, the mean recovery of T(3) added to plasma was 97.9% and both in vitro conversion of l-thyroxine (T(4)) to T(3) and cross-reaction between T(4) and the anti-T(3) antibody were undetectable (less than 0.1%). The assay procedure allowed the measurement of T(3) in up to 0.5 ml of plasma resulting in improved assay sensitivity (6 ng/100 ml). The mean plasma T(3) in normal subjects was 146+/-24 ng/100 ml (sd). Mean T(3) concentration was increased in hyperthyroidism (665+/-289 ng/100 ml) and decreased in hypothyroidism (44+/-26 ng/100 ml). In patients with severe hypothyroidism, plasma T(3) was between 7 and 30 ng/100 ml. Plasma T(3) concentration was relatively constant throughout the day in three euthyroid subjects. In contrast, in hypothyroid subjects on replacement therapy with T(3), a T(4): T(3) combination or desiccated thyroid plasma T(3) was markedly elevated for several hours after ingestion of the medication. Plasma T(3) was unchanged throughout the day in patients treated with T(4). Thus, insofar as plasma T(3) levels are concerned, replacement therapy with T(4) appears to mimic the euthyroid state more closely than other preparations.

Increased thyroxine turnover and thyroidal function after stimulation of hepatocellular binding of thyroxine by phenobarbital

Administration of phenobarbital to rats in a dosage schedule previously demonstrated to increase hepatocellular binding of thyroxine results in increased hormonal turnover, due both to increased deiodination and to fecal disposition of thyroxine iodine. The rate of biliary excretion of thyroxine iodine is roughly proportional to the hepatic content of exchangeable thyroxine. The enhanced peripheral disposition of thyroxine appears to lead to increased thyroidal function, as measured by isotopic iodine studies, and the maintenance of a normal nonradioactive serum PBI. On the other hand, thyroidectomized animals maintained on a constant replacement dose of L-thyroxine and treated with phenobarbital exhibit a marked fall in serum PBI. These findings suggest that increased thyroxine flux in phenobarbital-treated animals is secondary to primary stimulation of hepatocellular binding. Exchangeable intracellular thyroxine may thus be an important determinant of hormone turnover and, possibly, of hormonal action.

Nuclear Receptors and the Initiation of Thyroid Hormone Action

Publisher Summary This chapter provides an overview of nuclear receptors and the initiation of thyroid hormone action with the help of some studies. It discusses specific nuclear binding, biological relevance of nuclear sites, and speculations about post-binding mechanisms. It attempts to provide a general correlation between the concentrations of nuclear sites in individual tissues and the responsivity of such tissues to thyroid hormone and presents a comparison of the published thyromimetic activity of thyroid hormone analogs in bioassay with the relative nuclear binding affinities of such analogs. An effort is also made to determine whether maximal saturation of nuclear sites can lead to maximal hormonal tissue effect. The chapter presents three independent lines of evidence supporting the concept that nuclear binding sites act as true receptors: (1) the gross correlation between the density of nuclear sites in various tissues and responsivity of such tissues to thyroid hormone administration; (2) the correlation between affinity of nuclear binding and the established biological potency of thyroid hormone analogs; and (3) constraints to hormonal response at the tissue level imposed by the limited number of nuclear sites. It also presents a discussion confined to the relationship between hormone response and nuclear occupancy.

Replacement dosage of L-thyroxine in hypothyroidism. A re-evaluation.

Abstract The replacement dose of L-thyroxine was determined in 44 patients with hypothyroidism, 35 with primary thyroidal insufficiency and nine with pituitary failure. In patients with primary hypothyroidism the adequacy of replacement was confirmed by thyrotropin radioimmunoassay values within the normal range. The average daily replacement dose of thyroxine was 169 jug per day (2.25 μg per kilogram), and 89 per cent of the patients were maintained with doses between 100 and 200 μg per day. This amount is significantly less than recommended in current textbooks. The replacement dose correlated with body weight. The average serum thyroxine concentration in treated patients, 8.1 μg per 100 ml, was only slightly higher than that in 55 controls, 7.1 μg. Over 96 per cent of patients receiving replacement had thyroxine concentrations in the generally accepted normal range. The average serum concentration of tri-iodothyronine in treated patients was 130 ng per 100 ml — slightly less than that in control subjec...

A rapid, inexpensive, quantitative technique for the analysis of two-dimensional electrophoretograms

Abstract We describe a rapid, reproducible, and relatively inexpensive method for quantitative analysis of two-dimensional gel radiofluorograms. The equipment required includes a mediumresolution black and white video camera for data acquisition, a video digitizing circuit for data translation, and a microcomputer for data analysis. The system described can also be used to quantitate protein- or DNA-stained one- or two-dimensional gels or chromatograms.

Limited Binding Capacity Sites for l-Triiodothyronine in Rat Liver Nuclei: NUCLEAR-CYTOPLASMIC INTERRELATION, BINDING CONSTANTS, AND CROSS-REACTIVITY WITH l-THYROXINE

Further studies have been performed to define the kinetic characteristics of nuclear triiodothyronine (T(3)) binding sites in rat liver (J. Clin. Endocrinol. Metab. 1972. 35: 330). Sequential determination of labeled T(3) associated with nuclei and cytoplasm over a 4-h period allowed analysis of the relationship of T(3) in nuclear and cytoplasmic compartments. A rapid interchange of hormone between nuclei and cytoplasm was demonstrated, and in vitro incubation experiments with nuclei yielded no evidence favoring metabolic transformation of T(3) by the nuclei. In vivo displacement experiments were performed by subcellular fractionation of liver (1/2) h after injection of [(125)I]T(3) with increasing quantities of unlabeled T(3). The nuclear binding capacity for T(3) could be defined (0.52 ng/mg DNA). Analysis of these experiments also allowed an estimation of the association constant of nuclear sites for T(3) (4.7 x 10(11)M(-1)). The affinity of these sites for T(3) was estimated to be 20-40 fold greater than for thyroxine (T(4)). Chromatographic analysis of the nuclear radioactivity after injection of labeled T(4) indicated that the binding of T(4) by the nucleus could not be attributed to in vivo conversion of T(4) to T(3) but reflected intrinsic cross-reactivity of the two iodothyronines at the nuclear binding sites.