Outer and inner ring monodeiodination of thyroxine by dog thyroid and liver: a comparative study using a particulate cell fraction

In previous studies using perfused dog thyroids we have found evidence for a modulation of thyroid secretion by intrathyroidal thyroxine deiodinases. The purpose of the present study was to examine in detail the in vitro characteristics of thyroidal 5- and 5'-deiodinases of T4 and to make a comparison to T4 deiodinases from liver tissue. The sources of deiondinases were crude microsomal fractions prepared from thyroid and liver tissues obtained from the same dogs. Deiodinases from the two tissues behaved very similarly with respect to the time course of T3 and rT3 generation, dependency on the of added microsomes and substrate, and influence of temperature, pH, dithiothreitol, propylthiouracil, methimazol, ipodate, and added rT3. At pH 7.4, both T3 and rT3 were stable when added to the microsomal preparations. The apparent kinetic constants estimated from Lineweaver-Burk plots of T4 deiodination to T2 and rT3 by liver and thyroid microsomes, were similar for all reactions, (kinetic constant, 6.1-25.l1 microM). T3 generation was maximal at pH 6.2, and rT3 generation was maximal at pH 8.0. Even at pH 8.0, the generation of T3 was severalfold higher than that of rT3. In contrast to previous studies employing rat tissue, it was found that the addition of rT3 did not modify the deiodination of T4 to T3 unless the rT3 concentration exceeded that of T4. Thus, the canine thyroid contained T4 deiodinases very similar to those of a peripheral thyroid hormone target organ, the liver. These thyroidal deiodinases are probably responsible for the relatively high secretion of T3 and rT3 compared to that of T4 found in our previous studies. Further, the discrepancies between these and previous data obtained using rat tissues demonstrate that in such in vitro studies, great species variations are present in the stability of generated rT3 and the effect of rT3 on outer ring deiodination of T4 to T3.

Radioimmunological measurements of 3,5-diiodothyronine in hydrolysate and in effluent from perfused canine thyroids

A radioimmunoassay for 3,5-diiodothyronine (3,5-T2) was developed using inner ring labeled 125l-3,5-T2 and a 3,5-T2 antibody produced in rabbits by immunization with 3,5-T2 coupled to human albumin. Separation of free and bound 3,5-T2 was achieved by ascending ion-exchange wick-chromatography. The detection limit was 4 pmol/l. The only important cross-reaction was with T3. It amounted to 0.11 % (mol/mol) and contributed only little to the 3,5-T2 measured in thyroid hydrolysate and thyroid effluent. It was ascertained that no significant loss or generation of 3,5-T2 took place during storage of thyroid effluent samples and during hydrolysis of thyroid homogenate. In four experiments employing eight thyroid lobes, 3,5-T2 was measured in hydrolysate and effluent from perfused dog thyroid lobes during single passage perfusion with a synthetic hormone free medium. During perfusion with control medium 3,5-T2 in effluent was stable around 40 pmol/l. Infusion of 100 microunits/ml TSH induced after a latency period of approximately 20 min a gradual increase in 3,5-T2 release to a level of approximately 180 pmol/l. 3,5-T2 in thyroid hydrolysate was 1.47 +/- 0.05 pmol/mg thyroid wet weight (mean +/- SD). There did not seem to be any major generation of 3,5-T2 by intrathyroidal iodothyronine deiodination. Thus thyroidal secretion of 3,5-T2 is very low and would probably not contribute significantly to the amounts of circulating 3,5-T2.

3',5'-diiodothyronine release from perfused dog thyroids

A new radioimmunoassay for 3',5'-diiodothyronine (3',5'-T2) is described. The detection limit was 1-2 pmol/l with a sample size of 200 microliters. Cross-reaction with T4, T3, and rT3 was 0.005, 0.003, and 1.3% respectively. The intra- and interassay coefficients of variation were 5.1% and 9.2%. Dilution curves of effluent from perfused thyroid lobes of thyroid hydrolysate were parallel to the standard curve. By addition of 3',5'-T2 or rT3 standards to thyroid effluent sampling tubes or to thyroid homogenate before hydrolysis it was ascertained that no artifactual loss of 3',5'-T2 or generation of 3',5'-T2 from rT3 was taking place during collection and storage of thyroid effluent or during hydrolysis of thyroid homogenate. 3',5'-T2 was measured in effluent from perfused thyroids and in hydrolysate of the thyroids in six mongrel dogs. Only minute amounts of 3',5'-T2 was found: 1.66 +/- 0.41 pmol/mg thyroid wet weight (mean +/- SD) in hydrolysate, around 20 pmol/l in thyroid effluent during control perfusion and around 170 pmol/l during prolonged infusion of 100 micro U/ml bovine TSH. By measuring the apparent 3',5'-T2 concentration in perfusion medium with added T4 and rT3 standard corresponding to the amounts present in thyroid effluent samples, and dilutions of thyroid hydrolysates it was excluded that T4 and rT3 interference could account for more than maximally 20% of the measured 3',5'-T2. Thus 3',5'-T2 is present in the thyroid and secreted from the thyroid, but only in very small amounts.

Suppression of the night increase in serum TSH during development of ketosis in diabetic patients

The alterations in metabolic parameters, circulating iodothyronines and serum TSH were studied during a 21 h period of insulin withdrawal in 6 young patients with juvenile type diabetes mellitus. Concomitant with the derangement of metabolic state a significant fall in serum T3 (in average 27%), serum free T3 (28%), and T4 (12%) was observed. SErum free T4 remained unchanged. Before and after the period of ketosis the normal diurnal pattern of high serum TSH at night and low levels during the daytime period was observed. During the period of ketosis the night level of serum TSH was significantly depressed (46 +/- 9% lower at 23.00 h, p less than 0.01) while no significant alterations occurred in daytime TSH. Both the variations in T3, reverse T3 (rT3) and night TSH were correlated to the increase in blood-3-hydroxybutyrate. The depression of the night surge in serum TSH may be of importance for the fall in circulating levels of active iodothyronines during the initial phase of illness, together with the well known inhibition of T4 deiodination to T3 in peripheral tissues occurring in acute illness.

Enhanced release of T3 and rT3 compared to T4 from thyroglobulin during autolysis of dog thyroid homogenate

Previous studies on the secretion of thyroxine (T4), 3,5,3'-triiodothyronine (T3), and 3,3',5'-triiodothyronine (rT3) from perfused dog thyroids have indicated that a differential rate of secretion of various iodothyronines may take place. The aim of the present study was to evaluate whether the proteolysis of thyroglobulin taking place during secretion could be involved in this phenomenon. Homogenate from the same dog thyroid was incubated either at pH 3.6 for 18 h without added protease or with pronase at pH 8.4 for 18 h. Iodothyronines were measured radioimmunologically in ethanol extracts of the hydrolysates. No significant deiodination of T4 to T3 and rT3 took place during incubation. During acid autolysis 17.5 +/- 3.5% (mean +/- SE, n = 5) of the T4 found after pronase hydrolysis was liberated, while 31.6 +/- 4.8% of the T3 and 21.2 +/- 4.2% of the rT3 were liberated (both values were significantly higher than that found for T4). Since iodothyronines in thyroglobulin are released nearly quantitatively during pronase hydrolysis, the results indicate that thyroid proteases acting at acid pH, liberates T3 and rT3 more easily than T4 from thyroglobulin. This could be the mechanism behind the relatively high secretion of T3 and rT3 observed during acceleration of secretion from perfused thyroid lobes, and the relatively high secretion of T4 observed during deceleration of secretion.

Release of somatostatin-like immunoreactivity from the perfused canine thyroid. Selective stimulatory effect of calcium ions

It is well accepted that the C cells of the thyroid contain somatostatin, but the role in local endocrine function has not yet been firmly established in this organ, and it has not been proved that thyroidal somatostatin is released into the circulation. We have measured the contents of somatostatin-like immunoreactivity in the effluent of canine thyroid glands perfused without recirculation with a synthetic buffer medium. During basal conditions a definite release was consistently found in the order of 10 pg/ml corresponding to 12 pg/min. The somatostatin-like immunoreactivity was studied in dilution experiments and by gel-filtration chromatography, and found to have properties identical to those of synthetic cyclic somatostatin, which was also recovered quantitatively when added to sampling tubes. Various compounds were infused in concentrations that are highly active in pancreas perfusion experiments. 14-min infusion of arginine, 5 and 11.5 mmol/liter; isoproterenol, 10 and 23.7 nmol/liter and 68.7 mumol/liter; acetylcholine, 5 mumol/liter, carbamylcholine, 10 and 100 mumol/liter; glucagon, 1 and 30 nmol/liter; and porcine calcitonin, 1 and 100 ng/ml did not affect the basal release of somatostatin-like immunoreactivity significantly. Neither did an increase from the control level of 4 mmol/liter glucose of 10 or 20 mmol/liter, nor an increase in the control level of 4.4 mmol/liter K+ to 7.5 or 14.4 mmol/liter. Each of these compounds were tested in three or four dogs. The effect of an increase in Ca++ from the control level of 1.5 mmol/liter to 2.25, 3.0, and 4.5 mmol/liter was tested in random order in five thyroid lobes. All three doses elicited an immediate increase in effluent somatostatin-like immunoreactivity. In most experiments the response was biphasic with an early spike, followed by a stable level that was maintained during prolonged Ca++ infusion. The secretory response was not diminished through a series of repeated short pulses of calcium infusion. The response to 3.0 mmol/liter Ca++ (control period 8.4 +/- 1.5, test period 337 +/- 110 pg/ml, mean +/- SE) and 4.5 mmol/liter Ca++ (control period 9.5 +/- 1.4, test period 386 +/- 125) were significantly higher than 2.25 mmol/liter Ca++ (control period 7.2 +/- 1.0 test period 140 +/- 39), while there was no significant difference between responses to the two high doses. Infusion of salmon calcitonin, 10 ng/ml and 1 microgram/ml; or porcine calcitonin, 1 microgram/ml during calcium stimulation (2.25 mmol/liter of Ca++) did not induce alterations in the release of somatostatin-like immunoreactivity. The results demonstrate that thyroidal somatostatin is mobilizable, and it appears to be selectively sensitive to calcium stimulation, indicating a possible role in calcitonin release control.

Dynamics of inhibition of iodothyronine deiodination during propylthiouracil treatment of thyrotoxicosis

Serum 3,3',5'-triiodothyronine (rT3), 3,5,3'-triiodothyronine (T3), thyroxine (T4) and propylthiouracil (PTU) were measured before and at short intervals for 8 hours after oral administration of 200 mg PTU to six patients with untreated thyrotoxicosis. The study was repeated on the 4th day of treatment with 200 mg PTU every 8 hours. Six other patients with untreated thyrotoxicosis were studied after a single administration of 800 mg PTU. The results indicated that PTU inhibition of T4 deiodination to T3, evaluated by the fall in serum T3, was of the same duration as the PTU inhibition of rT3 deiodination, evaluated by the increase in serum rT3. After 200 mg PTU inhibition was maximal for only a few hours, and there was no cumulative effect of PTU during the first four days of treatment, when 200 mg PTU was given every 8 hours. After 800 mg PTU the full effect was maintained for the 9 hour period studied, after which serum T3 had fallen to 65 +/- 2% of the pretreatment level (mean +/- SE). Thus, to obtain a permanent full effect of PTU on iodothyronine deiodination during the treatment of thyrotoxicosis it is necessary to use large doses or frequent administration.

Secretion of 3,3'-diiodothyronine by the perfused canine thyroid isolated in situ

Previous studies employing perfused dog thyroid lobes have shown that thyroidal secretion is modulated by intrathyroidal deiodination of T4 to T3 and rT3. To characterize further the intrathyroidal iodothyronine-deiodinating processes 3,3'-diiodothyronine (3,3'-T2) and T4 were measured in hydrolysate and effluent from isolated dog thyroid lobes during single passage perfusion with a synthetic hormone-free-medium. In five experiments the concentrations in effluent from unstimulated thyroid lobes were: 3,3'-T2, 208 +/- 62 pmol/liter; and T4, 12.8 +/- 2.8 nmol/liter (mean +/- SE). During the infusion of 100 microU/ml TSH, there was a parallel increase in the secreton of 3,3'-T2 and T4. After approximately 90 min of TSH infusion, effluent concentrations were: 3,3'-T2, 2490 +/- 420 pmol/liter; and T4, 204 +/- 24 nmol/liter. In a pronase hydrolysate of the thyroid lobes, the amounts were: 3,3'-T2, 2.44 +/- 0.56 pmol/mg; and T4, 1070 +/- 125 pmol/mg. Expressed as a percent of T4, the amount of 3,3'-T2 in thyroid effluent was approximately 10 times higher than that in thyroid hydrolysate. This relative hypersecrection of 3,3'-T2 was abolished by the infusion of 10(-3) or 10(-5) M propylthiouracil, an inhibitor of peripheral and intrathyroidal iodothyronine deiodination. Thus, thyroid secretion contains small amounts of 3,3'-T2, most of which seems to originate from intrathyroidal deiodination of T3 and/or rT3.

24-h profile of serum rT3 and serum 3,3'-T2 in normal man

In a previous study we found a night surge in serum free T3 varying in parallel with that of serum TSH. In order to evaluate whether diurnal alterations in peripheral iodothyronine monodeiodination may be involved we have measured two products of peripheral deiodination, 3,3',5'-T3 (rT3) and 3,3'-T2 in serum samples obtained at short intervals during a 24-h period in 5 normal male subjects. Serum rT3 was rather stable during the period albeit with a trend towards lower levels during the night when the subjects were in bed. In order to obtain a measure of free rT2 a free rT2 index was calculated using the combined variations in per cent free T4 and free T3. Night and day levels of this rT3 index were found identical, suggesting a lack of diurnal variation in serum free rT3. Likewise serum 3,3'-T2 levels were identical during the day and night periods. The results suggest that variations in peripheral iodothyronine deiodinations are not involved in the night increase in serum free T3.

Somatostatin and the 24 h levels of serum TSH, T3, T4, and reverse T3 in normals, diabetics and patients treated for myxoedema

The influence of somatostatin on serum TSH, 3,5,3′-triiodothyronine (T3), free T3, thyroxine (T4), free T4 and 3,3′,5′-triiodothyronine (reverse T3, rT3) was studied in 5 healthy young subjects, in 7 young patients with juvenile type diabetes on a fixed daily insulin dose, and in 5 patients with treated myxoedema. Blood samples were taken hourly during a 24 h control period and during a 24 h somatostatin infusion period.

Somatostatin infusion obliterated the night increase in serum TSH in all three groups. However, the lower daytime serum TSH was not altered by somatostatin. It has earlier been shown that somatostatin inhibits the TSH secretion induced by TRH. Taken together these findings suggest that the high TSH level at night is induced by a hypothalamic TRH surge, while the lower day level of TSH is quite independent of hypothalamic TRH.

Somatostatin infusion was accompanied by a minor fall in serum T4 in all groups. In normal subjects and diabetics serum free T3 fell 23 ± 6% and 25 ± 6%, respectively, during 24 h somatostatin infusion. In patients with treated myxoedema, serum free T3 was not significantly affected. These results are consistent with a decrease in thyroidal secretion during somatostatin infusion, as the cause for the fall in serum T3 in normal subjects and in patients with diabetes, while the monodeiodination of T4 to T3 in peripheral tissues is unaffected. However, an extrathyroidal effect of somatostatin on iodothyronine metabolism might also exist since serum rT3 increased 49 ± 19% in patients with treated myxoedema. In normal subjects and diabetic patients only a trend towards such a variation was observed.

Lack of effect of somatostatin on iodothyronine release from the perfused canine thyroid

1. This study has evaluated the effect of somatostatin (0.1 or 3 micrograms/ml) on the release of thyroxine, 3,5,3'-tri-iodothyronine and 3,3',5'-tri-iodothyronine from perfused thyroid lobes. 2. Somatostatin did not affect either the total relase of iodothyronines or the ratio between various iodothyronines in thyroid effluent from unstimulated and thyrotrophic hormone stimulated thyroid lobes. 3. The results lend no support to the idea that somatostatin present in the parafollicular cells of the thyroid should have regulatory function on thyroidal secretion of iodothyronines.

Circulating iodothyronines in juvenile diabetic outpatients

Serum T3, rT3, T4 and TSH were measured in ten healthy subjects, in nine juvenile diabetic outpatients in fair metabolic control (fasting plasma glucose 6.5 mmol/1, 2.6-9.5 mmol/1 (mean, range), HbA1 10.1%, 8.1-12.2%), and in ten juvenile diabetic outpatients in poor metabolic control but without ketonuria (fasting blood glucose 15.1 mmol/l, 11.1-22.4 mmol/l, HbA1, 13,8% 10.8-16,2%. Serum T3 was identical in normal subjects and in diabetics in fair metabolic control. Diabetics in poor metabolic control had 13.0% lower serum T3 than normal subjects (2 p less than 0.05). No difference between groups was observed in serum rT3, T4 and TSH. These findings indicate that in diabetic patients, metabolic derangement induces alterations in serum T3 more easily than in serum rT3. However, alteration in serum T3 is not a sensitive indicator of moderate metabolic derangement in diabetic patients.

Iodothyronine release from the perfused canine thyroid following cessation of stimulation: rapid decline of triiodothyronines in comparison with thyroxine

The kinetics of thyroid secretion after termination of stimulation by 100 muU/ml bovine thyroid stimulating hormone (TSH) or 5 mM cyclic AMP (cAMP) were studied using perfused canine thyroid lobes. All experiments were performed as paired comparisons, one thyroid lobe acting as a control continuing to receive infusion of the stimulator. 2.5 h after termination of TSH infusion, the secretion of thyroxine (T(4)), 3,5,3'-triiodothyronine (T(3)), and 3,3',5'-triiodothyronine (rT(3)) was not significantly different from that of the control lobes. After cessation of cAMP infusion, the secretion of T(4) continued unaffected for approximately 40 min. Then a gradual decline in T(4) release occurred. The secretion of T(3) and rT(3) decreased somewhat earlier leading to a transient phase with increases in the T(4):T(3) and T(4):rT(3) ratios in the thyroid effluent. The persistently high secretion of iodothyronines despite cessation of TSH infusion is most likely the result of a continued stimulation by receptor-bound TSH. Because the clearance of intracellular cAMP is rapid and the concentration of cAMP used for stimulation in these experiments only exceeded the concentration necessary for eliciting a secretory response modestly, it is reasonable to assume that stimulation of colloid droplet formation stopped shortly after termination of cAMP infusion. The bulk of iodothyronines secreted thereafter thus originated from continued hydrolysis of thyroglobulin engulfed by the follicular cells during the preceding cAMP infusion. The pattern of an earlier decline in secretion of T(3) and rT(3) than of T(4) from this intracellular pool of thyroglobulin points to a more rapid liberation of triiodothyronines than of thyroxine from thyroglobulin during intracellular hydrolysis.

Parathyroid hormone release from the perfused canine thyroid-parathyroid complex isolated in situ

A model for direct measurement of hormone release from the canine parathyroid gland is described. The two separate thyroid-parathyroid gland complexes were isolated in situ and perfused independently using a synthetic medium with a welldefined concentration of ionized calcium (Ca++). The concentration of parathyroid hormone (PTH) in the effluent was measured by radioimmunoassay, using an antiserum to bovine PTH that cross-reacts with canine PTH. Displacement curves of dilutions of effluent samples were similar to those of bovine PTH (1-84). During infusion of 1.49 mmol/l Ca++ the PTH release was constant for more than 4 h. Variations in Ca++ from 1.56 to 1.15 or 2.15 mmol/l induced rapid and sustained stimulation or suppression of PTH release. The retained ability of the preparation was ascertained by recording the response to infusion of calcium free medium at the end of each experiment.

Dynamics of serum rT3 and 3,3'-T2 during rT3 infusion in patients treated for thyrotoxicosis with propylthiouracil or methimazole

rT3 metabolism in patients treated for thyrotoxicosis with prophylthiouracil (PTU), or methimazole (MMI) was studied by infusion of rT3 and measurements of the increase in serum rT3 and serum 3,3'-diiodothyronine. The results indicate that the high serum rT3 observed during treatment with PTU is not due to an increase in rT3 production, but to a decrease in the metabolic clearance rate of rT3. rT3 infusion was followed by an increase in serum 3,3'-T2 which was similar whether PTU or MMI was given. However, after stopping rT3 infusion there was a more rapid fall serum 3,3'-T2 during MMI treatment, compatible with an inhibitory effect of PTU on 3,3'-T2 degradation.

The effect of propylthiouracil on thyroid-stimulating hormone-induced alterations in iodothyronine secretion from perfused dog thyroids

The aim of this study was to see whether the inhibitory effect of propylthiouracil on thyroidal secretion of 3,5,3'-triiodothyronine (T3) and 3,3',5'-triiodothyronine (rT3) could be reproduced in intensively stimulated thyroids, and to elucidate whether an increase in the fractional deiodination of thyroxine (T4) to T3 and rT3 during iodothyronine secretion might be responsible for the transient fall in the T4/T3 and T4/rT3 ratios in thyroid secretion seen in the early phase after stimulation of thyroid secretion. For this purpose T4, T3 and rT3 were measured in effluent from isolated dog thyroid lobes perfused in a non-recirculation system using a synthetic hormone free medium. 1 mmol/1 propylthiouracil induced a significant reduction in thyroid-stimulating hormone (TSH) stimulated T3 and rT3 release while the release of T4 was unaffected. This supports our previous conclusion that T4 is partially monodeiodinated to T3 and rT3 during thyroid secretion. Infusion of 1 mmol/l propylthiouracil for 30 min or 3 mmol/l propylthiouracil for 120 min did not abolish the transient fall in effluent T4/T3 and T4/rT3 induced by TSH stimulation. Thus, this phenomenon seems not to depend on intrathyroidal iodothyronine deiodinating processes.

rT3 production in normal man, assessed from variations in serum rT3 during short-term rT3 infusion

The metabolic clearance rate (MCR) of 3,3'5'-triiodothyronine (reverse T3, rT3) was estimated in normal human subjects by a modified noncompartmental method using the integrated increase in serum rT3 following intravenous infusion of 0.10 nmol/min rT3 for 4 hr. The MCR-rT3 was calculated to be 102.8 +/- 17.01/day and the daily rT3 disposal to be 33.0 +/- 9.5 nmol (mean +/- SD, n = 6). The MCR-rT3 compares well to that of previous studies employing tracer kinetic methods. The disposal rate of rT3 estimated in the present study is considerably lower than found in some previous studies. The discrepancy is due to differences in the measured levels of serum rT3 in normal subjects.

Low serum triiodothyronine and high serum reverse triiodothyronine in old age: an effect of disease not age

Serum concentrations of T4, T3, T3, free T4, free T3, and TSH were determined in four groups of adult subjects: group A, 27 healthy young volunteers (aged 18-29 yr); group B, 24 carefully selected healthy elderly subjects (aged 70-90 yr); group C, 41 subjects living at a municipal nursing home for the elderly (aged 70-90 yr); and group D, 35 hospitalized patients (aged 70-90 yr). Identical levels of iodothyronines in serum were found in the young and in the elderly healthy subjects. Moderate and severe disease induced alterations in iodothyronine concentrations in serum comparable to those reported earlier. Serum T3 and free T3 levels were low and serum rT3 levels were high in groups C and D subjects; serum free T4 was elevated in group D, while serum T4 was low. Serum TSH was lower in the healthy elderly subjects than in the young subjects. Serum TSH was higher in the elderly sick subjects (groups C and D) than in the healthy subjects (group B). We conclude that advanced age per se is not accompanied by alterations in free or total serum iodothyronine levels.

Selective inhibition of the secretion of triiodothyronines from the perfused canine thyroid by propylthiouracil

In order to assess a possible influence on the relative proportions of T4, T3, and rT3 in thyroid effluent, the thyroid-blocking agents, propylthiouracil (PTU) and methylmercaptoimidazol (MMI), were administered to perfused canine thyroids. The dog has two distinct thyroid lobes which were both perfused independently, one acting as a control for the other. T4, T3, and rT3 in thyroid effluents and thyroid hydrolysates were measured radioimmunologically. During control perfusion the T4:T3 and T4:rT3 ratios in thyroid effluent were considerably lower than in thyroid hydrolysates [t4:t3 (wt/wt), 6.1 +/- 2.0 vs. 12.8 +/- 4.4; T4:rT3, 23.0 +/- 5.9 vs. 69.7 +/- 29.1; mean +/- SD; n = 8; P less than 0.001], i.e. T3 and rT3 were secreted preferentially to T4. Infusion of PTU (1 mmol/liter) induced a gradual increase in both the T4:T3 and T4:rT3 ratios in thyroid effluent. The variation in the T4:rT3 ratio appeared somewhat earlier than that in the T4:T3 ratio. Both ratios eventually approached the ratios in thyroid hydrolysates. The change in ratios was caused by a PTU-induced decrease in the release of T3 and rT3 while the release of T4 was practically unaffected. In contradistinction, infusion of MMI did not affect the T4:T3 or T4:rT3 ratios in thyroid effluent. As PTU but not MMI inhibits T4 deiodination in peripheral tissues, the results of the present study suggest that part of the T3 and rT3 secreted from the thyroid stems from T4 deiodinated during the secretion by a mechanism similar to the T4-deiodinating processes in peripheral tissues.

The relative distribution of thyroxine, triiodothyronine and 3,3',5'-(reverse)-triiodothyronine in various fractions of thyroglobulin

Thyroglobulin fractions rich and poor in new thyroglobulin were separated by means of DEAE-cellulose chromatography of dog thyroid extracts and by zonal ultracentrifugation in a sucrose gradient of guinea pig thyroid extract incubated at low temperature. The distrubtion of thyroxine, triiodothyronine and 3,3',5'-(reverse)-triidothyronine in hydrolysates of the different fractions was estimated by radioimmunoassays. Following DEAE-cellulose chromatography there was a small but statistically significant increase in T4/T3 ratio in thyroglobulin fractions eluted at high ionic strength--that is fractions relatively rich in stable iodine but poor in fresh thyroglobulin. There was no differences in the T4/rT3 ratios between the different fractions. The ratios between iodothyronines were almost identical in the various thyroglobulin fractions following zonal ultracentrifugation in a sucrose gradient of cold treated guinea pig thyroid extract. These findings lend no support to the possibility that a relatively high content of triiodothyronines in freshly synthesized thyroglobulin modulates the thyroid secretion towards a preferential secretion of triiodothyronine and 3,3',5'-(reverse)-triidothyronine at the expense of the secretion of thyroxine.

Dynamics of hormone release from the perfused canine thyroid during cyclic AMP infusion

In order to try to characterize the sequence of processes leading to hormone secretion from the stimulated thyroid, the effect of cyclic 3'5' adenosine monophosphate (cAMP) and related compounds were examined in 15 two-sided perfusions of canine thyroids isolated in situ. T4 and T3 concentrations in the effluent were measured radioimmunologically. cAMP 5 mM and TSH 100 muU per ml induced the same pattern of hormone release from the thyroid. After a latency period of 15--25 minutes a steep increase occurred in both T4 and T3 release. During the initial part of the stimulation the rise in T4 relase was somewhat slower than that of T3 release. The prolonged latency period before response earlier recorded in the same preparation during infusions of low concentrations of TSH was not observed during infusions of decreasing concentrations of cAMP (1, 0.8, 0.5 and 0.2 mM) or theophylline (5 and 1 mM). Either there was no response or the latency period was of the same length as that observed after a strong stimulus. These findings suggest that the latency period can be divided in two parts: () a variable, dose dependent satency period confined to the early part of the process sequence leading to secretion--i.e. before cAMP exerts its effect, and 2) an obligatory latency period related to the processes taking place after the formation of pseudopods. The duration of these late processes seems to be independent of the degree of stimulation.

Non-parallel variations in the preferential secretion of 3,5,3'-triiodothyronine (T3) and 3,3',5'-triiodothyronine (rT3) from dog thyroid

rT3, T3, and T4 were measured in effluents from perfused dog thyroids and in hydrolysates of the thyroids. rT3 was determined by a newly developed sensitive radioimmunoassay (sensitivity approximately 0.1 pg rT3/tube). The rT3 and T3 contents of thyroid effluent were always much higher than would be expected from the relative amounts of T4, T3, and rT3 in thyroid hydrolysate. In four two-sided thyroid perfusions the T4/rT3 ration (wt/wt) in thyroid hydrolysates was 59.3 +/- 26.3 (mean +/- SD) and in effluents from unstimulated thyroids it amounted to 15.1 +/- 7.8. The T4/T3 ratio in thyroid hydrolysates was 10.3 +/- 1.9 and the ratio in effluents from unstimulated thyroids was 6.0 +/- 0.3. A continuous infusion of TSH, 100 micromicron/ml, enhanced the secretion of all three iodothyronines. During the stimulation a biphasic response was observed in the T4/T3 and T4/rT3 ratios in the effluent. At first the preferential secretion of T3 and rT3 was enhanced. Then the T4/T3 ratio returned to prestimulation values while there was a further increase in the T4/rT3 ratio to nearly twice the prestimulation values. Dog thyroids stimulated and examined in vivo showed a similar preferential secretion of rT3 and T3. Thus, both rT3 and T3 were secreted preferentially to T4 from the thyroid. The preferential secretions of rT3 and T3 did not vary in parallel. During a prolonged TSH stimulation the degree of preferential secretion of T3 was maintained despite a considerable increase in hormone secretion. On the contrary, the preferential secretion of rT3 decreased during prolonged TSH infusion.

Opposite variations in serum T3 and reverse T3 during propylthiouracil treatment of thyrotoxicosis

Blood samples for determination of serum total and free reverse triiodothyronine (rT3), triiodothyronine (T3) and thyroxine (T4) were obtained daily in 6 previously untreated thyrotoxic patients during periods of propylthiourazil (PTU) (600 mg per day) or methimazol (MMI) (45 mg per day) administration. PTU induced about 60 per cent increase in both total and free serum rT3. This was accompanied by a rapid decrease in serum T3 and a more gradual decline in serum T4. MMI administration to untreated patients was followed by a gradual parallel decrease in rT3, T3 and T4. Turn from PTU to MMI produced a rapid decrease in serum rT3 and increase in serum T3 in all 6 patients. The relative variations in the free and total concentrations of iodothyronines were practically identical. The increase in serum rT3 after PTU is most likely explained either by enhanced deiodination of T4 to rT3 or by an inhibitory effect of PTU on rT3 degradation.

Radioimmunological determination of reverse triiodothyronine in unextracted serum and serum dialysates

A sensitive radioimmunoassay for measurements of 3,3'-5'-triiodothyronine (reverse T3, rT3) in small amounts of unextracted serum is described. The interference of rT3 binding proteins in serum was precluded by addition of 8-anilino-1-naphthalene sulphonic acid (ANS). The cross reaction of T4 with the rT3 anti-serum varied with the concentration of T4 in the samples. At 50 per cent inhibition of [125I]rT3 binding, the T4 cross reaction was 0.075% (mol/mol). All values were corrected for T4 cross reaction. By the present method total rT3 averaged 0.37 nmol/l in thirty-two normal subjects. Higher values (0.81-1.98 nmol/l) were obtained in seventeen thyrotoxic patients, while the rT3 was very low (less than or equal to 0.046 nmol/l) in ten patients with severe primary hypothyroidism. A modification of the total rT3 assay was used for measurements of rT3 in serum dialysates (free rT3). The sensitivity was 0.46 pmol/l. This sensitivity did not allow detection of free rT3 in all normal subjects.

The relative contribution of thyroxine and triiodothyronine to the hormone secretion from the perfused canine thyroid during various degrees of stimulation

The effect of constant infusions of bovine TSH in different concentrations on the pattern of T4 and T3 secretion was examined in perfused canine thyroids isolated in situ. Each of the thyroid lobes was isolated and perfused in each animal. It was thus possible to examine the effect of two different concentrations of TSH in the same animal. The latency period was found to be shorter and the increase in hormone release was steeper when a high concentration of TSH (100 microU/ml) was infused than after a low concentration of TSH (2 microU/ml). The increase in T4 and T3 release started simultaneously; however, at all TSH concentrations employed a phase of relatively high secretion of T3 was induced. Despite continued infusion of TSH, the T4 T3 ratio in the effluent always returned to near prestimulation values at the end of the experiment. Accordingly, there was no support for a sustained enhancement of the preferential secretion of T3 from the thyroid during prolonged stimulation of the thyroid.

T4 and T3 release from the perfused canine thyroid isolated in situ

A method for once-through perfusion of the canine thyroid isolated in situ is described. The perfusion medium was a modified Krebs Ringer buffer with 4% dextran added. In 4 control experiments of the T4 and T3 concentratios in effluent were stable or slightly falling during 3 h perfusion. There were no significant alterations in the T4/T3 ratio in the effluent during these experiments. A 10-min infusion of bovine TSH (1 mU/ml) caused an increase in the release of T4 and T3 after 15-25 min. The T4/T3 ration in the effluent was significantly reduced after TSH stimulation. However, the ratio returned to pre-stimulation values while the hormone release was still very high. T4 and T3 content of the contralateral thyroid was determatio in the homogenate was twice as high as the T4(3 ratio in the effluent during control perfusion. Thus there was a preferential secretion of T3 from the perfused canine thyroid and this was increased after TSH stimulation.

Diural TSH variations in hypothyroidism

There is a circadian variation in serum TSH in euthyroid subjects. A similar diurnal variation has been demonstrated in patients with hypothyroidism. In the present study the 24-hour pattern of serum TSH was investigated in eight patients with hypothyroidism of varying severity and in five hypothyroid patients treated with thyroxine (T4). There was a circadian variation in serum TSH in patients with hypothyroidism of moderate degree, and in patients treated for severe hypothyrodism with thyroxine. The pattern was similar to that found in normal subjects, i.e., low TSH levels in the daytime and higher levels at night. In severely hypothyroid patients, no diurnal variation in serum TSH was observed. A practical consequence is that blood samples for TSH measurements in patients with moderately elevated TSH levels are best taken after 1100 h, when the low day levels are reached.

T3 release from thyroid slices as an assay for thyroid stimulators

A new in vitro bioassay for thyroid stimulators is described. The method is based on radioimmunological measurement of triiodothyronine (T3) release from thyroid slices. Small thyroid slices were cut from several porcine thyroids. The slices were pooled in incubation vessels. The stimulating effect of human thyroid-stimulating hormone (TSH), bovine TSH, and serum from a patient with Grave's disease was investigated. After 3 hr the T3 concentration was significantly higher in the vessels containing the thyroid stimulators than in the control vessels. For maximal sensitivity 16 hr of incubation was used in the assay. Bovine TSH, human TSH, and serum from a patient with Grave's disease gave linear dose response curves in a log-log system. For bovine TSH the curve was linear from 5 to 125 muU/ml incubation medium. The dilution curves for bovine TSH and serum from a patient with Grave's disease were parallel. The dilution curve for human TSH was considerably steeper. In 5 assays the lambda values for the bovine TSH curves were 0.10-0.16 (mean, 0.12). In 3 assays the lambda values for the human TSH curves were 0.05-0.07.

Sarcoid reactions in pulmonary neoplasms

In a consectuive series of 734 patients with malignant tumorous of the lung, sarcoid reactions were observed in the mediastinal lymph nodes in 20 cases, i.e. in 3.2% of the 630 patients in whom these nodes were studied. Among these 20 patients, sarcoidosis was suspected in three. In two of these, granulomata were revealed in the tumour. Another two had metastases in the affected lymph nodes. All the common histological types of lung tumour were represented in the patients, but squamous-cell carcinomata showed a statistically significant preponderance as compared with anaplastic carcinomata. Three causes of such sarcoid reactions have been suggested: 1. an immunological reaction to substances released by the tumour and transported along the lymphatics; 2. an unrecognized sarcoidosis predisposing to lung cancer; 3. the co-existence of sarcoidosis and malignant tumour, possibly due to a common aetiological factor. The significant preponderance of squamous-cell carcinomata observed in this study is in favour of the first theory, because the slower growth and higher tendency to necrosis of this tumour type may be assumed to give rise to a more vigorous and longer-lasting stimulation of the regional lymph nodes.