Effect of thyrotropin on conversion of T4 to T3 in perfused rat liver

Effects of Thyrotropin on Peripheral Thyroid Hormone Metabolism and Serum Lipids

BACKGROUND

Subclinical hypothyroidism is associated with dyslipidemia and atherosclerosis. Whether these effects are in part mediated via direct effects of thyrotropin (TSH) on peripheral thyroid hormone (TH) metabolism and/or concentrations of serum lipids is not clear.

OBJECTIVE

This study examined whether TSH has direct effects on peripheral TH metabolism and serum lipids.

METHODS

Eighty-two patients with differentiated thyroid cancer were retrospectively analyzed. All patients had undergone total thyroidectomy and ¹³¹I remnant ablation. During follow-up, two successive injections of recombinant human TSH (rhTSH) were administered to patients on a stable dose of levothyroxine. In all patients, TSH, thyroxine (T4), free T4 (fT4), triiodothyronine (T3), reverse T3 (rT3), total cholesterol, high-density lipoprotein cholesterol, low-density lipoprotein cholesterol, apolipoprotein B, lipoprotein(a), and triglyceride levels were measured immediately before the first and approximately 72 hours after the second injection of rhTSH.

RESULTS

After rhTSH stimulation, T3 values decreased (from 1.91 to 1.81 nmol/L; p < 0.001). T4, fT4, and rT3 did not change. After rhTSH, median apolipoprotein B increased from 0.90 to 0.92 g/L (p = 0.03), lipoprotein(a) from 0.21 to 0.24 g/L (p < 0.001), and triglycerides from 1.98 to 2.50 mmol/L (p < 0.001). Serum high-density lipoprotein cholesterol decreased from 0.98 to 0.81 mmol/L (p < 0.001). Multiple regression analysis showed that the changes in lipids were most closely associated with the decrease in T3 levels.

CONCLUSIONS

TSH has direct effects on peripheral TH metabolism by decreasing T3 levels in levothyroxine-treated thyroidectomized patients. This decrease in T3 levels is accompanied by unfavorable changes in serum lipids.

Regulation of tissue iodothyronine deiodinase activity in a model of prolonged critical illness

BACKGROUND

The low plasma triiodothyronine (T3) observed during prolonged critical illness can be explained in part by suppressed hepatic deiodinase type I (D1) and increased D3 activity. Infusion of thyrotropin-releasing hormone (TRH) can restore D1 and D3 activity in critically ill rabbits, but it remains unknown whether this is a direct effect of TRH or the TRH-induced rise in circulating thyroxine (T4) and T3.

METHODS

To answer this specific question, burn-injured rabbits randomly received a 4-day treatment with saline, T4, T3, T4+T3, or TRH, started on day 4 of the illness. Plasma iodothyronine concentrations, D1 and D3 activity, and T3-responsive gene expression were quantified in liver and kidney.

RESULTS

Infusion of T4, T3, or TRH increased circulating T3 levels and hepatic D1 activity. Co-infusion of T3 with T4 enhanced T4 to T3 conversion as demonstrated by lower T4, higher T3, and lower reverse T3 (rT3) levels and tended to further increase hepatic D1 activity. Hepatic D1 activity correlated positively with circulating T3 and the T3/rT3 ratio, but not with T4, rT3, or thyroid-stimulating hormone.

CONCLUSIONS

During prolonged critical illness, D1 activity is primarily regulated via changes in circulating T3, suggesting that the low plasma T3 concentrations may be important in sustaining low D1 activity in this condition.

Thyroid hormone production rates in rat liver and intestine in vivo: a novel graph theory and experimental solution

We develop a novel method for finding sufficient experimental conditions for discriminating and quantifying individual biomolecule production sources in distributed, inhomogeneous multisource systems in vivo, and we apply it experimentally to a complex, unsolved problem in endocrinology. The majority of hormonal triiodothyronine (T3) is produced from prohormone thyroxine (T4) in numerous nonthyroidal organs and, with one exception, the T3 production rate has not been fully resolved in any single extrathyroidal organ of any species. Using a readily generalized graphic method called cut-set analysis, we show here that measured steady-state responses in several organs to three independent tracer infusions, two into blood and one directly into the organ(s) of interest, are sufficient to resolve this problem for organs fully accessible to direct infusion in vivo. We evaluated local T3 production in rat liver and intestine, which also required T3 bile flux measurements, and we found that liver produces approximately 31% and whole intestine approximately 6% of whole body T3 from T4. With thyroidal production included, liver contributes approximately 15% and intestine approximately 3% of whole rat T3 production. This new methodology is broadly applicable, especially to biosystems that include molecular interconversions at multiple sites.

Thyrotropin dysregulation during a non-thyroidal illness: transient hypothalamic hypothyroidism?

Both the basal TSH concentration and the TSH response to iv TRH administration are noted to be decreased at the peak of an acute critical illness. Moreover, an impaired release from hypothalamus has been documented in rats with uncontrolled diabetes, suggesting hypothalamic dysfunction in a non-thyroidal illness. However, the exact inference and mechanism of this impaired TSH secretary pattern is not well defined in humans during a non-thyroidal illness. Therefore, this study assessed hypothalamic pituitary thyroid axis by determination by T4, T3, and T3 resin uptake prior to and TSH concentrations, prior to, as well as following, iv TRH administration at an interval of 30 min up to 2 hours on three successive mornings during a severe, critical, fatal illness in five previously known euthyroid subjects. TSH response to iv TRH administration was expressed as a maximal absolute change (delta TSH) and a cumulative response (CR TSH), calculated as the sum of changes from the basal level at each specific time period for up to 120 min. Serum T4, T3 and TSH concentrations on day 1 of the TRH administration were significantly lower than normal values as well as the values documented previously in the same individuals prior to hospitalization. T3 resin uptake was increased simultaneously. Moreover, serum T4, T3, and T3 resin uptake remained significantly unaltered on three successive days of iv TRH administration. However, basal serum TSH rose significantly with a parallel TSH response to iv TRH administration, as reflected by a progressive rise in delta TSH as well as CR TSH over this three-day period, with normalization of the TSH responses by the third day. Therefore, impaired TSH secretary pattern and altered thyroid hormone concentrations noted in subjects with acute critical illness may be attributed to the presence of a transient hypothalamic hypothyroidism.

Efflux of T4 from the in situ perfused liver of rainbow trout: effect of T4, dithiothreitol and cysteine in the perfusate

A Cortland saline-perfused rainbow trout (Oncorhynchus mykiss) liver model was used to study aspects of T4 efflux from the intact organ system. There was a consistent efflux of T4 in the absence of T4 in the perfusate, and the T4 efflux was increased in the presence of T4 in the perfusate, but the efflux was not T4-dose dependent. The addition of the thiol-containing compound dithiothreitol (DTT, 2 mM) to the perfusate had no significant effect on the flux of T4 from the liver, whereas the addition of cysteine (2 mM), a thiol-containing amino acid suppressed T4 efflux. The results are consistent with the known mechanisms of thyroid hormone trafficking across cell membranes, and suggest that organ systems, such as the liver, may act as a major reserve of hormone, thus participating in plasma thyroid hormone homeostasis.

Effects Of Carbamazepine On The Hypothalamic-Pituitary-Thyroid Axis

OBJECTIVE

To evaluate the effects of treatment with carbamazepine on the hypothalamic-pituitary-thyroid axis and thyroid hormone concentrations in patients with epilepsy.

METHODS

We undertook a prospective study in seven subjects in whom the diagnosis of epilepsy had been established and a regimen of carbamazepine (200 mg three times a day) was initiated. All participants underwent 24-hour 123 I thyroid uptake studies, as well as assessment of the basal thyrotropin concentration and the thyrotropin response to intravenous administration of thyrotropin-releasing hormone. In addition, thyroxine (T(4)), free T(4) index, triiodothyronine (T(3)), free T(3) index, reverse T(3), and T(3) resin uptake were determined before initiation of carbamazepine treatment and again after 3 to 4 months of carbamazepine therapy.

RESULTS

Serum T(4), T(3), and free T(4) index decreased significantly (P<0.05) after therapy with carbamazepine, whereas no significant alterations were noted in T(3) resin uptake, free T(3) index, and reverse T(3) concentrations. A significant increase occurred in basal serum thyrotropin level (P<0.05). Moreover, the peak thyrotropin concentration, the absolute change from the basal thyrotropin level, and an integrated thyrotropin response expressed as cumulative response to thyrotropin-releasing hormone stimulation were all significantly increased after carbamazepine therapy in comparison with values documented before initiation of therapy (P<0.05 for all comparisons). Finally, 24-hour 123 I uptake by the thyroid gland increased slightly after carbamazepine therapy, but the change was not statistically significant. All values determined both before and after carbamazepine therapy remained within normal ranges.

CONCLUSION

Carbamazepine therapy induces significant alterations in serum thyroid hormone concentrations as well as in the hypothalamic-pituitary-thyroid axis. Nevertheless, patients receiving carbamazepine treatment continue to remain euthyroid despite these significant changes.

The effect of recombinant human thyrotropin (rhTSH) on thyroid function in mice and rats

The vast majority of studies to determine the biological activity of recombinant human thyrotropin (rhTSH) have been carried out in the mouse. We have recently reported that 0.1 mg of rhTSH IM (one-ninth the dose given in thyroid cancer patients) given to normal subjects elicits a brisk rise in serum thyroxine (T4), triiodothyronine (T3), and thyroglobulin (Tg) concentrations. In contrast, in initial studies in the rat, a low dose of rhTSH failed to increase serum T4 or T3 concentrations. The present study was, therefore, carried out to determine the biological activity of rhTSH in euthyroid and in T3-treated, TSH-suppressed rats and mice. Doses of rhTSH based on body weight were used and resulted in similar serum human thyrotropin (hTSH) concentrations in the two species. Euthyroid and TSH-suppressed mice responded briskly to rhTSH administration. In contrast, serum T4 did not increase after rhTSH administration in euthyroid rats. In TSH-suppressed rats, the increase in serum T4 was similar to that observed in TSH suppressed mice. These observations suggest that rhTSH more readily displaces endogenous TSH from the mouse than from the rat thyroid TSH receptor, because equal responses were observed when endogenous TSH was suppressed.

The application of an in vitro perfused liver preparation to examine the effects of epinephrine and bovine thyroid-stimulating hormone on triiodo-L-thyronine release from the liver of rainbow trout (Oncorhynchus mykiss)

An isolated, perfused rainbow trout liver preparation was developed to investigate the action of nonthyroidal hormones on hepatic thyroid hormone metabolism. Several assessments were made of the stability and viability of the preparations under a range of conditions, including measures of lactate dehydrogenase flux and tissue ATP and glycogen content, all of which indicated that the perfused liver was stable for the 60-min perfusion period. Moreover, the liver preparations were responsive to an epinephrine challenge and, throughout the series of experiments, sustained hepatic glucose release. Triiodo-L-thyronine (T3) flux from the liver preparation was significantly increased by the provision of thyroxine (T4) substrate. Epinephrine and bovine thyroid stimulating hormone (TSH) were perfused alone and in combination with T4 to evaluate the effect of these hormones on T3 flux from the liver. Both epinephrine and TSH significantly enhanced hepatic T3 flux in the absence of T3 substrate, but neither had an additional effect on T3 flux when perfused in combination with T4. The results of the study suggest that a relationship exists between the circulating levels of nonthyroid hormones and peripheral thyroid hormone metabolism that may be receptor-mediated.

Chronic fasting reduces the response of the thyroid to growth hormone and TSH, and alters the growth hormone-related changes in hepatic 5'-monodeiodinase activity in rainbow trout, Oncorhynchus mykiss

Chronically fasted rainbow trout (Oncorhynchus mykiss) had significantly lower plasma L-thyroxine (T4) and triiodo-L-thyronine (T3), and higher plasma growth hormone (GH) concentrations than fed animals. Fasted and fed trout were administered bovine thyrotropic hormone (bTSH), native ovine GH (oGH), or recombinant human GH (rhGH) alone, or GH in combination with bTSH to further study the effects of food deprivation on the activity of the pituitary-thyroid axis and on the control of hepatic T3 production. Although the fasted rainbow trout retained the ability to respond to bTSH challenge, the resultant elevation in plasma T4 concentration was significantly lower than that of fed animals; there was no plasma T3 response to bTSH challenge in either fed or fasted trout, except for a significant elevation in fed bTSH-injected fish and a significant depression in fed saline-injected fish sampled 2.5 hr after the injection. GH when administered alone had no significant effect on plasma T4 concentrations of either fed or fasted animals, and stimulated an increase in plasma T3 concentration and an increased hepatic T3 content only in the fed fish, despite a significant stimulation by both oGH and rhGH of in vitro hepatic 5'-monodeiodinase activity (MDA) in both fed and fasted groups. bTSH appeared to suppress rhGH- and oGH-stimulated MDA in fasted groups, and rhGH-stimulated MDA in fed trout. The data suggest that chronic fasting induced a down-regulation of the response of thyroid tissue to bTSH challenge, and of the GH-stimulation of T3 production, in vivo, although in vitro hepatic MDA was elevated following GH administration to both fed and fasted rainbow trout.

Evidence of hypothyroidism in the genetically epilepsy-prone rat

A number of neurochemical and behavioral similarities exist between the genetically epilepsy-prone (GEPR) rat and rats made hypothyroid at birth. These similarities include lower brain monoamine levels, audiogenic seizure susceptibility and lowered electroconvulsive shock seizure threshold. Given these similarities, thyroid hormone status was examined in GEPR rats. Serum samples were collected from GEPR-9 and non-epileptic control rats at 5, 9, 13, 16, 22, 31, 45, 60, 90, 150 and 350 days of age. Serum thyroxine (T4) levels were significantly lower in GEPR-9 rats compared to control until day 22 of age. GEPR-9 thyrotropin (TSH) levels were significantly elevated during the period of diminished serum T4. GEPR-9 triiodothyronine (T3) levels were lower than control throughout the first year of life. The data indicate that the GEPR-9 rat is hypothyroid from at least the second week of life up to 1 year of age. The critical impact of neonatal hypothyroidism on brain function coupled with the development of the audiogenic seizure susceptible trait by the GEPR-9 rat during the third week after birth suggests that neonatal hypothyroidism could be one etiological factor in the development of the seizure-prone state of GEPR-9 rats.

Effect of intestinal factors on extraction of insulin in perfused rat liver

To study the direct effect of intestinal factors on hepatic extraction of insulin, an investigation was made into the extraction of insulin from isolated rat liver perfused with portal venous effluent (PVE) obtained from the isolated perfused rat intestine. Rat intestine was perfused with Krebs-Ringer bicarbonate medium for 45 min, and the PVE was collected from glucose-, lipid-, or NaCl-treated and untreated control intestines. The PVE, after adjustment of its glucose (180 mg/dl) and insulin (200 microU/ml) concentrations, was used as the perfusing medium for the liver of a different rat. The liver was perfused without recirculation with the PVE not containing insulin for 15 min and then perfused with the PVE containing insulin for the next 30 min. Insulin removal from liver perfused with PVE from lipid- or NaCl-treated intestine (52.6 +/- 5.4 or 46.6 +/- 4.1%) was similar to that from comparable controls (49.7 +/- 2.8 or 48.2 +/- 2.9%), respectively. However, that from glucose-treated intestine (39.7 +/- 6.2%) was significantly (P less than 0.01) lower than that from control intestine (51.1 +/- 2.5%). These results indicate that an intestinal factor secreted after glucose ingestion significantly reduces hepatic extraction of insulin and that at least a part of the incretin phenomenon is due to a decreased hepatic extraction of insulin after oral glucose administration.