Inhibition of hepatic outer ring monodeiodination of thyroxine and 3,3',5'-triiodothyronine by sodium salicylate

Effects of deracoxib and aspirin on serum concentrations of thyroxine, 3,5,3′-triiodothyronine, free thyroxine, and thyroid-stimulating hormone in healthy dogs

OBJECTIVE

To evaluate the effects of deracoxib and aspirin on serum concentrations of thyroxine (T4), 3,5,3'-triiodothyronine (T3), free thyroxine (fT4), and thyroid-stimulating hormone (TSH) in healthy dogs.

ANIMALS

24 dogs.

PROCEDURE

Dogs were allocated to 1 of 3 groups of 8 dogs each. Dogs received the vehicle used for deracoxib tablets (PO, q 8 h; placebo), aspirin (23 to 25 mg/kg, PO, q 8 h), or deracoxib (1.25 to 1.8 mg/kg, PO, q 24 h) and placebo (PO, q 8 h) for 28 days. Measurement of serum concentrations of T4, T3, fT4, and TSH were performed 7 days before treatment (day -7), on days 14 and 28 of treatment, and 14 days after treatment was discontinued. Plasma total protein, albumin, and globulin concentrations were measured on days -7 and 28.

RESULTS

Mean serum T4, fT4, and T3 concentrations decreased significantly from baseline on days 14 and 28 of treatment in dogs receiving aspirin, compared with those receiving placebo. Mean plasma total protein, albumin, and globulin concentrations on day 28 decreased significantly in dogs receiving aspirin, compared with those receiving placebo. Fourteen days after administration of aspirin was stopped, differences in hormone concentrations were no longer significant. Differences in serum TSH or the free fraction of T4 were not detected at any time. No significant difference in any of the analytes was detected at any time in dogs treated with deracoxib.

CONCLUSIONS AND CLINICAL RELEVANCE

Aspirin had substantial suppressive effects on thyroid hormone concentrations in dogs. Treatment with high dosages of aspirin, but not deracoxib, should be discontinued prior to evaluation of thyroid function.

Nonthyroidal illness syndrome or euthyroid sick syndrome?

OBJECTIVE

To characterize the nonthyroidal illness syndrome (NTIS) and to discuss various underlying potential biochemical mechanisms for this condition.

METHODS

The pertinent medical literature was reviewed, and studies of thyroid function in systemic non-thyroidal illnesses were summarized.

RESULTS

Abnormalities of thyroid function in the NTIS have been classified into four major categories: (1) low triiodothyronine (T3) syndrome, (2) a combination of low T3 and low thyroxine (T4), (3) high T4 syndrome, and (4) other abnormalities. The NTIS has been noted in essentially all severe systemic illnesses and after caloric deprivation, major operations, and administration of some drugs. Some mechanisms that may contribute to low serum T3 in the NTIS are decreased type I 5 -monodeiodinase in tissues, decreased uptake of T4 by tissues, decreased serum binding, increased reverse T3, alterations in selenium status, cytokines, and a decrease in thyrotropin. Decreased thyrotropin may also contribute to low T4 levels in NTIS, as may decreased serum T4-binding proteins, abnormalities in T4-binding globulin, and circulating inhibitors of binding of T4 to serum proteins. Although T4 treatment of patients with NTIS has yielded little improvement, administration of T3 has produced some beneficial effects.

CONCLUSION

Further studies should be conducted to determine appropriate patient populations, dose-response ratios, and possible adverse effects of treatment of the NTIS with T3.

Changes in thyroid function tests during short-term salsalate use

Although long-term administration of salsalate depresses blood levels of both total thyroxine (T4) and total triiodothyronine (T3) and at least transiently decreases serum thyrotropin (TSH), changes in thyroid function tests have not been fully characterized during its short-term use. It is also unclear if the observed changes are solely the result of decreased hormone binding to carrier proteins or if reduced hepatic 5'-monodeiodinase activity is important. Blood was sampled at baseline (day 0) and after 24 hours (day 1) and 72 hours (day 3) in eight subjects taking a therapeutic dose of salsalate 1,500 mg twice daily. Total T4 decreased from 90.1+/-7.7 nmol/L (mean+/-SD) on day 0 to 82.9+/-8.6 nmol/L on day 1 (P=.1 v baseline) and 68.6+/-8.7 nmol/L on day 3 (P=.0001). Total T3 decreased from 1.76+/-0.20 nmol/L to 1.61+/-0.16 nmol/L on day 1 (P<.05) and 1.31+/-0.27 nmol/L on day 3 (P=.002). The T4/T3 ratio was 51.7+/-7.7 at baseline and remained unchanged after 3 days. Levels of reverse T3 (rT3) were reduced from 0.24+/-0.05 nmol/L to 0.18+/-0.02 nmol/L on day 3 (P<.05). While the free T4 index (FTI) declined in parallel with total T4, the free T4 level by direct equilibrium dialysis (FTD) was unchanged after 3 days. Serum TSH decreased from 1.47+/-0.47 mU/L to 0.91+/-0.27 mU/L after 1 day (P<.05) and remained suppressed after 3 days (0.95+/-0.49 mU/L, P<.05). In conclusion, (1) therapeutic doses of salsalate significantly decrease serum concentrations of total T4, total T3, and rT3 to about 75% of baseline levels after 3 days without altering the T4/T3 ratio; (2) although the FTD does not change, serum TSH concentrations remain suppressed; and (3) the proportionate decrease in total thyroid hormone levels suggests that inhibition of hormone binding to serum proteins is more important in producing these changes than reduced hepatic 5'-monodeiodinase activity.

Nutritional and prognostic significance of serum hypothyroxinemia in hospitalized patients with liver cirrhosis

BACKGROUND/AIMS

A variety of severe illnesses can induce changes in thyroid hormone metabolism, leading to findings referred to as "sick euthyroid syndrome". In several groups of patients the reduction of serum thyroxine concentration (T4), characteristic of the low-T4 variant of sick euthyroid syndrome, has been found to be a good predictor of survival. Although the pathophysiology of hormonal alterations has not yet been defined, nutritional deficits have been suggested to play a role. The study aimed to define the prognostic and nutritional significance of serum thyroxine in liver cirrhosis.

METHODS

Thyroid hormones and nutritional status were evaluated in a group of 75 consecutive hospitalized patients with cirrhosis, followed-up clinically for 12 months.

RESULTS

A low-T4 variant of sick euthyroid syndrome was found in 23 of the 75 enrolled patients with cirrhosis (30.6%). Serum T4, but not serum T3, correlated with mid-arm muscle circumference (p < 0.01), an indicator of muscle protein compartment. While both serum T3 and T4 correlated directly with serum proteins and inversely with Child-Pugh score, only T4 was predictive of outcome. Patients with the low-T4 variant of sick euthyroid syndrome showed significantly lower short- and long-term survival rates compared to those with normal serum T4 concentrations (p < 0.008 at 3 months, p < 0.001 at 6 months and 1 year). A multivariate analysis using the proportional hazards Cox's regression procedure showed that serum T4, but not serum T3 or nutritional parameters, improves the prognostic capacity of Child-Pugh score (p < 0.01).

CONCLUSIONS

These data indicate that the low T4-variant of sick euthyroid syndrome distinguishes a subgroup of patients with cirrhosis at risk for decreased survival. The inclusion of T4 in the Child-Pugh score, by improving its prognostic power, may optimize the selection of patients with advanced cirrhosis to receive specific therapy such as transplantation.

Case report: abnormal thyroid function tests in a patient and two normal volunteers treated with salsalate

There are only three prior reports of abnormal thyroid function tests in patients who have received salsalate, the salicylate ester of salicylic acid. The authors report an elderly clinically euthyroid man who had thyroid function tests suggestive of central hypothyroidism while taking salsalate but whose thyroid tests returned to normal after the drug was discontinued. They also studied thyroid function tests, including free thyroxine (FT4) and reverse (T3), in two normal volunteers who took salsalate 750 mg twice daily for 1 week. In the normal subjects, total T4 and FT4 began to fall within 24 hours after the first dose of salsalate, and remained suppressed for at least 24 hours after the drug was discontinued. This rapidity of effect by salsalate is previously undescribed. There was also a fall in FT4, probably due to the use of diluted serum in the equilibrium dialysis FT4 assay. Because FT4 measurement using diluted serum or equilibrium dialysis may cause falsely low FT4 measurements, the authors believe ultrafiltration may be the only reliable method of measuring FT4 in these patients.

The effects of drugs on tests of thyroid function

Many drugs affect tests of thyroid function through alterations in the synthesis, transport and metabolism of thyroid hormones, as well as via influences on thyrotrophin (TSH) synthesis and secretion. Despite effects on circulating thyroid hormone and TSH levels, few drugs result in important changes in clinical thyroid state, but difficulty in interpretation of thyroid function tests often results. Commonly prescribed drugs including anti-convulsants, non-steroidal anti-inflammatory drugs, beta-adrenoceptor antagonists, steroid hormones and heparin may result in abnormal thyroid function tests in the absence of clinical features of thyroid dysfunction. In contrast, lithium and iodine containing drugs, including radiographic contrast agents and amiodarone, may result rarely in overt thyroid disease.

Thyromimetic effects of 3,5-dimethyl,3'-isopropyl thyronine (DIMIT) and 3,5-diethyl,3'-isopropyl thyronine (DIET) in various tissues of the rat

We have examined the effects of treatment of the rat with 3,5-dimethyl-3'-isopropyl thyronine (DIMIT) and 3,5-diethyl-3'-isopropyl thyronine (DIET) on serum thyrotropin (TSH) concentration, heart weight, hepatic outer ring (5'-) monodeiodination of T4 to T3, cardiac outer ring monodeiodination of 3'-5'-diiodothyronine (3',5'-T2) to 3'-monoiodothyronine (3'-T1), and cerebral cortical monodeiodination of 3,5-T2 in the inner ring to 3-T1. Groups of four to seven rats were injected intraperitoneally either once a day or at eight-hour intervals for three days with saline or thyronines. Serum TSH was measured by radioimmunoassay. The various monodeiodinations were studied in homogenates of the tissues at optimal pH (7.35 for outer ring monodeiodinations and 8.0 for inner ring monodeiodination) and temperature (37 degrees C) in the presence of an excess of dithiothreitol. DIMIT was clearly active in all thyromimetic effects studied. It was about 10% as active as T4 in suppression of TSH. It stimulated hepatic monodeiodination of T4 to T3 in a similar manner to T4. DIMIT was also more comparable to T4 than to T3 in effect on heart weight and cardiac metabolism of 3',5'-T2 to 3'-T1. It was highly active, apparently more so than either T4 or T3, in cerebral cortical stimulation of metabolism of 3,5-T2 to 3-T1. In distinction to DIMIT, DIET had very little thyromimetic activity.(ABSTRACT TRUNCATED AT 250 WORDS)

One enzyme for the 5'-deiodination of 3,3',5'-triiodothyronine and 3',5'-diiodothyronine in rat liver

Many studies suggest that one enzyme is involved in the phenolic ring deiodination of iodothyronines in rat liver and kidney and another one in the tyrosyl ring deiodination. This study describes some characteristic of the phenolic ring (5'-) deiodination of rT3 and 3',5'-T2 by rat liver microsomes. At pH 7.2 the Km values of the 5'-deiodination of rT3 and 3',5'-T2 were 0.103 and 0.77 microM, respectively. 3',5'-T2 and rT3 inhibited the respective 5'-deiodination reactions competitively, the Ki values being 1.05 and 0.134 microM, respectively. Several radiographic contrast agents markedly inhibit the 5'-monodeiodination or rT3 and 3',5'-T2, the type of inhibition being competitive. Of these compounds iopanoic acid, ipodic acid and iophenoxic acid are the most potent inhibitors with Ki values of approximately 2 microM for both reactions. The non-iodine containing compound 8-anilino-1-naphthalene sulphonic acid (ANS) appeared to be a very strong competitive inhibitor of both 5'-deiodinations (Ki 4.3-4.7 microM), whereas salicylic acid, which as ANS inhibits the binding of iodothyronines to T4-binding globulin, inhibited these reactions to a much lesser extent (Ki 300-500 microM). On the other hand, diiodosalicylic acid was a very strong inhibitor. The beta-adrenergic blocker D,L-propranolol was a weak noncompetitive inhibitor of both 5'-deiodinations (Ki 0.4-0.7 mM). These reactions were also inhibited by various 2,6-diiodophenol derivatives, triiodophenol being the strongest and diiodotyrosine the weakest inhibitor tested. Comparing the Ki values of various inhibitors for the 5'-deiodination of rT3 and 3',5'-T2, a positive correlation between these values was found (r = 0.97). It was concluded that rT3 (to 3,3'-T2) and 3'-5'-T2 (to 3'-T1) monodeiodinating activities are very similar to each other and that there may just be one monodeiodinase catalyzing the 5'-deiodination of iodothyronines in rat liver.

Pharmacological interference with in vitro tests of thyroid function

Numerous drugs may cause changes in the serum concentrations of T4 and of T3. If such alterations are not recognized an incorrect diagnosis may result. In moderate degrees of hypo- and hyperthyroidism thyroid hormone levels may be spuriously normal, or the influence of pharmacological substances may lead to false diagnosis of thyroid disease in euthyroid patients. Since prediction of such alterations remains uncertain, it may be necessary to perform additional investigations when a potential artefact is recognized. On the other hand many pharmacological agents, especially those which interact with neurotransmitters, may influence TSH secretion, too. The TRH-test may show an increase or decreased TSH response, although complete suppression is only rarely seen during high-dose glucocorticoid treatment when low TRH doses are applied. Because of TRH-test gives such wide separation between different clinical states false interpretations are generally less likely than with drug-induced changes in T4 and T3 values.