Amiodarone inhibits T4 to T3 conversion and alpha-glycerophosphate dehydrogenase and malic enzyme levels in rat liver

Thyroid Dysfunction under Amiodarone in Patients with and without Congenital Heart Disease: Results of a Nationwide Analysis

Background: Amiodarone has a profound adverse toxicity profile. Large population-based analyses quantifying the risk of thyroid dysfunction (TD) in adults with and without congenital heart disease (ACHD) are lacking. Methods: All adults registered with a major German health insurer (≈9.2 million members) with amiodarone prescriptions were analyzed. Occurrence of amiodarone-associated TD was assessed. Results: Overall, 48,891 non-ACHD (37% female; median 73 years) and 886 ACHD (34% female; median 66 years) received amiodarone. Over 184,787 patient-years, 10,875 cases of TD occurred. The 10-year risk for TD was 38% in non-ACHD (35% ACHD). Within ACHD, compared to amiodarone-naïve patients, the hazard ratio (HR) for TD was 3.9 at 4 years after any amiodarone exposure. TD was associated with female gender (HR 1.42, p < 0.001) and younger age (HR 0.97 per 10 years, p = 0.009). Patients with congenital heart disease were not at increased risk (HR 0.98, p = 0.80). Diagnosis of complex congenital heart disease, however, was a predictor for TD (HR 1.56, p = 0.02). Amiodarone was continued in 47% of non-ACHD (38% ACHD), and 2.3% of non-ACHD (3.5% ACHD) underwent thyroid surgery/radiotherapy. Conclusions: Amiodarone-associated TD is common and comparable in non-ACHD and ACHD. While female gender and younger age are predictors for TD, congenital heart disease is not necessarily associated with an elevated risk.

Amiodarone and thyroid physiology, pathophysiology, diagnosis and management

Although amiodarone is considered the most effective antiarrhythmic agent, its use is limited by a wide variety of potential toxicities. The purpose of this review is to provide a comprehensive "bench to bedside" overview of the ways amiodarone influences thyroid function. We performed a systematic search of MEDLINE to identify peer-reviewed clinical trials, randomized controlled trials, meta-analyses, and other clinically relevant studies. The search was limited to English-language reports published between 1950 and 2017. Amiodarone was searched using the terms adverse effects, hypothyroidism, myxedema, hyperthyroidism, thyroid storm, atrial fibrillation, ventricular arrhythmia, and electrical storm. Google and Google scholar as well as bibliographies of identified articles were reviewed for additional references. We included 163 germane references in this review. Because amiodarone is one of the most frequently prescribed antiarrhythmic drugs in the United States, the mechanistic, diagnostic and therapeutic information provided is relevant for practicing clinicians in a wide range of medical specialties.

Inhibition of the type 2 iodothyronine deiodinase underlies the elevated plasma TSH associated with amiodarone treatment

The widely prescribed cardiac antiarrhythmic drug amiodarone (AMIO) and its main metabolite, desethylamiodarone (DEA), have multiple side effects on thyroid economy, including an elevation in serum TSH levels. To study the AMIO effect on TSH, mice with targeted disruption of the type 2 deiodinase gene (D2KO) were treated with 80 mg/kg AMIO for 4 wk. Only wild-type (WT) mice controls developed the expected approximate twofold rise in plasma TSH, illustrating a critical role for D2 in this mechanism. A disruption in the D2 pathway caused by AMIO could interfere with the transduction of the T4 signal, generating less T3 and softening the TSH feedback mechanism. When added directly to sonicates of HEK-293 cells transiently expressing D2, both AMIO and DEA behaved as noncompetitive inhibitors of D2 [IC(50) of >100 μm and ∼5 μm, respectively]. Accordingly, D2 activity was significantly decreased in the median eminence and anterior pituitary sonicates of AMIO-treated mice. However, the underlying effect on TSH is likely to be at the pituitary gland given that in AMIO-treated mice the paraventricular TRH mRNA levels (which are negatively regulated by D2-generated T3) were decreased. In contrast, AMIO and DEA both exhibited dose-dependent inhibition of D2 activity and elevation of TSH secretion in intact TαT1 cells, a pituitary thyrotroph cell line used to model the TSH feedback mechanism. In conclusion, AMIO and DEA are noncompetitive inhibitors of D2, with DEA being much more potent, and this inhibition at the level of the pituitary gland contributes to the rise in TSH seen in patients taking AMIO.

Biochemistry, cellular and molecular biology, and physiological roles of the iodothyronine selenodeiodinases

The goal of this review is to place the exciting advances that have occurred in our understanding of the molecular biology of the types 1, 2, and 3 (D1, D2, and D3, respectively) iodothyronine deiodinases into a biochemical and physiological context. We review new data regarding the mechanism of selenoprotein synthesis, the molecular and cellular biological properties of the individual deiodinases, including gene structure, mRNA and protein characteristics, tissue distribution, subcellular localization and topology, enzymatic properties, structure-activity relationships, and regulation of synthesis, inactivation, and degradation. These provide the background for a discussion of their role in thyroid physiology in humans and other vertebrates, including evidence that D2 plays a significant role in human plasma T(3) production. We discuss the pathological role of D3 overexpression causing "consumptive hypothyroidism" as well as our current understanding of the pathophysiology of iodothyronine deiodination during illness and amiodarone therapy. Finally, we review the new insights from analysis of mice with targeted disruption of the Dio2 gene and overexpression of D2 in the myocardium.

Thyroid hormone alpha1 and beta1 receptor mRNA are downregulated by amiodarone in mouse myocardium

Amiodarone, a powerful antiarrhythmic drug, may exert its effect by antagonism of the thyroid hormone, probably at the receptor level. The aim of this study was to investigate whether amiodarone affects the levels of thyroid hormone receptor (TR) messenger RNA (mRNA) subtypes in mouse hearts. Mice were treated with 10, 25, and 50 mg/kg body weight (BW) amiodarone or vehicle (propyleneglycol) intraperitoneally, daily for 14 days. The heart rate dose-dependently decreased in the 25 mg/kg BW (p < 0.05) and 50 mg/kg BW (p < 0.005) amiodarone-treated mice compared with control. Serum T3 levels were significantly decreased by 25% (4.2 +/- 0.7 pM) in the 50 mg/kg BW amiodarone group in comparison to control (5.6 +/- 1.4 pM; p < 0.05). The serum T4 levels were 1.3 times higher in 50 mg/kg BW amiodarone-treated mice (13.2 +/-1.6 pM) compared with the control (10.3 +/- 1.3 pM; p < 0.005). Determination of TRalpha1, alpha2, beta1, and beta2 mRNA in the heart were performed by reverse transcriptase-polymerase chain reaction (RT-PCR)/enzyme-linked immunosorbent assay (ELISA). Both in treated and untreated mice, TRalpha2 mRNA had the highest density in mouse heart, whereas TRbeta2 mRNA had the lowest density. Amiodarone dose-dependently downregulated the levels of TRalpha1 and beta1 mRNA in comparison to the control. There were, however, no differences in the TRalpha2 and TRbeta2 mRNA levels in the mice heart treated with different doses of amiodarone in comparison with the control group. In conclusion, this study shows that amiodarone subtype selectively downregulates the TR mRNA levels in mouse myocardium in a dose-dependent manner. These results support a thyroid hormone-dependent action of amiodarone.

Interaction of amiodarone and triiodothyronine on the expression of beta-adrenoceptors in brown adipose tissue of rat

1. This study was undertaken to evaluate in vivo the influence of amiodarone on the effects of triiodothyronine (T3) in brown adipose tissue (BAT) which are independent of thyroid hormone synthesis and of the conversion of thyroxine (T4) to T3. Thyroidectomized rats were given a replacement dose of T3 (0.5 mg kg(-1) p.o. daily for 3 days) with or without amiodarone (50 mg kg(-1) p.o. daily for 1 week). 2. As assessed by RT-PCR, treatment of thyroidectomized rats with T3 caused a 2 fold decrease in beta3-adrenoceptor (beta3-AR) mRNA levels and a 2 fold increase in beta1-AR mRNA levels. 3. Binding studies using [3H]-CGP 12177 as a ligand showed that treatment of thyoidectomized rats with T3 resulted in a 70% decrease in beta3-AR number and in an 80% increase in beta1-AR in BAT membranes. 4. T3-treatment abolished the increase in BAT adenylyl cyclase (AC) activity induced by CGP12177 in thyroidectomized rats. It also decreased the amount of Gi protein (ADP-ribosylation) by 30%. 5. At variance with the literature on the heart, amiodarone administration did not inhibit the positive effect of T3 on beta1-AR expression in BAT in thyroidectomized rats. However, it antagonized the effect of T3 on beta3-AR number, but not on AC activity or on Gi expression. 6. These results indicate that the effects of thyroid hormones on the responsiveness of BAT to catecholamines involves both receptor and post-receptor mechanisms, they also suggest that interaction between amiodarone and thyroid hormones is highly tissue-specific and depends on the beta-AR subtype.

Amiodarone and the Thyroid

OBJECTIVE

To review the amiodarone-associated alterations in thyroid hormone metabolism and thyroid function and compare them with the effects of inorganic iodide. To clarify the pathophysiologic features and treatment of amiodarone-associated hypothyroidism and thyrotoxicosis.

SUMMARY

Amiodarone, an iodinated benzofuran, is an important antianginal and antiarrhythmic medication. It also alters thyroid hormone metabolism and may precipitate hypothyroidism or hyperthyroidism. Amiodarone-associated hypothyroidism (AAH) is similar to iodine-induced hypothyroidism. Amiodarone-associated thyrotoxicosis (AAT) has a complex pathophysiology. Type I AAT is due to increased thyroid hormone synthesis and release and occurs in patients with multinodular goiter or Graves' disease. Therapeutic interventions may include discontinuation of amiodarone, thionamide therapy, perchlorate, or surgery. In type II AAT, hyperthyroidism is the consequence of a destructive thyroiditis with release of preformed thyroid hormone. Prednisone therapy is the treatment of choice. The distinction between these two entities is of considerable clinical and therapeutic importance.

Comparison of the effects of propylthiouracil, amiodarone, diphenylhydantoin, phenobarbital, and 3-methylcholanthrene on hepatic and renal T4 metabolism and thyroid gland function in rats

We studied the effects of propylthiouracil (PTU), amiodarone (AMIO), diphenylhydantoin (DPH), phenobarbital (PB), and 3-methylcholanthrene (MC) on thyroid histomorphology, on the hepatic and renal enzymes involved in endogenous and exogenous metabolism, and on the plasma levels and pharmacokinetics of thyroid hormones after 7 and 14 days of treatment. PTU and PB, by decreasing both serum tetraiodothyronine (T4) and triiodothyronine (T3), induced a massive increase in serum thyrotropin (TSH) and thus induced thyroid hypertrophy. AMIO and MC, by decreasing respectively serum T3 and T4, also induced an increase of TSH, but to a lesser extent, not sufficient to induce thyroid hypertrophy. Hepatic 5'-deiodinase activity was decreased in all treated rats. Inhibition of this enzyme by PTU was demonstrated in vitro; AMIO also decreased the enzyme activity by a still unelucidated mechanism, which probably requires intact cell plasma membranes, whereas in PB- and MC-treated rats the decrease in enzyme activity certainly resulted from decreased serum concentrations of T4. In PTU-treated rats, and probably in MC-treated rats, decreases in circulating thyroid hormones were primarily due to impairment of synthesis and/or of secretion by the thyroid. In contrast, in PB-treated rats, the decrease in serum thyroid hormone levels seems to be due to increased excretion of these hormones, as T4 serum clearance was significantly increased. PB, a microsomal enzyme inducer, increased the cytochrome b5 and P450 content as well as the cytochrome P450-dependent O-depentylation of pentoxyresorufin. The other type of enzyme inducer, MC, did not affect cytochrome b5 and P450 levels, but did increase the cytochrome P450 dependent O-deethylation of ethoxyresorufin. PB increased the glucuronidation of morphine, whereas MC increased the glucuronidation of 1-naphthol. However, serum T4 clearance, mainly determined by its hepatic conjugation rate, was increased only in PB-treated rats. It appears from this study that the close metabolic relationship between the liver/kidney and the thyroid should be taken into consideration when the findings of chronic toxicology and carcinogenicity studies are interpreted.

The effects of amiodarone on thyroid hormone function: a review of the physiology and clinical manifestations

Amiodarone, an iodinated benzofuran derivative, is used for treatment of refractory cardiac arrhythmias. Certain features of the drug's structure resemble those of the biologically active thyroid hormone, triiodothyronine (T3). In addition, the drug has a variety of complex effects on thyroid hormone physiology, including a number of possible antagonistic effects on thyroid hormone function at the cellular level. The drug occasionally causes clinically overt hyperthyroidism and hypothyroidism. We review these effects and discuss their clinical implications.

Amiodarone and thyroid function

Amiodarone blocks the action of thyroid hormone by the inhibition of 5'-deiodinase which reduces production of T3 in peripheral tissues and possibly by blocking nuclear binding of T3. Since the drug inhibits peripheral conversion of T4 to T3, many patients taking amiodarone have abnormal thyroid function studies (increased T4 and rT3; decreased T3) despite being euthyroid. Treatment of patients with amiodarone generates an iodine excess, which contributes greatly to the significant incidence of altered thyroid status in this population. The diagnosis of hyperthyroidism and hypothyroidism can be difficult. However, using the overall clinical picture and the tolerance limits of hormone levels determined for patients remaining euthyroid on amiodarone therapy, the accurate diagnosis of clinically significant thyroid dysfunction can almost always be made. To screen for thyroid disease, thyroid function should be assessed before initiating therapy, semiannually during therapy or whenever clinical features of thyroid dysfunction occur. Subclinical hypothyroidism as denoted by modest increases in TSH levels do not require treatment or the discontinuation of amiodarone therapy. An appreciation of the mechanism of the interaction between amiodarone and thyroid hormone metabolism allows the clinician to recognize thyroid dysfunction at an early stage and initiate appropriate therapy, thereby minimizing the morbidity associated with forms of amiodarone toxicity.