Male Wistar rats of eight weeks of age were obtained from the breeding centre of the Central Pharmacy of Tunis, weighing of 103±8g. Food (standard diet, supplied by the ‘Société industrielle des concentrés’, SICO, Sfax, Tunisia), and water were available ad libitum.

The animals were acclimated for one week under the same laboratory conditions of photoperiod (12-h light: 12-h dark cycle), a minimum relative humidity of 40% and room temperature 23±2°C. The rats were randomly divided into three groups of eight males, each as follows:

• • group 1, comprising eight individuals and serving as control;
• • group 2, whose individuals received 0.05% BTU in drinking water for five weeks to induce hypothyroidism;
• • group 3, whose rats were treated with 0.0012% of l-thyroxine sodium salt in their drinking water for five weeks to induce hyperthyroidism.

The total rat body weights were daily recorded throughout the experimental period. The rats were sacrificed by decapitation. Their hearts and livers were removed and weighed after the removal of the surrounding connective tissues, and then they were placed in a buffer solution of TBS (Tampon buffer saline, 50 mM Tris, 150 mM NaCl, pH = 7.4) at a temperature of 4 °C. One gram of each organ was homogenised in 2 ml of TBS in an ultrasound homogeniser. Homogenates were centrifuged at 9000 rpm for 15 min at 4 °C, and the supernatants were divided into aliquots and stored at −20 °C before being used. Blood samples were immediately collected before being centrifuged at 3000 rpm for 15 min; plasma were then separated and stored at −20 °C before being used for hormones assays.

T₃ and T₄ were determined by radioimmunoassay (commercial RIA Kit, CIS International, France), as described by López-Torres et al. (2000) [17].

MDA concentrations were estimated according to the method of Esterbauer et al. [18], while hydroperoxides were determined following the method of Heath and Tappel [19]. On the other hand, vitamin E was evaluated using an HPLC procedure, as described by Lang et al. [20]. The reduced glutathione content was estimated using a colorimetric technique, as mentioned by Weckbeker and Corey [21], while the glutathione peroxidase activity was measured following the method of Flohe and Gunzler [22].

The glutathione peroxydase activity was expressed by the quantity of oxidized GSH having disappeared, while that of glutathione reductase was determined following the method of Horn [23]. The activity of catalase was measured according to the method of Aebi [24], whereas that of superoxide dismutase was achieved by the nitrobluetetrazolium test (NBT), according to the method of Asada et al. [25]. Protein contents of various samples were determined by the method of Bradford [26], using bovine serum albumin as a standard.

The results were expressed as mean values ± SEM. Comparisons between mean values were made using the one-way analysis of variance (ANOVA), followed by the Student–Newmann–Keuls test when necessary.

Changes in T₃ concentrations are illustrated in Table 1. T₃ underwent a highly significant increase (p⩽0.01) in the hyperthyroid rats compared to the hypothyroid and the control ones, whereas in hypothyroid animals the concentrations were remarkably reduced (p⩽0.01) compared to those of untreated animals. However, T₄ plasma levels did show a significant decrease (p⩽0.01) in both hyperthyroidism and hypothyroidism, even though the decrease was more pronounced in the latter case (p⩽0.001) in comparison with the controls.

Changes in total body weight are also shown in Table 1. The total body weight showed a pronounced reduction (p⩽0.01) in rats of both treated groups. Besides, a significant rise (p⩽0.01) in mean heart weights of hyperthyroidism was recorded, compared only to the case of hypothyroid rats, while a significant decrease (p⩽0.01) in mean liver weights of both hypothyroid and hyperthyroid rats was noted compared to the controls.

Lipid peroxidation products such as malondialdehyde and hydroperoxide concentrations did not show any significant change as regards the hearts and livers in the hypothyroid individuals compared to controls (Tables 1 and 2), whereas those of hyperthyroid rats were significantly higher (p⩽0.01) than those of controls (Tables 1 and 2). Finally, vitamin E levels underwent a significant increase (p⩽0.01) in hyperthyroid rats, compared to those of the controls (Tables 1 and 2).

Changes in GSH concentrations, GPx, GR, SOD and CAT activities are illustrated in Tables 2 and 3. A significant reduction (p⩽0.01) in GSH levels accompanied by an increase (p⩽0.01) of GPx activity in hyperthyroid rats was observed compared to those of controls.

The GR activity of hyperthyroid rats was remarkably higher (p⩽0.01) than that recorded in hypothyroid animals and even more pronounced (p⩽0.05) than that of the controls.

On the other hand, SOD and CAT activities in hyperthyroid rats did undergo a significant increase (p⩽0.01) compared to that recorded in both hypothyroid and control rats.

The preliminary results arising from the current investigation reveal that thyroid hormones affect the effectiveness of antioxidants in the rat hearts and livers. Most if not all the studied biochemical parameters related to the oxidizing stress are in favour of such a hypothesis.

The overall changes in the thyroid hormones profile reflect the observed decline in the total body weight, as well as the relative weights of hearts and livers. Nonetheless, the present results show that thyroxine depletion (treatment with BTU) did induce a loss in body weight. Such an observation does not agree with some previous studies where body gain has been reported to occur under such conditions, even though few studies confirm our present results [17,27–29]. Moreover, the altered levels of T₃ and T₄ might be considered as a sound argument in the induction of either hypo- or hyperthyroidism.

Besides, hyperthyroidism leads to a rise in oxygen consumption, inducing in turn, an increase in the level of oxidative stress (singlet oxygen ¹O₂) in the heart and in the skeletal muscles [4,17], as well as in the liver [10,29,30]. Consequently, the formation of lipid peroxidations and hydroperoxides increases.

The high increase in the level of MDA and hydroperoxides in hyperthyroidism compared to those of both hypothyroidism and controls might be due to the possible changes in the cellular respiration of target tissues, which are undoubtedly related to any alteration in the thyroid function, knowing the major role of the thyroid hormones in the control (acceleration) of the mitochondrial respiration rate [1,2,28]. From a biochemical point of view, the provoked hyperthyroidism, and via a variety of mechanisms, mitochondrial respiratory chain activity is altered, leading to an increase in electrons transfer from the respiratory chain through the acceleration of the cellular metabolism rate, resulting in the increased generation of superoxide (O2•−) at the site of ubiquinone [7]. Superoxide radicals can lead to the formation of many other reactive species, including hydroxyl radicals (OH•), which can readily start the free-radical process of lipid peroxidation [3,6].

It is worth mentioning that the inhibition of the thyroid gland activity leads not only to cellular metabolism inhibition, but also to other negative effects, such as a reduction of the effectiveness of antioxidants that have a crucial role in tissue protection [4,5].

Since vitamin E has a hydrophobic character, it adheres to cell-membrane fatty acids and lipoproteins. Vitamin E is also known to have a phenolic function with an antioxidant capacity. Therefore, such a vitamin protects and prohibits the propagation of lipid peroxidation, arising from the oxidative stress. Besides, the observed increase in vitamin E concentrations in rats suffering from hyperthyroidism, might be due to an adaptation against the oxidative stress provoked by the thyroid hyperactivity. Such results are in total agreement with other findings [29–33].

GSH is a tripeptide with a broad spectrum of effects. It is involved in several biological activities including neutralisation of reactive oxygen species (ROS), detoxification of xenobiotics, lipid peroxidations, and the hydroperoxides issued from the respiratory chain [33,34]. The reduction in GSH levels in hyperthyroid rats reflects its consumption through the oxidative stress, associated with the increase in GPx activity, ensuring the elimination of lipid peroxidations. This does not only confirm the main role of the thyroid hormones in regulating the oxidative stress in target cells, but is also in agreement with that of Morini et al. [9], where an increase in GPx activity in hyperthyroid rats was observed. In contrast, Asayama et al. [4] and Fernandez et al. [10] reported a significant reduction in GPx activity of rats suffering from hyperthyroidism. The difference in GPx enzyme activity was probably due to the age (eight weeks) of the rats used in the current investigation. The physiological state of the thyroid gland, the dose and the duration of treatment are also of a major influence on antioxidants enzymes. It was reported in previous studies that the level of lipid peroxidation in the heart was affected by both the age and the state of the thyroid gland, in hyperthyroid rats [35]. From another point of view, the above-mentioned effects might involve an accumulation of superoxide anion that inhibits CAT activity, giving rise to hydrogen peroxide (H₂O₂) concentrations [1].

The reduction in GR activity in hypothyroid rats seems to be due to the lack of the reduced equivalents of NADPH coenzyme, necessary for the activity of glucose-6-phosphate dehydrogenase. A key enzyme in the pentose phosphate pathway in in-vivo conditions, since GR activity is dependent on the availability of these reducing equivalents [33]. Similar observations have been reported in recent studies carried out on rabbits [36]. Yet, there has been an increase in the activity of GR in the hearts and livers of hyperthyroid rats. This accounts for the accumulation of toxic peroxides. Since GSH is widely involved in oxido-reduction reactions, in the presence of the GPx, it oxidizes GSH to GSSG, affecting the natural ratio of GSH/GSSG and, consequently, GR interferes in maintaining this ratio through the neo-formation of GSH.

The acquired results reveal also a marked increase in SOD activity in hyperthyroidism, indicating the presence of oxidative stress due to the increasing mitochondrial oxidation rate, characterised by an overproduction of superoxide anion. The latter is known for its harmfulness to the cell membrane. The SOD is also known for its role in transforming (O2−) into inorganic hydroperoxide (H₂O₂), which will, in turn, be reduced by both CAT and GPx enzymes [1,35,37]. Accordingly, an increase in CAT activity in the homogenates of hyperthyroid rats is noted. This accelerates the speed of the formation of superoxides and the renewal of H₂O₂ quantity (substrate of CAT), which increases CAT activity until the dismutation of hydrogen peroxide [33,34,38]. Both SOD and CAT function together in a way linked to the dissociation and formation of H₂O₂, and their activities are adjusted by their variation in the thyroid gland's activity.

In the case of the thyroid gland inhibition, one might expect a fall in cellular respiration and, by analogy, it does not have an effect on the SOD activity, showing the possible effect of thyroid hormones in the determination of the antioxidant enzyme levels. Similar assumptions have already been made by other authors [33,35].

To conclude, it is assumed from the obtained results that the formation of lipid peroxidation in tissues depend on the physiological state of the thyroid gland, where the administration of an antithyroid drug does not affect peroxide products, whereas the treatment with exogenous thyroxine leads to their increase.