The Influence of Hypothermia on Thyroid Function in Rats
Subgroup: Volume 1, Issue 1, Winter
Date: March 2003
Type: Original Article
Start Page: 27
End Page: 32
- A Shirpour Departments of Physiology, Oroumieh University of Medical Sciences, Oroumieh, I.R. Iran
- S Khameneh Departments of Physiology, Oroumieh University of Medical Sciences, Oroumieh, I.R. Iran
- N Zarghami Departments of Biochemistry, Oroumieh University of Medical Sciences, Oroumieh, I.R. Iran
- M. Eskandari Departments of Physiology, Oroumieh University of Medical Sciences, Oroumieh, I.R. Iran
City, Province: Oroumieh,
Country: I.R. Iran
He study was designed to investigate the influence of hypothermia on thyroid gland function and its role in metabolic balances.
Materials and Methods: Superficial hypothermia, bringing the body temperature to 25°C, was induced in ten rats (albino, Wistar race) with a mean age of 8 months. Serum levels of FT3, FT4, T3, T4, and TSH were determined before and just after hypothermia, repeated every 24hours for 4 days.
Results: Hormone levels, measured by radioimmunoassay, changed during the study. On different days of study, TSH levels altered, although not significantly, from basal values. Other hormones, decreased significantly after hypothermia, except for T3 that increased significantly on day 3 compared to basal levels. FT3 and FT4 showed the most decrease. Serum T4 level decreased significantly until 48 hours following hypothermia, after which the decrease was not significant and thereafter began to rise and return to basal levels. Although the body temperature of rats decreased significantly after hypothermia, it increased the day after hypothermia and reached closer to body basal temperature (37°c) after 96 hours.
Conclusion: Our findings indicate reduced activity of the thyroid gland and hypothalamus-pituitary axis during hypothermia, this being more prominent in the thyroid gland.
Keywords: thyroid gland function;hypothermia;Wistar rat
The thyroid hormones exert various effects on cardiovascular function.1 Differing states of disease verify these influences.2 Heart rate, stroke volume and contractile force, which are results of imbalance in enzymatic function and alterations in protein synthesis,4 decrease in hypothyroidism.3 This evidence shows a significant association between thyroid hormones and cardiac function. Studies show that cardiopulmonary bypass (CPB) and hypothermia result in noticeable decrease in circulatory thyroid hormones.5 Muller et al. revealed that cardiopulmonary bypass with hypothermia alter thyroid hormones in a way that TSH increases during operation but normalizes during first postoperative day. T3 also decreases and returns to normal after several days.5 Mitchell et al. reported that in infants undergoing CPB, T3, T4 and TSH decrease while FT4 increases.6 Studying infants, Mainwaring et el. showed that FT4 increases transiently in CPB and hypothermia while T3, T4, TSH and TRH decrease dramatically immediately following the operation.7 After 5 days, influenced by TSH, serum levels of T3, FT3 and T4 returned to normal values. They suggested that concurrent CPB and hypothermia give rise to a transient attenuation of pituitary-thyroid axis in infants. The aim of this study is to determine the influence of hypothermia per se, without the resultant stress associated with Cardiopulmonary bypass.
Materials and Methods
Ten male rats (albino, Wistar) with an average weight of 285 gr. and age of 8 months were enrolled. To induce hypothermia, we used a device designed in the physiology department of Tabriz University of Medical Sciences. The rats after being anesthetized were placed in a specific section of the device and the desired temperature was set. The device had a sensor connected to a thermostat at one end and to the rat rectum at the other. The sensor was also connected to a digital screen, displaying the body temperature of the rat throughout the procedure. First, rectal temperature of the animals was determined. General anesthesia was induced by sub peritoneal injection of chloral hydrate (50 mg/100 gr), following which 2mL of blood was withdrawn from the heart. Body temperature was measured rectally and the rats were placed in the hypothermia device, the temperature of which had initially been set to 37°C. The temperature was then gradually decreased to 25°C, at which temperature the animals were kept for 2 hours. Following this, blood samples were taken as before. After 24 hours a transient state of drowsiness was induced in the rats by placing the rats with ether-soaked swabs in desiccators and removing them immediately following induction of drowsiness for blood sampling, which was repeated every 24 hours for 96 hours.
To separate sera, all blood samples were centrifuged (2500 rpm, 20 min). Serum samples were kept in paraffin-sealed test tubes and stored at -22°C. Thyroid hormones were measured by radio-immuno assay kits (Kavoshyar Co, Iran) at the hormone laboratory of Imam Khomeini hospital in Tabriz. The hormone levels before induction of hypothermia (time 0) were considered as the basal levels, which were then compared with the levels obtained, after induction and maintenance of hypothermia, using SPSS software (Windows), and the paired t-test. P<0.05 was considered significant.
In this study investigating the influence of hypothermia per se, without the effects of stress associated with CPB or cardiac arrest on the thyroid gland activity in the animal models, our findings were as follows:
Basal level of FT3 in the physiologic state was 3.53±0.219 pg/mL, decreasing after induction of hypothermia and reaching its nadir during the 24 hours following hypothermia (2.12±0.28 pg/mL) significantly lower than physiologic levels (p<0.002). FT3 level, then, increased gradually, but it was significantly lower than in the physiologic state during the first 78 hours after hypothermia, reaching 2.83±0.204 pg/mL after 96 hours which was not significantly different from the basal level. (Fig.1)
Fig. 1. Serum FT3 levels before (0) and just after hypothermia (2 hours) 24, 48, 72, and 96 hours after hypothermia.
Before hypothermia the basal level of FT4 was measured at 0.89±0.042 ng/dL, compared to which, significant differences in levels of FT4 were observed just after
hypothermia, the differences remaining significant for 96 hours, during which time FT4 level reached 0.68±0.08 ng/dL (p<0.016) (Fig. 2).
Fig. 2. Serum FT4 levels before (0) and just after hypothermia (2 hours) and 24, 48, 72, and 96 hours after hypothermia.
Prior to hypothermia, the basal level of T3 was 110±13.59 ng/dL, its mean level
showing no significant difference from basal levels in the 24 hours following hypothermia. It however increased to 121±14 ng/dL after 48 hours, which was significantly higher than basal levels (p<0.045). After 72 hours, it decreased and reached 115±8 ng/dL 96 hours after hypothermia, which did not differ significantly from basal levels (Fig.3).
Fig. 3: Serum T3 levels before (0) and just after hypothermia (2 hours) and 24, 48, 72, and 96 hours after hypothermia.
Basal T4 level was 4.87±0.59 µg/dL. Significant alteration of T4 level was seen just after hypothermia lasting for 48 hours after hypothermia (p<0.014). After 48 hours T4 levels began to increase and reached 4.57±53 µg/dL 96 hours after hypothermia(Fig. 4).
Fig. 4: Serum T4 levels before (0) and just after hypothermia (2 hours) and 24, 48, 72, and 96 hours after hypothermia.
Basal TSH level, prior to hypothermia, was measured at 0.037±0.002 µIU/mL and although alterations were seen within first 96 hours after hypothermia, they were nonsignificant (Fig. 5).
Fig. 5: Serum TSH levels before (0) and just after hypothermia (2 hours) and, 48, 72, and 96 hours after hypothermia.
Alterations of temperature:
Body temperature is normally 37±0.5°C and body temperature during hypothermia alterations in thyroid hormones, in a way that serum levels of FT3 and T4 decreased dramatically compared to basal values,
during and after hypothermia. T3 level did not change within 24 hours of hypothermia, while it increased significantly compared to the levels measured before hypothermia. TSH level was seen to alter during the study, though its alterations were not significant.
Our findings indicate that serum T4 level diminished after hypothermia, reaching its nadir 24 hours thereafter. It then increased to approximate its basal level in 96 hours. The decrease of serum T4 level, seen in the pituitary-thyroid axis attenuation6.7 In this study, serum TSH level, the stimulator of thyroid hormones synthesis and secretion, did not differ significantly. Thyroid hormones are hormones most stored in the body, a storage that could easily prevent thyroid failure for a long time, and thereby be expected to maintain normal serum T4 level. This was not the case in and thyroid hormones decreased significantly. This study on the other hand, it has been reported that in hypothermia, serum norepinephrine levels increase to 74 times their normal levels8. As norepinephrine increases the formation of endocytic cysts from droplets of follicular constituents as well as the secretion of thyroid hormones,8 drastic decrease of circulating T4 indicates the inability of T4 to be released from its source. It shows that hypothermia probably affects the secretion of T4 directly. Another reason could be the excessive conversion of T4 to T3 under the stressful conditions resulting from hypothermia. Unlike T4, on the third day, T3 showed a significant rise in comparison to basal levels (p<0.045).
Several studies have shown serum T3 to decrease postoperatively in hypothermia, returning to preoperative basal values within 3 to 5 days.6,7,9,10 The investigators considered the fall in T3 level to be a result of decreased T3 production in its peripheral source (e.g. was 25±0.5°C. The body temperature of the rats was seen to decrease after hypothermia followed by an increase toward normal body temperature during the following days, eventually reaching 36±0.22°c, 96 hours after hypothermia. Body temperature of rats differed significantly from normal values during this period (p<0.001) (Fig. 6).
Fig. 6: Mean body temperature of rats before (0) and just after hypothermia (2 hours) and 24, 48, 72 and 96 hours after hypothermia.
This study investigates the influence of hypothermia on thyroid function in animal models. Our findings indicate remarkable liver), stress and acute illness, a defect in the hypothalamus-pituitary axis, reduction of the activity of 5`-deiodinase (type 2) and low secretion of TSH. In this study, the alterations in TSH levels are in contrast to the findings of other studies. Various factors may be responsible for this difference. For instance, in rats, circulatory T3 results from the conversion of T4 to T3 in thyroid gland by 5´- deiodinase (type 1),12 while 5´-deiodinase type 1 mediates the conversion of T4 to T3 in peripheral tissues.13 Type 1 5´-deiodinase is specifically activated by TSH while 5´deiodinase type 2 is not under the influence of TSH and acts more by stress, other peripheral factors and the amount of available substrate (T4).
Considering the aforementioned details, we may presume that hypothermia increases type 1 enzyme activity by an unknown self-regulating mechanism in a way that despite decreased secretion of T4, serum T3 remains constant. The process may reduce serum T4 further, because T3 production is still normal in spite of low absorption of follicular fluid, which results in low T4 production. The organism probably compensates for the low metabolism following hypothermia by preventing the reduction of a more metabolically potent hormone, T3.
FT4 began to decrease significantly following hypothermia, a decrease that was significant even after 4 days of hypothermia. A similar decrease was also observed in FT3, persisting for the 3 following days. Some earlier studies have reported an increase of FT4. According to different researchers, different factors such as addition of heparin to FT4 in test tubes, decreased binding of protein to the hormone due to CPB and hypothermia are thought to have contributed to increase of FT4.7 Previous studies have also reported a reduction in FT3.7-9
Bremmer et al. and Mainwaring et al. considered the low conversion of T4 to T3 to be the cause of low FT3 levels owing to the decreased activity of 5`-deiodinase.7-9 Our findings differ from the two previous studies, which may be a result of not using heparin in the present study. Also, frozen plaoma was not used in this study. Both of these factors cause a false increase of FT4. It could, therefore, be said that our study shows the direct effect of hypothermia on FT4 and FT3 levels without being influenced by artificial phenomena. When secretion of T4 from thyroid is low and thyroxin binding globulin level (TBG) does not change during hypothermia (as in other plasma proteins), a balance exists between free and protein bound forms of hormone, keeping FT4 at a low level, because in this situation, low T4 could not perform its buffer role to keep the level of free active form of hormone.
Regarding the decrease in FT3, despite increased T3, this may be due to more uptake of FT3 by peripheral cells and definitely requires further investigations and more studies. TSH alterations during and after hypothermia in this study are not consistent with those of previous investigations. Previous studies have shown that TSH decreases significantly as compared to basal values before CPB and hypothermia. Investigators have suggested various factors as being responsible for the low secretion of TSH.
Mainwaring et al. considered these phenomena to be a result of FT4 increase or steroid injection preoperatively leading to decreased secretion of TSH.7 They also believe that anesthesia, the severity of hypothermia, the duration of cardiac arrest and the perfusion procedure in CPB may contribute to hypothalamus- pituitary-thyroid axis response. In this study, TSH level did not alter significantly while FT4 and T4 decreased significantly. Normally in the case of a healthy hypothalamus-pituitary axis, its activity leads to a rise in TSH levels. On the other hand, since in hypothermia, norepinephrine levels increase to 4-7 times their normal levels, this can increase TRH secretion through α-adrenergic receptors in the paraventricular nucleus (PVN), leading to a rise of TSH synthesis and secretion.16 Hypothermia seems to partially impair the hypothalamus-pituitary-thyroid axis function, an impairment that does not, however, suppress TSH secretion completely, but restricts secretion of the hormone. amounts.
Overall, hypothermia results in limited serum levels of thyroid hormones, which we believe is due to decreased activity of the thyroid gland and the hypothalamus-pituitary axis, more so in the former. Further investigations measuring TRH, rT3 and related enzymes are deemed necessary to confirm the results of this study.
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