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The Influence of
Hypothermia on Thyroid Function in Rats
Shirpour Aa, Khameneh Sa, Zarghami N b, Eskandari M.a
Departments of a Physiology and b Biochemistry,
Oroumieh University of Medical Sciences, Oroumieh, I.R. Iran.
Correspondence: Department of Physiology Oroumieh
University of Medical Sciences,
Nazlou Road, Oroumieh, I.R. Iran.
E-mail:
Shirpoor@hotmail.com
Introduction: THe 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 radio-immunoassay, 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.
Key Words: thyroid gland function, hypothermia, Wistar rat.
Introduction
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 hypo-thermia, 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.
Results
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:
FT3:
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.
Fig.
2. Serum FT4 levels before (0) and just after
hypothermia (2 hours) and 24, 48, 72, and 96 hours after hypothermia
FT4:
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).
T3:
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).
T4:
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. 3: Serum T3 levels before (0) and
just after hypothermia (2 hours) and 24, 48, 72, and 96 hours after
hypothermia
Fig. 4: Serum T4 levels before (0) and
just after hypothermia (2 hours) and 24, 48, 72, and 96 hours after
hypothermia
TSH:
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 non- significant (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 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.
Discussion
This study investigates the influence of hypothermia on thyroid
function in animal models. Our findings indicate remarkable 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 studies of Mitchell and Mainwaring, was considered to
be an effect of hypothalamus-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. 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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