Download IJEB 45(6) 549-553

Indian Journal of Experimental Biology
Vol. 45, June 2007, pp. 549-553
Thyroid dysfunction modulates glucoregulatory mechanism in rat
Sudipta Chakrabarti, Srikanta Guria, Ipsita Samanta & Madhusudan Das*
Department of Zoology, University of Calcutta, 35 Ballygunge Circular Road, Kolkata 700 019, India
Received 3 January 2006; revised 21 February 2007
The role of the thyroid gland in glucose homeostasis remains incompletely understood. To get a better insight hypoand hyperthyroid conditions were experimentally induced in rat and found severe defects in glucose homeostasis. While
blood glucose level returned to normal level after 2.5 hr of oral glucose challenge in control rats the blood glucose level
remained high even after 24 hr of glucose load in both hypo- and hyperthyroid rats. These experimentally manipulated rats
displayed higher levels of liver glycogen (10.45-22.8-fold) and serum glutamic pyruvic transaminase (1.48-9.8-fold). Liver
histology of hyperthyroid treated rats revealed hepatotoxicity. From the results it can be concluded that thyroid gland plays
an important role in glucose homeostasis.
Keywords: Blood glucose, Glycogen, Hyperthyroid, Hypothyroid Liver
Thyroid hormones [(primarily 3,5,3’5’-l-tetraiodothyronine (T4), and to a lesser extent 3,5,3’-l-triiodothyronine (T3)] regulate a variety of biochemical
reactions in virtually all tissues. These hormones are
known as important factors in gene regulation in
tissues such as brain, liver, muscles and adipose
tissue1. They are involved in the control of resting
metabolism2. Thyroid hormone status is also
important for glucose homeostasis. In thyrotoxic
subjects, glucose turnover and oxidation rates are
increased, whereas non-oxidative glucose turnover is
unchanged3, decreased4 or increased5. In contrast,
glucose
production
is
decreased
in
hypothyroidism4,6,7. Impaired glucose tolerance is a
frequent complication of hyperthyroidism. This
alteration changes both insulin secretion and
degradation in humans3. Impairment in the insulininduced suppression of glucose production in
hyperthyroid patients has been reported3,8. At low
insulin levels, insulin-stimulated glucose disposal is
usually unaffected3,9, whereas it has been reported to
be decreased3, unchanged8,11 or even increased12,14 at
high insulin levels. In addition, thyroid hormones also
blunt the insulin-induced increases in the total
distribution volume of the exchanging pool of
glucose, possibly by accelerating intracellular glucose
degradation9.
However, the role of thyroid gland in glucose
——————
*Correspondent author
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homeostasis has remained elusive. Therefore, to
uncover this complex interaction an attempt has been
made by generating hypothyroidism (by treatment
with methimazole) and hyperthyroidism (by treatment
with thyroxine) in rats to study the effects on glucose
levels during oral glucose tolerance test and changes
in serum glutamic pyruvic transaminase (SGPT) have
been examined. in addition, morphological and
biochemical changes in liver have been observed.
Higher glucose levels were noted in both hypo- and
hyperthyroid rats in response to oral glucose tolerance
test, higher liver glycogen content, and elevated levels
of SGPT. In addition, profound alteration in liver
histology were observed. These findings may help to
better understand the complex relationship between
thyroid dysfunction and glucose homeostasis.
Materials and Methods
Young adult rats, Rattus rattus (8-10 weeks: 70-80
g) were housed in polypropylene cages and were
acclimatized in the laboratory condition for a week
with standard food and water in natural light and dark
schedules. Rats were divided into following 4 groups:
group I (for hypothyroid studies), group II (for
hyperthyroid studies) and their respective control
groups. While Group I animals were treated with
Methimazole15 (20 mg/kg body weight in 1 ml water /
day) for 14 days, Group II animals received
thyroxine16 (600 μg/kg body weight in 1 ml of
water/day) for 14 days. Control rats received 1 ml of
water only. Rats were anesthetized with chloroform
and blood was collected directly from the heart for
serum T3 and T4 hormone level. T3 or T4 concentra-
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INDIAN J EXP BIOL, JUNE 2007
tions in rat serum were determined by radioimmunoassay (RIA). 125I-labeled thyroid hormone (either T3
or T4) competes with thyroid hormone in the serum
sample and for antibody sites on the tube, in the
presence of blocking agents for thyroid hormone
binding proteins. Thyroid gland and liver were
dissected out, fixed in Bouin’s fixative, and processed
for routine histology. Liver glycogen was determined
by the colorimetric method17 by digesting 1 g fresh
liver in 30% KOH and treatment with the anthrone
reagent. Serum SGPT was measured colorimetrically
using the method of Reitman18. For oral glucose
tolerance test (OGTT) blood was collected first from
the tail veins of control and treated (hypo- and
hyperthyroid) rats after 18 hr of fasting followed by
challenge with glucose (25 mg glucose/100 g body
weight) and at the following time point after oral
glucose infusion: 0.5, 2.5 and 24 hr. Blood glucose
was measured estimated using a blood glucose
monitoring system (ACON Laboratories, Inc. San
Diego, USA).
Results
T3 and T4 levels, body weight and liver glycogen—
Rats with hypothyroidism showed lower levels of
thyroid hormones T3 and T4 and with higher levels in
hyperthyroid rats as compared to control rats (Fig. 1).
Rats with hypothyroidism also showed increase in
body weight (~18%) while those with hyperthyroidism exhibited a decrease in body weight (~18%).
Liver glycogen increased dramatically in both
hypothyroid (by ~22.8-fold) and hyperthyroid (by
~10.45-fold) (Fig. 2).
Oral glucose tolerance test— While hypothyroid rats
displayed low glucose level (by 18%) no change in
glucose level was seen for hyperthyroid rats (Fig. 3). In
the control rats the blood glucose level returned to the
normal level after 2.5 hr of glucose feeding. Like control
rats, in hypothyroid rats glucose level increased by 90%
after 0.5 hr of glucose challenge but the elevated
glucose didn’t return to control level even after 24 hr of
glucose challenge (Fig. 3). Unlike control and
hypothyroid
rats,
the increments in glucose level after 0.5 hr of glucose
challenge was modest (by ~22%) in hyperthyroid
rats. However, like hypothyroid rats, the glucose
remained elevated even after 24 hr of glucose feeding
(Fig. 3).
Serum SGPT — Hyperthyroid rats showed dramatic
increments in SGPT level (by ~9.8-fold) as compared
to a modest (by ~0.5 fold) increase in hypothyroid
rats (Fig. 4).
Liver histology — Hyperthyroid rats exhibited
marked changes in the general cytomorphology of
Fig. 1—Plasma concentrations (ng/dL) of T3 (A) and T4 (B) in
hypothyroid and hyperthyroid rats. Values are expressed as mean
± SE from 6 rats. P values: *<0.001; **<0.01
Fig. 2— Liver glycogen content (mg/g wet tissue) in hypothyroid
and hyperthyroid rats. Values are expressed as mean ± SE from 6
rats. P values: *<0.001; **<0.01
CHAKRABARTI et al.: THYROID AND GLUCOSE HOMEOSTASIS
551
hepatocytes as evidenced by the enlargement of
central terminal hepatic venule and disorientation of
the nuclei and the loss of the individual boundary.
Small numbers of hepatocytes were found to be
picnotic. Few uniform sized cell bodies stained in
eosin were found in the periphery of central terminal
hepatic venule (Fig. 5). No discernible change in liver
histology was detected in hypothyroid rats (data not
shown).
Discussion
Fig. 3—Blood glucose level in response to oral glucose tolerance
test in control, hypothyroid and hyperthyroid rats. Values are
expressed as mean ± SE from 6 rats. P values: *<0.001; **<0.01
Fig. 4—Serum glutamic pyruvic transaminase (SGPT) level (U/L)
in normal, hypothyroid and hyperthyroid rats. Values are
expressed as mean ± SE from 6 rats. P values: *<0.001; **<0.01
The T3 and T4 levels are low in hypothyroid and
high in hyperthyroid rats (Fig. 1). A complex
relationship exists between thyroid disease, body
weight and metabolism. It is well known that
hyperthyroidism causes extensive weight loss despite
normal or increased calorie intake19,20. The weight loss
is related to the severity of the overactive thyroid. In
congruence with these findings 18% loss of body
weight was observed in hyperthyroid rats. Weight loss
reflects not only a depletion of body adipose tissue
stores but also a loss of muscle mass caused by
accelerated catabolism and heat elimination. Because of
low BMR hypothyroidism is generally associated with
some weight gain20. There was an 18% increase in
body weight in the present experimental hypothyroid
which is in line with reported observation.
Hypothyroidism is associated with decreased
gluconeogenesis and glycogenolysis resulting
in increment in glycogen in liver. In addition,
hypothyroidism lowers the glycogen phosphorylase
Fig. 5—Cytomorphology of normal and hyperthyroid liver. Arrow indicates changes in liver sinusoids.
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INDIAN J EXP BIOL, JUNE 2007
activity in the liver21. Consistent with these findings
profound increments in liver glycogen in hypothyroid
rats were observed. In contrast, the hyperthyroid state
is associated with low hepatic glycogen levels, but
paradoxically with a high activity of glycogen
synthase and low activity of glycogen phosphorylase.
In hyperthyroidism, insulin-stimulated rates of
glucose utilization in muscle to form lactate are
increased mainly because of a decrease in glycogen
synthesis. In the present study, elevated glycogen
content in the liver of hyperthyroid rats is paradoxical
and may be due to the high activity of glycogen
synthase22.
In hyperthyroid humans as well as in experimental
thyrotoxicosis in animals, glucose turnover and
hepatic glucose production are increased due to
increased metabolic rate and peripheral glucose
utilization23. Experimental as well as spontaneous
hyperthyroidism in humans cause increased glucose
production and impaired suppression of glucose
production by insulin3,24. Consistent with these
previous reports increments in blood glucose levels
were observed after OGTT and the hyperglycemia
persisted even 24 h after glucose load. It is believed
that insulin response to a glucose load is relatively
decreased in hyperthyroidism, and that an inability to
increase their insulin response further impairs glucose
tolerance in low β-cell responder25,26. It is possible
that T4 causes deleterious effects on β-cell function
thereby impairing insulin secretion after an oral
glucose load. In line with this thought, Lenzen28
reported that injections of T4 (50-2000 μg/kg/day for
5 days) dose-dependently inhibited glucose-induced
insulin secretion in the isolated pancreas. It has also
been reported that hyperthyroid patients with younger
age (<30 years) showed an increased secretion of
insulin to compensate hyperglycemia after glucose
load, but older patients (>31 years), who may have a
low β-cell reserve, showed a blunted response of
insulin29. In addition, insulin sensitivity is altered in
hyperthyroid patients30,31. In the present study, the
reason behind impaired glucose tolerance in response
to glucose overload in hypothyroid rats is not known
and further experiments needs to be performed.
Liver maintains a stable blood glucose level by
taking up and storing glucose as glycogen, breaking
this down to glucose when needed and forming
glucose from non-carbohydrate sources such as amino
acids. Because of the impairment of glucose tolerance
in both hypo- and hyperthyroid rats we found
dramatic changes in SGPT enzyme levels in these
rats. It is believed that abnormal liver enzyme levels
may cause liver damage. Profound alteration in liver
histology in hyperthyroid rats is a reflection of
changes in SGPT level in the liver an observation that
is unique to this study.
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