The Journal of Clinical Endocrinology & Metabolism Vol. 82, No. 1 78-81
Copyright © 1997 by The Endocrine Society
Serum Interleukin-6 and Bone Metabolism in Patients with Thyroid Function Disorders1
Peter Lakatos,
Janos Foldes,
Csaba Horvath,
Laszlo Kiss,
Agnes Tatrai,
Istvan Takacs,
Gabor Tarjan2 and
Paula H. Stern
1st Department of Medicine (P.L., J.F., C.H., A.T.,I.T.),
Semmelweis University Medical School, Budapest, Hungary, H-1083;
Hetenyi Geza County Hospital (L.K.), Szolnok, Hungary, H-5000; and
Department of Molecular Pharmacology and Biological Chemistry,
Northwestern University Medical School (P.L., G.T., P.H.S.) Chicago,
Illinois 60611
Address all correspondence and requests for reprints to: Peter Lakatos, 1st Department of Medicine, Semmelweis University Medical School, Koranyi 2/A, Budapest, H-1083, Hungary.
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Abstract
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To determine the possible involvement of interleukin-6 (IL-6) in the
bone loss of hyperthyroidism, relationships between thyroid status,
biochemical and densitometric parameters of bone metabolism, and IL-6
were studied in female subjects. Patients with hyperthyroidism caused
by either toxic nodular goiter or Graves disease had significantly
higher serum IL-6 concentrations than normal controls. Within the
control group, serum IL-6 was higher in postmenopausal than in
premenopausal women, but this influence of menopausal status was not
seen in the hyperthyroid patients. The production of IL-6 by blood
mononuclear cells was higher in cells from the hyperthyroid women. Bone
turnover was increased in the hyperthyroid patients based on serum
osteocalcin and urinary deoxypyridinoline excretion, and the
hyperthyroid group also had reduced radius bone mineral content (BMC).
A subgroup of hyperthyroid patients who had the lowest BMC (values more
than 1 SD below normal age-matched controls) also had serum
IL-6 concentrations significantly greater than those of hyperthyroid
patients showing less reduction of BMC. The correlations observed in
this study support the possibility that IL-6 plays a role in mediating
the bone loss that results from excess thyroid hormone.
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Introduction
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THYROID hormones are necessary for normal
skeletal growth. However, their excess may result in bone loss,
especially in adulthood. Hyperthyroidism is accompanied by osteoporosis
(1) and increased fracture rate (2). Although there is ample evidence
for increased bone turnover in hyperthyroidism (3, 4, 5, 6), the exact
mechanism of the deleterious effect of thyroid hormones has not yet
been elucidated.
Locally produced factors are important in maintaining normal bone
metabolism (7). Interleukin-6 (IL-6), particularly, has a major
influence on bone turnover. IL-6 stimulates differentiation and
proliferation of osteoclasts (8), thus leading to increased bone
resorption (9, 10, 11). Increased serum IL-6 level has been reported in
several pathological states. Among these, multiple myeloma (12) are
characterized by excessive osteoclast development and bone loss.
IL-6 is produced in a variety of tissues, such as bone (13), the
thyroid (14), and blood mononuclear cells (15). These mononuclear cells
also express thyroid hormone receptors (22). The effect of thyroid
hormones on serum IL-6 and its relation to thyroid hormone-stimulated
bone loss have not been elucidated. To address this question, we
studied serum IL-6 levels, IL-6 production by blood mononuclear cells,
and bone metabolism in patients with thyroid disorders.
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Subjects and Methods
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Subjects
The study population consisted of 42 hyperthyroid (age: 2466
yr; 23 pre- and 19 postmenopausal) and 19 hypothyroid (age: 2864 yr;
9 pre- and 10 postmenopausal) Caucasian women before antithyroid or
replacement therapy, respectively, as well as 30 healthy euthyroid
controls (age: 2265 yr; 15 pre- and 15 postmenopausal). The
hyperthyroid group included 22 patients with toxic nodular goiter (TNG)
(age: 3466 yr) and 20 patients with Graves disease (GD) (age:
2447 yr). Hypothyroidism was caused by autoimmune thyroiditis in 10
patients, radioisotope treatment in 5 patients and surgery in 4
patients. Subjects in all groups were free of any diseases or were not
taking any medication known to affect calcium metabolism. Informed
consent was obtained from all subjects.
Mononuclear cell cultures
Blood mononuclear cell cultures were prepared as described by
Pacifici et al. (16). Briefly, mononuclear cells separated
on Ficoll/Hypaque were resuspended in medium at a concentration of
106 cells/mL and 1-mL aliquots were placed in a 24-well
tissue culture plate for 2 h at 37 C, in a humidified atmosphere
containing 5% CO2/95% air. The adherent cells were then
incubated for 48 h in 1 mL DMEM (Sigma Chemical Co., St. Louis,
MO) supplemented with 5% heat-inactivated FBS ± 1 µg/mL
phytohemagglutinin (PHA) (Sigma Chemical Co.). Supernatants from
quadruplicate cultures were stored at -20 C until assayed.
Biochemical measurements
Blood was drawn after an overnight fast, centrifuged, and serum
was stored at -20 C until determination. IL-6 was assayed from sera
and mononuclear cell culture supernatants by ELISA (Quantikine, R&D
Systems, Minneapolis, MN). The sensitivity of the method was 0.7 pg/mL.
When assaying mononuclear cell culture supernatants, results were
expressed as picograms per 106 adherent cells. Serum
osteocalcin (OC) was measured by NovoCalcin ELISA (Metra Biosytems,
Mountain View, CA), sensitivity was 0.45 ng/mL. Urinary free
deoxypyridinoline cross-link excretion (DPD) was measured by ELISA
(Pyrilinks-D; Metra Biosystems) from the first morning void with a
sensitivity of 3 nM. Final results were expressed as DPD
nM/creatinine mM.
Bone densitometry
Bone mineral content (BMC) was measured at the lumbar spine and
femoral neck by dual-enengy x-ray absorptiometry (Norland XR-26;
Norland, Fort Atkinson, WI) and at the radius midshaft by single-photon
absorptiometry (NK-364; Gamma Ltd., Budapest, Hungary).
Statistical analysis
Data were expressed as mean ± SEM and analyzed
by two-tailed Studentss t test and one-way ANOVA, as
appropriate. Subsequent mean comparison tests were performed by
Tukeys significant difference test.
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Results
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Serum IL-6 values (Fig. 1
) were significantly
increased in the hyperthyroid group compared with the controls
(18.2 ± 0.8 vs. 6.2 ± 1.3 pg/mL;
P < 0.01). No difference was found between TNG and GD
patients. IL-6 concentrations of hypothyroid patients (5.3 ± 2.0
pg/mL) did not differ statistically from control subjects. There was no
correlation between individual IL-6 concentrations and thyroid hormone
levels in either group. When the values from the pre- and
postmenopausal women were compared (Fig. 2
),
significantly higher serum IL-6 was found in healthy postmenopausal
controls (2.3 ± 0.7 vs. 10.1 ± 0.9 pg/mL;
P < 0.01) but not in postmenopausal patients with
hyperthyroidism [17.4 ± 0.9 vs. 18.6 ± 1.0
pg/mL; not significant (NS)] or hypothyroidism (5.2 ± 0.9
vs. 5.4 ± 1.1 pg/mL; NS).

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Figure 1. Serum IL-6 in thyroid disorders. Data
represents mean ± SEM. **, P <
0.01 vs. control. Hyper, Hyperthyroidism; hypo,
hypothyroidism.
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Figure 2. Serum IL-6 in thyroid disorders: effect of
menopause. Data represents mean ± SEM. **,
P < 0.01 vs. premenopausal
controls; ++, P < 0.01 vs.
postmenopausal controls. Solid bars, Premenopausal;
hatched bars, postmenopausal.
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BMC (Table 1
) was reduced significantly only at the
radius midshaft of the hyperthyroid patients. There was no difference
in BMC between TNG and GD subjects (0.70 ± 0.03 vs.
0.68 ± 0.04 g/cm; NS). BMC of the hypothyroid group was not
different from controls. Serum IL-6 values did not correlate with BMC
in either group. However, serum IL-6 of those patients with
hyperthyroidism whose BMC was lower than -1 SD below the
normal age-matched population values (17) was significantly higher than
of those whose BMC was above that threshold (23.8 ± 1.1
vs. 16.8 ± 0.7; P < 0.05; Fig. 3
).

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Figure 3. Serum IL-6 and radius BMC. Data represents
mean ± SEM. -1.0 SD means that BMC is
below the age-matched normal population values by 1 SD.
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Basal IL-6 production of blood mononuclear cells in hyperthyroid women,
either with TNG and GD, was higher than in controls (367 ± 132
and 413 ± 112 pg/106 cells, respectively,
vs. 158 ± 87 pg/106 cells;
P < 0.05; Fig. 4
). There was no
significant difference between the TNG and GD groups. In response to
PHA, controls and GD patients showed a significant increase in IL-6
production (802 ± 176 and 846 ± 181 pg/106
cells; P < 0.05 vs. no PHA of respective
group; Fig. 4
), whereas TNG patients did not exhibit a further
enhancement (391 ± 155 pg/106 cells). The basal IL-6
production by mononuclear cells in hypothyroid subjects (139 ± 63
pg/106 cells) was not statistically different from controls
and it could not be stimulated with PHA (161 ± 79
pg/106 cells).

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Figure 4. IL-6 production of blood mononuclear cells
in hyperthyroid women. Data represent mean ± SEM.
Solid bars, -PHA; hatched bars, +PHA. *,
P < 0.05 vs. -PHA control; +,
P < 0.05 vs.-PHA of respective
group.
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Serum OC values of the hyperthyroid group was significantly higher than
that of controls (19.6 ± 1.6 vs. 5.5 ± 0.4
ng/mL; P < 0.01; Table 2
). Urinary DPD
excretion of these patients was also increased (15.8 ± 1.1
vs. 6.1 ± 0.7 nM/mM
creatinine; P < 0.01; Table 2
). However, subjects with
BMC below -1 SD of age-matched normal population values
had higher OC (24.9 ± 0.7 vs. 16.8 ± 0.9 ng/mL;
P < 0.05) and DPD (18.3 ± 1.0 vs.
13.1 ± 0.6 nM/mM creatinine;
P < 0.05) concentrations than of those who were above
-1 SD. There was no difference in these parameters between
TNG and GD patients. No correlation was seen between serum IL-6 and OC
or DPD. OC and DPD values were not different from controls in the
hypothyroid group (Table 2
).
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Discussion
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We have shown that IL-6 levels are increased in the circulation in
hyperthyroid women independent of the etiology of thyroid
hyperfunction. Hypothyroidism did not significantly reduce serum IL-6.
In hyperthyroidism, increased serum IL-6 could originate from several
sources including the thyroid gland, blood mononuclear cells, and bone
tissue.
Serum IL-6 has been reported to be increased in subacute thyroiditis
(18). Serum IL-6 normalizes on remission of the disease, indicating
that it is related to follicular cell damage. When these cells are
damaged as a consequence of the presence of antithyroid antibodies,
IL-6 is released into the circulation. It has been suggested that the
magnitude of the increase may reflect the severity of the disease (19).
In GD, although follicle cells are also able to express certain
cytokines, intrathyroidal lymphocytes are the main source of IL-6
production (20). Weetmann et al. (14) found increased
intrathyroidal but not serum IL-6 levels in a small group of patients
with GD. Celik et al. (21) reported enhanced IL-6 serum
levels in both GD and toxic multinodular goiter, which returned to
normal after euthyroidism was restored with propylthiouracil treatment.
Thus, increased intrathyroidal production of IL-6 in thyroid
hyperfunction is one source of elevated IL-6 levels in serum.
Blood mononuclear cells produce IL-6 under normal basal conditions
(15). These cells express thyroid hormone receptors (22). In the
present study, we showed that basal IL-6 secretion from mononuclear
cells is increased in both types of thyroid hyperfunction, but it can
be further stimulated by PHA only in GD patients. In TNG, PHA was not
effective in stimulating IL-6 production. This difference between GD
and TNG might be because of the underlying immune disorders present in
GD, however, this remains to be established. Our data raise the
possibility that blood mononuclear cells make a major contribution to
the increased serum IL-6 levels in hyperthyroidism. May et
al. (23) has demonstrated complexed forms of IL-6 in human blood
as a consequence of binding to plasma proteins. This binding could
camouflage IL-6 immunoreactivity in serum in our assay and modify the
results. However, we measured significantly increased levels of IL-6 in
both serum and medium of mononuclear cell cultures of our patients.
IL-6 is produced by bone cells, especially by osteoblasts (13). IL-6
stimulates bone resorption by enhancing osteoclast proliferation and
differentiation (8). In vitro, thyroid hormones do not
stimulate IL-6 production directly in fetal rat limb bones (24).
However, in the presence of physiological concentrations of thyroid
hormones, the IL-1-stimulated IL-6 response, and bone resorption is
greatly increased in these cultures (24). Thus, the increased levels of
serum IL-6 in our present study may be at least partly a result of this
permissive action of thyroid hormones in bone.
IL-6 has been suggested to serve as a paracrine mediator of estrogen
action on bone cells (7). Estrogen withdrawal induces elevated IL-6
production by murine bone cells (25). Estrogen also inhibits IL-6
production in human osteoblastic cells (13). This effect seems to be
mediated by an inhibitory effect of estrogen on the IL-6 gene promoter
(26). In our study, the effect of menopause, i.e. estrogen
deficiency, resulted in increased serum IL-6 levels in healthy
postmenopausal women. It is interesting that no difference was seen in
pre- and postmenopausal IL-6 levels in the hyperthyroid group. It may
be that in hyperthyroidism IL-6 production is maximally stimulated even
before menopause and cannot be further stimulated by estrogen
deficiency. The lack of increase in IL-6 levels after menopause in the
hypothyroid group may reflect the necessity of thyroid hormones for
IL-6 production.
Appendicular BMC of hyperthyroid women was reduced in our study. High
bone turnover was reflected in this group by increased biochemical
markers of bone metabolism, i.e. serum osteocalcin and
urinary cross-link excretion. Although there was no correlation between
serum IL-6 and BMC, patients with the lowest BMC had the most elevated
IL-6 levels. These data are consistent with studies implicating IL-6 in
the elevated bone turnover resulting from estrogen deficiency (27, 28, 29).
On the other hand, another study found no difference in IL-6 production
by osteoblasts cultured from osteoporotic and nonosteoporotic normal
women (30). In the osteoporotic group, the amount of IL-6 produced by
osteoblasts did not correlate with the bone loss of patients as
assessed by bone histomorphometry. We likewise could not show a
correlation between serum IL-6 and bone loss, but did find
significantly increased levels in the osteoporotic hyperthyroid
patients. Serum OC and urinary DPD were also significantly increased in
this subset of patients compared with hyperthyroid patients with normal
BMC. The increased bone turnover in osteoporotic hyperthyroid patients
accompanied by significantly elevated serum IL-6 raises the possibility
that this cytokine plays a major role in thyroid hormone-stimulated
bone loss.
Thyroid hormones have both anabolic and catabolic effects on bone.
Anabolic effects, which are important for normal growth and the
maintenance of bone tissue integrity, may be mediated through
insulin-like growth factors (31). Based on our data, deleterious
effects of thyroid hormones on bone tissue might be mediated by
IL-6.
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Acknowledgments
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We thank Ms. Veronika Reiner Szabone for excellent technical
assistance.
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Footnotes
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1 This work was supported by NIH Grants Fogarty R03-TW-00167-01,
OTKA-1051, and ETK-K-126/95. 
2 Recipient of stipend support from NIH Grant DK-45269. 
Received July 8, 1996.
Revised August 22, 1996.
Accepted August 27, 1996.
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