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Journal of Clinical Endocrinology and Metabolism
Copyright © 1997 by The Endocrine Society
Vol. 82, No. 2
Printed in U.S.A.
Short-Term Hyperthyroidism Has No Effect on Leptin
Levels in Man*
CHRISTOS S. MANTZOROS†, HAROLD N. ROSEN‡, SUSAN L. GREENSPAN,
JEFFREY S. FLIER, AND ALAN C. MOSES
Department of Medicine, Divisions of Endocrinology and Metabolism, Gerontology, and Bone and
Mineral Metabolism, Charles A. Dana Research Institute and the Harvard-Thorndike Laboratory of
the Beth Israel Hospital, Boston, Massachusetts 02215
ABSTRACT
Leptin, a 16-kDa adipocyte-derived protein whose circulating levels reflect energy stores, increases the resting metabolic rate and
thermogenesis in rodents. Thyroid hormones also increase the basal
metabolic rate, but nothing is known about possible interactions between leptin and thyroid hormone. Activation of ␤-adrenergic receptors decreases leptin levels in rodents. To test the hypothesis that
thyroid hormones, by causing a “functional hyperadrenergic” state,
result in decreased leptin concentrations in humans, we studied 22
normal healthy men before and after the administration of T3 for 1
week to induce moderate hyperthyroidism. Short term thyroid hormone excess does not alter circulating leptin concentrations despite
a demonstrated effect on heart rate, systolic blood pressure, cholesterol levels, and metabolic indexes of bone turnover. Elucidation of the
apparently separate pathways by which thyroid hormones, ␤-agonists, and leptin regulate energy expenditure and food intake may
have important implications for our understanding of the mechanisms for regulating energy homeostasis in health and disease.
(J Clin Endocrinol Metab 82: 497– 499, 1997)
T
HE REGULATION of body weight requires a balance
between energy intake and energy expenditure. A major advance in our understanding of the physiological system
that is responsible for the long term regulation of energy
balance and body weight (1, 2) was the recent discovery of
leptin (2). Leptin is produced by adipocytes (3–5), and its
circulating levels reflect energy stores in adipose cells (4, 5).
Genetic defects that either impair the production of this
molecule (2, 6) or produce resistance to its actions (7, 8) cause
severe obesity in rodents (2, 7, 8). Administration of leptin
decreases food intake and increases the resting metabolic rate
and thermogenesis (9 –12), providing evidence that leptin
plays a key role in a feedback loop maintaining energy
balance.
Thyroid hormones directly increase the basal metabolic
rate in man and have a permissive effect on adaptive thermogenesis in small animals (13, 14). The potential mechanisms responsible for thyroid hormone-controlled energy
expenditure, including uncoupled oxidative phosphoryla-
tion, are complex and not yet fully elucidated (13, 14). Moreover, the potential interaction of thyroid hormones with the
leptin system remains to be explored.
Recent studies indicate that stimulation of ␤-adrenergic
receptors decreases leptin expression in rodent adipocytes
(12, 15). The current study was conducted to test the hypothesis that thyroid hormones would decrease leptin concentrations by causing a “functional hyperadrenergic” state,
by decreasing energy stores as fat, or through other
mechanisms.
Subjects and Methods
Subjects
Twenty-two healthy male volunteers between the ages of 18 –35 yr
were recruited. Subjects were excluded if they had either a medical
condition that predisposed them to adverse effects from treatment with
excess thyroid hormone or any condition that could alter the metabolic
end points of the study. The study protocol was approved by the Beth
Israel Hospital committee on clinical investigations, and written informed consent was obtained. The part of this study relating to the effect
of moderate hyperthyroidism on bone metabolism has been described
previously (16).
Received July 11, 1996. Revision received September 10, 1996. Accepted September 26, 1996.
Address all correspondence and requests for reprints to: Dr. Alan C
Moses, Clinical Research Center, GZ 800, Beth Israel Hospital, Harvard
Medical School, 99 Brookline Avenue, Boston, Massachusetts 02215.
E-mail moses@sprcore. bih.harvard.edu.
* This work was supported by NIH Grant DK-28082 (to J.S.F.), USPHS
Bureau of Health Professions Faculty Training Project in Geriatric Medicine (to H.N.R.), Grant D31-PE-91000, Boots Pharmaceuticals, the Endocrine Fellows Foundation, Smith Kline Beecham Pharmaceuticals,
Ciba-Geigy Pharmaceuticals, and Beth Israel Hospital General Clinical
Research Center Grant M01-RR-01032.
† Member of the Clinical Investigator Training Program, Beth Israel
Hospital, Harvard-Massachusetts Institute of Technology Health Sciences and Technology, in collaboration with Pfizer, Inc.
‡ Recipient of the Clinical Associate Physician Award through the
General Clinical Research Center.
Protocol
Subjects were admitted to Boston’s Beth Israel Hospital General Clinical Research Center (GCRC), and after an overnight fast they were
instructed to void and then drink 0.5 L water at 0600 h. At 0800 h on day
1, a fasting blood sample and a 2-h fasting urine sample were obtained.
The hydroxyproline to creatinine ratio was measured in the urine. Leptin, osteocalcin, total cholesterol, T3, T4, T4-binding globulin (TBG), TSH,
complete blood count (CBC), and differential and serum chemistry profile were determined in the blood.
On day 7, subjects returned to the GCRC at 1700 h for a repeat of the
testing performed on days 0 –1. Subjects began taking T3 (Cytomel,
Smith-Kline and French, Philadelphia, PA: 25-␮g tablets, two tablets
twice daily, i.e. 100 ␮g daily) on day 8 and continued until day 15
inclusive. Compliance was verified by daily telephone calls and pill
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498
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Vol 82 • No 2
MANTZOROS ET AL.
counts on day 15. On days 15 and 29, subjects returned to the GCRC at
1700 h for a repeat of the protocol performed on days 7– 8.
Measurements
All serum specimens, with the exception of CBC and chemistry profiles, were frozen at ⫺70 C after collection and assayed at the end of the
study at once to avoid interassay variability.
Serum
TSH, T3, T4, and TBG were measured by a commercially available
fluoroimmunometric assay using a Delfia kit (Wallac, Gaithersburg,
MD). The free T4 index was calculated as the product of total T4 ⫻
20/TBG. Serum osteocalcin levels were determined by RIA as described
previously (17). CBC and automated serum chemistry profiles were
performed by Bioran (Cambridge, MA).
Serum leptin was measured by RIA (Linco Research, Indianapolis,
IN). The limit of detection was 0.5 ng/mL, the within-assay coefficient
of variation was 4.39% for low levels (2.9 ng/mL) and 5.66% for high
levels (14.1 ng/mL), and the between-assay variations were 6.9% and
9%, respectively. All assays were performed in duplicate.
Urine
Measurement of urinary creatinine was based on the modified Jaffe
method (19). Urinary hydroxyproline was determined using the Hypernosticon kit (Organon-Teknika, Boxtel, Holland) (16).
returned to baseline levels (Table 1). Additionally, the free T4
index was not significantly different from baseline.
Effect of T3 treatment on heart rate and blood pressure
levels
After treatment with T3, heart rate increased by 15% compared to the baseline level (P ⬍ 0.01 vs. baseline; Table 1),
systolic blood pressure increased by ⬃5% (P ⬍ 0.05 vs. baseline; Table 1), and diastolic blood pressure did not change
(Table 1).
Effect of T3 treatment on serum cholesterol and indexes of
bone metabolism
After treatment with T3, serum cholesterol concentrations
fell to 70% of the baseline level (P ⬍ 0.05 vs. baseline; Table
1), but were not different from the baseline on study day 30
(Table 1). After treatment with T3, serum osteocalcin and the
urinary hydroxyproline to creatinine ratio rose significantly
on day 16 (P ⬍ 0.05 vs. baseline), as described previously (16).
However, bone turnover indexes were not different from the
baseline on study day 30 (16).
Effect of T3 treatment on serum leptin concentrations
Statistical analysis
Values are reported as the mean ⫾ sem. Repeated measures ANOVA
followed by multiple comparison testing were performed using the SAS
statistical program (SAS Institute, Cary, NC).
Serum leptin did not change significantly during thyroid
hormone administration (Table 1).
Side-effects of treatment
Results
Baseline characteristics
The mean (⫾ sem) age of participants in this study was
23.36 ⫾ 1.08 yr (range, 18 –29 yr), and their mean (⫾sem)
height was 176.19 ⫾ 2.6 cm (range, 160 –197). Their weight
was 75.91 ⫾ 3.79 kg (range, 60.4 –104.7) and remained stable
for the duration of the study. Baseline serum leptin and
thyroid hormone indexes were normal (Table 1).
Effect of treatment with T3 on thyroid hormone indexes
After 7 days of treatment with T3 (study day 16), mean
serum T3 values tripled (P ⬍ 0.01 vs. study day 1; Table 1),
and mean serum TSH decreased to 15% of the baseline value
(P ⬍ 0.01 vs. study day 1; Table 1). By study day 30 (2 weeks
after the discontinuation of T3), thyroid hormone indexes
TABLE 1. Mean and
SEM
Two of the 22 subjects reported mild insomnia while taking thyroid hormone. No symptoms consistent with overt
hyperthyroidism were reported.
Discussion
This study investigated the possible interrelationships between two molecules that are involved in the physiological
regulation of energy homeostasis, leptin, and thyroid hormone. Leptin is a circulating adipocyte-derived molecule
whose levels reflect the magnitude of fat stores (2–5). The
demonstrated actions of leptin in rodents include inhibition
of food intake and stimulation of energy expenditure (3, 5,
9 –11). Leptin gene expression is regulated by direct actions
on the adipocyte by glucocorticoids, insulin, and, recently,
activation of ␤-adrenergic receptors (12, 15). Thyroid hor-
of thyroid and metabolic parameters on days 1–30
Parameter
TSH (mU/L)
T3 (nmol/L)
FT4 index
Heart rate (beats/min)
Systolic BP (mm Hg)
Diastolic BP (mm Hg)
Cholesterol (mmol/L)
Leptin (ng/mL)
Study day
1
8
16
30
1.65 (0.22)
106
(17.7)
10.04 (0.60)
70.8 (2.3)
116.43 (2.7)
69.57 (1.8)
4.75 (0.28)
3.42 (0.57)
1.65 (0.19)
108.5 (16)
9.73 (0.63)
72.43 (2.4)
118.1 (2.5)
68.91 (2.3)
4.34 (0.19)
3.22 (0.51)
0.25 (0.14)a
327.5 (65.7)a
6.39 (0.52)a
82.38 (2.8)a
123.96 (2.0)b
69.91 (2.4)
3.41 (0.19)a
3.57 (0.59)
1.8 (0.2)
107
(20.6)
9.01 (0.58)
ND
ND
ND
4.63 (0.23)
3.43 (0.66)
ND, Not documented.
a
Significantly different from baseline, P ⬍ 0.01.
b
Significantly different from baseline, P ⬍ 0.05.
SHORT TERM HYPERTHYROIDISM AND LEPTIN LEVELS
mone increases the resting metabolic rate in man and has a
permissive effect on adaptive thermogenesis in small animals (13, 14). The potential mechanisms responsible for thyroid hormone-controlled energy expenditure are complex
and have not been fully elucidated (13, 14).
Thyroid hormones produce a hyperresponsiveness of peripheral tissues to adrenergic hormones. Given our previous
findings in mice, supporting an acute effect of a ␤3-adrenergic agonist to increase energy expenditure and decrease
serum leptin concentrations (12), we hypothesized that thyroid hormones might produce a similar effect. Alternatively,
it is possible that the effect of thyroid hormones to increase
energy expenditure could be mediated in part by a thyroidinduced increase in leptin concentrations. We, therefore, examined the potential effects of a short term increase in the
level of T3 on the serum leptin concentration in humans.
Exogenously administered T3 produced a hyperthyroid
state, as assessed by T3 levels, suppressed TSH, and several
metabolic indexes, including decreased cholesterol concentrations and increased indexes of bone formation and resorption. Hyperthyroidism was also assessed by a functional
hyperadrenergic state, as indicated by the increases in heart
rate and systolic blood pressure. Despite these changes, thyroid hormone excess did not change serum leptin concentrations in this group of young men.
We previously have shown that stimulation of ␤3-adrenergic receptors in mice, which causes increased energy expenditure, acutely suppresses the expression and circulating
levels of leptin (12). Other work shows that this is a cAMPdependent process in white adipose tissue (15). A decrease
of leptin concentrations might, therefore, have been expected
in response to the functional hyperadrenergic state produced
by T3 in this study, assuming that a hyperadrenergic state
exists at the level of the adipocyte. A previous study has
shown that leptin messenger ribonucleic acid expression is
significantly decreased after T4 administration to Zucker rats
(20). However, the rats had lost a significant amount of
weight (20), and the reduced leptin expression in this case
most likely reflects significantly decreased adipose stores.
What are the implications of the fact that no changes in
leptin concentrations were observed in the present study?
First, it is likely that the ability of thyroid hormones to regulate energy expenditure does not operate through increases
in leptin levels in humans. We might have expected thyroid
hormone-induced hypermetabolism to cause a fall in leptin
levels. However, as no weight change was seen with the short
term hyperthyroidism, no alteration in leptin levels would be
expected on the basis of changes in fat mass. It might be
argued that the dose of thyroid hormone and/or the duration
of treatment were insufficient for an effect on leptin concentrations to be observed. However, definite biological effects
of thyroid hormone treatment were documented in this
499
study. It remains to be determined whether higher doses of
T3 or a longer duration of treatment would produce changes
in leptin concentrations.
In summary, a short term thyroid hormone excess of sufficient magnitude to affect heart rate, systolic blood pressure,
serum cholesterol concentrations, and biochemical indexes
of bone turnover does not alter circulating early morning
leptin concentrations in young men.
Acknowledgments
We thank Janet Hurwitz for performing the RIA analyses of thyroid
hormones and TSH. Without the efforts of the nursing staff of the GCRC,
this study would not have been possible.
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