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Association Between Thyroid Dysfunction and Total Cholesterol Level in an Older Biracial Population
The Health, Aging and Body Composition Study
Alka M. Kanaya, MD;
Fran Harris, MS;
Stefano Volpato, MD;
Eliseo J. Pérez-Stable, MD;
Tamara Harris, MD;
Douglas C. Bauer, MD
Arch Intern Med. 2002;162:773-779.
ABSTRACT
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Background Thyroid dysfunction increases with age. Less is known about the prevalence
of thyroid disease in older black adults and whether an association between
thyroid function and serum cholesterol level exists, as in older white adults.
Methods A cross-sectional study of 2799 well functioning white and black participants,
aged 70 to 79 years, were recruited for a population-based study. Participants
underwent thyrotropin, free thyroxine, and total cholesterol testing; a medical
history; and physical measurements.
Results Among the entire cohort, 94% were euthyroid based on biochemical testing
results. Approximately 10% were taking thyroid hormones. Subclinical hypothyroidism
was the most prevalent disorder (3.1% of all participants not taking thyroid
hormones), but black men and women had lower rates of this condition than
white men and women. After excluding those taking thyroid or lipid medication
and adjusting for potential confounders, an elevated thyrotropin level (>5.5
mIU/mL) was associated with a 9 mg/dL (0.23 mmol/L) higher cholesterol level,
and a suppressed thyrotropin level (<0.35 mIU/mL) was associated with a
19 mg/dL (0.49 mmol/L) lower cholesterol level.
Conclusion Healthy community-dwelling older black adults have a lower prevalence
of thyroid dysfunction compared with older white adults, but the association
between increased thyrotropin and increased cholesterol levels is similar
in both races.
INTRODUCTION
WITH THE advent of sensitive assays for thyrotropin, isolated elevation
and suppression of thyrotropin levels have been increasingly recognized in
older populations. Subclinical hypothyroidism, defined as an elevated thyrotropin
level with a normal serum free thyroxine (FT4) concentration, occurs
in 5% to 10% of patients older than 65 years and is especially prevalent in
older white women.1-9
Less information exists regarding the prevalence of subclinical hyperthyroidism
in older black adults; estimated prevalences range from 2% to 12%, but results
from population-based studies often have included patients receiving thyroxine
treatment, which may overestimate the true prevalence.10
Many contend that identifying and treating older patients with subclinical
hypothyroidism may retard deleterious effects on the cardiovascular system,
mostly via beneficial changes on lipoprotein levels.11
The relationship between overt hypothyroidism and lipid abnormalities
has been recognized for decades. Frank hypothyroidism is associated with elevated
total cholesterol and low-density lipoprotein levels.12-13
Studies investigating the relationship between lipids and subclinical hypothyroidism
have shown variable results, ranging from no correlation14-16
to deleterious effects on total cholesterol, low-density and high-density
lipoprotein cholesterol, lipoprotein(a), and apolipoproteins B and A1.1, 17-20
A meta-analysis21 found that subclinical hypothyroidism
is 2 to 3 times more frequent in persons with elevated cholesterol levels
and that total cholesterol levels are slightly elevated in patients with subclinical
hypothyroidism. Similarly, intervention studies of thyroxine treatment for
subclinical hypothyroidism have found mixed results on lipid profile, although
the meta-analysis of 13 treatment studies found that thyroid substitution
therapy resulted in an overall decrease in cholesterol of 15 mg/dL (0.4 mmol/L).
Little is known about the prevalence of thyroid dysfunction in older
black adults and whether a similar association between thyroid function and
cholesterol level exists among black adults, as in white adults. A cross-sectional
study7 of middle-aged and older volunteers
screened at health fairs found that white participants had a higher prevalence
of abnormally suppressed or increased thyrotropin levels compared with black
adults. We are unaware of any study that has examined the relationship between
thyrotropin and cholesterol levels among older black adults.
To examine the prevalence of self-reported thyroid disease and biochemically
measured thyroid dysfunction and the relationship between thyroid disorders
and total cholesterol levels by race, we used data from the Health, Aging
and Body Composition study, a study of well functioning white and black men
and women aged 70 to 79 years. We hypothesized that the black participants
in this cohort might have a lower prevalence of measured thyroid dysfunction
than the white participants, and that a significant relationship between thyrotropin
and cholesterol levels would exist for both racial groups.
PARTICIPANTS AND METHODS
STUDY DESIGN AND SETTING
Participants in this cross-sectional study were enrolled in the Health,
Aging and Body Composition study, an ongoing prospective cohort study of 3075
community-dwelling men and women, aged 70 to 79 years, enrolled or recruited
at 2 clinical centers, in Memphis, Tenn, and Pittsburgh, Pa. To be eligible
for this study, participants had to report no difficulty walking 0.4 km (one
quarter mile) or up a flight of stairs. For our analyses, we used data gathered
from 2799 participants who had thyrotropin test specimens, medical history,
medication inventory, and physical measurements obtained at the second annual
visit. All Health, Aging and Body Composition study participants were interviewed
and examined at one of the clinical centers during the first and second annual
visits between April 28, 1997, and June 14, 1999.
MEASUREMENTS
Questionnaire variables gathered from the first annual visit included
self-identified racial group, level of education completed, and estimated
family income. Physical activity was assessed at this visit by self-report
of walking and exercise, assigning kilocalories per week to categories based
on the Harvard Alumni Health Study variables.22
In addition, participants reported smoking history (never, former, or current
smoker) and alcohol use (number of drinks per week). Detailed data about physician-diagnosed
medical conditions were collected at the first visit as well. Participants
were asked specifically about previous diagnosis of Graves disease, hyperthyroidism,
hypothyroidism, or an overactive or underactive thyroid gland. Participants
were classified as having diabetes if they reported physician-diagnosed diabetes,
were taking medication to control their diabetes, had a fasting serum glucose
level greater than 125 mg/dL (>6.9 mmol/L), or had a 2-hour oral glucose tolerance
test result 200 mg/dL or greater than ( 11.1 mmol/L) at the baseline or
second annual visit. At the second visit, their weight was measured using
standardized protocols,23 and body mass index
(BMI) was calculated using the height measurement from the first visit. Trained
interviewers confirmed medication used in the preceding 2 weeks by examination
of pill bottles at the second visit as well.
Fasting serum specimens were collected from each participant at the
second annual visit. We measured thyrotropin levels by immunoassay (ACS; Chiron
Diagnostics Corp, Emeryville, Calif) in 2799 participants. The normal range
of thyrotropin by this assay is 0.35 to 5.5 mIU/mL. The lower limit of detection
of thyrotropin was 0.03 mIU/mL. The coefficient of variation for thyrotropin
was 4.1% at a level of 18.94 mIU/mL and was 3.6% at a level of 1.26 mIU/mL.
We routinely measured FT4 by competitive immunoassay (ACS; Chiron
Diagnostics Corp) on all of the participants with a thyrotropin value of 0.1
mIU/mL or less or greater than 7.0 mIU/mL. The normal range of FT4
for this assay is 0.8 to 1.8 ng/dL (10-23 pmol/L). We determined a fasting
total cholesterol level (VITROS; Johnson & Johnson, Rochester, NY) for
all participants at this visit as well.
DEFINITION OF THYROID DISEASE AND DYSFUNCTION
Biochemically documented hypothyroidism was defined as a thyrotropin
level of 7.0 mIU/mL or higher and an FT4 value of less than 0.8
ng/dL (<10 pmol/L), while subclinical hypothyroidism was defined as a thyrotropin
value of 7.0 mIU/mL or higher with a normal FT4 level. Biochemically
documented hyperthyroidism was defined as a thyrotropin level of 0.1 mIU/mL
or less with an FT4 value of greater than 1.8 ng/dL (>23 pmol/L),
and subclinical hyperthyroidism was defined as a suppressed thyrotropin level
( 0.1 mIU/mL) with a normal FT4. All participants with a thyrotropin
value of greater than 0.1 mIU/mL or less than 7.0 mIU/mL were considered to
be euthyroid. To distinguish between participants who were currently being
treated with thyroid hormone therapy or antithyroid medications from those
with laboratory evidence of a thyroid disorder, we termed the former group
as those with thyroid disease and the latter group
as those with thyroid dysfunction. Suboptimal thyroid
treatment was evaluated for those with existing thyroid disease (hypothyroidism)
who were taking thyroid hormone therapy. Participants who were not euthyroid
while taking thyroid hormones were included in the analysis of suboptimal
thyroid treatment.
STATISTICAL ANALYSIS
Means and SDs of the baseline characteristics were determined. Bivariate
associations were examined by  2 test, Fisher exact test, or t test, when appropriate. Differences were considered significant
at P.05.
In analyzing the biochemically measured thyroid status of participants
in each sex and race group, we stratified our analyses by use of thyroid hormone.
Among the cohort taking thyroid hormones, we performed a stepwise logistic
regression analysis to assess characteristics associated with suboptimal treatment
with thyroid hormones. In addition, we evaluated the relationship of mean
cholesterol levels adjusted for age, BMI, physical activity, current smoking,
alcohol use, oral estrogen use, and diabetes with levels of thyroid dysfunction
among those who were not taking thyroid medications or lipid-lowering drugs.
We calculated a P for trend for this relationship
for each sex and race subgroup.
The relationship between thyrotropin and total cholesterol levels was
analyzed using multivariate linear regression models, adjusting for potential
confounders. Confounders were determined based on biologic plausibility (age
and BMI) or strong association with serum cholesterol levels (alcohol use,
oral estrogen use, and diabetes) or thyrotropin level (current smoking). Thyrotropin
level was analyzed as a categorical, continuous, and log-transformed variable.
All statistical analyses were performed using commercially available software
(SAS, version 6.12; SAS Institute, Inc, Cary, NC).
Because there were small numbers of persons with thyroid dysfunction
in each sex and race subgroup and because a similar trend was seen between
cholesterol level and thyroid function regardless of sex or race, we performed
multivariate analyses of the entire group and adjusted for age, sex, race,
BMI, current smoking, alcohol use, oral estrogen use, and diabetes. We excluded
persons who were taking lipid-lowering medications, thyroid hormones, or antithyroid
medications in these analyses. We performed similar analyses stratified by
sex as well.
RESULTS
Among the entire cohort, 2799 (91%) had laboratory tests performed at
the second annual visit. Sixty percent of them were white, and 51% were women.
The mean age was similar among all subgroups (Table 1). White men and women were more educated and reported higher
family income than black men and women, respectively. Men of both races had
similar mean BMIs, while black women had the highest BMIs of all. Overall,
white men had the highest activity levels, and black women were the least
active. Women of both races reported more known diagnoses of thyroid disease
and increased use of thyroid hormones compared with men. Compared with black
women, white women reported a significantly greater prevalence of known hypothyroidism
(16.5% vs 6.2%, P = .001), while black women reported
a higher prevalence of hyperthyroidism (9.7% vs 6.0%, P = .01). However, among the entire study population, only 2 black women
were currently taking antithyroid medications. Significantly more white participants
were taking lipid-lowering medications in both sex groups, and more white
women were taking oral estrogen than black women.
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Table 1. Characteristics of the Participants at the First and Second
Annual Visits*
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Mean values and ranges for thyrotropin, FT4, and cholesterol
for all participants are reported in Table
2. On average, FT4 and cholesterol means were similar
for both races in each sex group, but women had higher mean total cholesterol
levels than men. The mean thyrotropin values were significantly higher for
white men and women compared with black men and women.
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Table 2. Laboratory Values of Participants at Second Annual Visit*
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Most (94%) of the cohort were euthyroid based on biochemical testing
results (Table 3). The overall
frequency of overt hypothyroidism and hyperthyroidism in the whole cohort
was low (<1% for each condition). Subclinical hypothyroidism was the most
prevalent thyroid disorder in the entire cohort (3.9%), with white women having
a significantly higher rate compared with the other 3 subgroups.
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Table 3. Prevalence of Thyroid Abnormalities by Race and Sex*
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Among the 271 participants taking thyroid hormones (10% of the entire
cohort), most (83%) were euthyroid. Black women taking thyroid hormones appeared
to have a lower percentage (76%) with euthyroid status compared with the other
3 sex and race subgroups (84% for the 3 subgroups combined), but this difference
was not statistically significant (P = .34). Again,
subclinical hypothyroidism was the most prevalent thyroid disorder overall
(11.1%). We determined predictors of suboptimal treatment with thyroid hormones
by stepwise logistic regression analysis. Low family income was the strongest
predictor for having an abnormal thyrotropin level (P
= .003), and those with the highest reported physical activity were least
likely to be euthyroid (P = .03). Neither sex nor
race was associated with suboptimal treatment of hypothyroidism.
In analyses based on those who were not taking thyroid hormones or antithyroid
medications, again subclinical hypothyroidism was the most prevalent thyroid
abnormality, with white women having a significantly higher prevalence than
black women (4.6% vs 1.7%, P = .005). There was a
trend toward a higher prevalence of subclinical hypothyroidism in white men
compared with black women (P = .06) and an equal
prevalence among black men and black women.
Table 4 shows adjusted mean
cholesterol values by thyroid status for each sex and race strata of participants,
excluding those who were taking lipid-lowering medications, thyroid hormones,
or antithyroid medications. Participants with overt hypothyroidism had higher
mean cholesterol values than those who were euthyroid among all subgroups.
For all subgroups, there was a significant trend seen with decreasing cholesterol
values for distinct levels of thyroid function, from frank biochemically documented
hypothyroidism to hyperthyroidism. The only subgroup that had significantly
elevated cholesterol levels associated with subclinical hypothyroidism, compared
with the euthyroid state, was black women (P<.001).
The unadjusted cholesterol values by thyroid status for all sex and race groups
were similar to these adjusted values.
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Table 4. Cholesterol Levels by Thyroid Function for Those Not Taking
Thyroid Hormones*
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We performed multivariate linear regression analysis to determine whether
an association existed between thyrotropin and cholesterol levels after adjusting
for age, sex, race, BMI, current smoking, alcohol use, oral estrogen use,
and diabetes. Similar results were obtained using thyrotropin level as a continuous,
categorical, or log-transformed explanatory variable. We examined the results
with thyrotropin levels categorized into high, normal, or low, using 2 definitions
for euthyroid state (>0.1 to <7.0 mIU/mL and 0.35 to 5.5 mIU/mL). For the
entire cohort, a high thyrotropin level (>5.5 mIU/mL) was associated with
a 9 mg/dL (0.23 mmol/L) higher total cholesterol, while a low thyrotropin
level (<0.35 mIU/mL) was associated with a 19 mg/dL (0.49 mmol/L) lower
total cholesterol (Table 5). Separate
models were constructed for each sex, which showed similar associations between
continuous and log-transformed thyrotropin levels, examined relative to cholesterol
levels, while the relationship between categorical thyrotropin levels and
cholesterol levels trended in the same direction, but did not reach statistical
significance for many of the categories. Separate analyses of participants
taking thyroid hormones but not lipid-lowering medications found significantly
higher cholesterol levels for those with high thyrotropin values (15 mg/dL
[0.39 mmol/L] higher total cholesterol if the thyrotropin level was >5.5 mIU/mL)
and a trend to lower cholesterol levels with low thyrotropin values, but these
did not reach statistical significance.
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Table 5. Multivariate Results for the Association Between Thyrotropin
and Cholesterol Levels*
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Among the 271 participants taking thyroid hormones, only 60 (22%) were
taking lipid-lowering medications. Similarly, among the 96 participants who
had subclinical or overt hypothyroid dysfunction, 15 (16%) were taking lipid-lowering
medications. The thyroid dysfunction group was not more likely to be treated
with lipid-lowering agents than was the thyroid disease group (P = .17).
COMMENT
In this study of healthy community-dwelling older adults, the overall
prevalence of any unrecognized thyroid dysfunction was lower than previously
reported, and the use of thyroid supplementation was more common. Subclinical
hypothyroidism was the most prevalent disorder, but black men and women had
lower rates of this condition than white men and women. The lower prevalence
of thyroid dysfunction may be partially explained by the selection of healthy
participants who were able to walk 0.4 km (one quarter mile) and climb stairs
at baseline as part of the Health, Aging and Body Composition study. We also
confirmed an association between thyrotropin and cholesterol levels and extended
this association to older black adults. For men and women in both racial groups,
an elevated thyrotropin level was associated with a 9 mg/dL (0.23 mmol/L)
higher total cholesterol, and a suppressed thyrotropin level was associated
with a 19 mg/dL (0.49 mmol/L) lower total cholesterol.
The prevalence of thyroid dysfunction in older populations has varied
by ethnicity. There are limited published data reporting prevalence rates
of subclinical thyroid disease in nonwhite populations. Lindeman et al24 found that non-Hispanic white women have higher rates
of subclinical hypothyroidism compared with Hispanic women. However, there
was insufficient power to examine the relationship between thyroid status
and lipid abnormalities among the different sex and ethnicity strata in this
study. The only prior publication reporting the prevalence of biochemically
measured hypothyroidism or hyperthyroidism in blacks is a cross-sectional
analysis of community-dwelling volunteers, 55 and older, at community health
fairs.7 The authors reported a lower prevalence
of hypothyroidism and hyperthyroidism in blacks vs whites, and both ethnicities
had higher prevalences of the 2 disorders in women. Our results support this
finding, although we found much lower prevalences of overt thyroid disease
in all 4 sex and ethnicity strata overall. Likewise, older ambulatory Chinese
patients in Hong Kong had low frequencies (<2%) of abnormal thyrotropin
values, with Chinese women having more thyrotropin abnormalities compared
with men.25
Among the participants who were taking thyroid hormones, most were euthyroid
based on biochemical testing results. Neither sex nor race was associated
with being overtreated or undertreated with thyroid hormones among this subgroup.
Those with lower family income were less likely to be euthyroid, possibly
because of poorer access to health care, expense of successive laboratory
testing, and prescription drug costs. Those with higher levels of physical
activity were also less likely to be euthyroid, an observation that is unexplained.
In our evaluation of thyrotropin abnormalities and cholesterol levels,
we found that a similar association exists between these 2 biochemical test
results for both sexes and ethnicities. However, despite finding elevated
cholesterol levels associated with high thyrotropin levels, we did not detect
overtreatment of hypercholesterolemia in patients with thyroid dysfunction.
Similar proportions of older adults with existing treated hypothyroidism and
those with thyroid dysfunction detected on laboratory screening were taking
cholesterol-lowering medications. Although screening older adults with increased
cholesterol levels for thyroid dysfunction seems prudent, our data do not
show overuse of lipid-lowering drugs for those with thyroid dysfunction.
We were limited to using questionnaire results and physical measurements
obtained for participants at the first annual visit and using thyrotropin,
FT4, and cholesterol test results from the second annual visit
for our analyses. It is unlikely that there was a significant change in most
self-reported variables, such as income, physical activity, smoking, or alcohol
use, during one year. With regard to the biochemical tests, serum lipid levels
have high day-to-day variability, and a more accurate association between
thyroid function and cholesterol level may be seen with multiple fasting levels
of the test specimens drawn simultaneously. However, this additional variability
would only obscure the association that we observed. We also had limited power
for some associations, such as race-specific relationships between thyrotropin
and cholesterol levels.
When a participant's thyrotropin level was 0.1 mIU/mL or less or greater
than 7.0 mIU/mL, FT4 test results were obtained. Other cut points
might have given slightly different results. By using our conservative cut
points for ordering FT4 testing, we were unable to determine a
biochemically defined thyroid status for 41 persons with thyrotropin levels
of less than 0.35 mIU/mL but greater than 0.1 mIU/mL and for 106 participants
with thyrotropin levels of greater than 5.5 mIU/mL but less than 7.0 mIU/mL.
Because the overall prevalences of frank hypothyroidism and hyperthyroidism
were so low in persons with more strikingly abnormal thyrotropin values, we
doubt that we have misclassified overt thyroid disease. However, by using
these conservative variables, we may have failed to recognize more subclinical
thyroid dysfunction and may have underestimated the true prevalence of subclinical
hypothyroidism and hyperthyroidism.
We conclude that, in individuals who were not taking thyroid hormones,
overt thyroid dysfunction was less prevalent than subclinical hypothyroidism
in an ambulatory biracial older population. Black women have equally low prevalences
of subclinical thyroid disease compared with white and black men. There is
an association between thyrotropin and cholesterol levels, with an elevated
thyrotropin being associated with a 9 mg/dL (0.23 mmol/L) increase in cholesterol
and a suppressed thyrotropin being associated with a 19 mg/dL (0.49 mmol/L)
decrease in total cholesterol. Future studies should examine the association
between thyrotropin levels and specific lipoprotein fractions and investigate
whether thyroid autoantibodies explain the differences in thyroid dysfunction
prevalences between racial groups.
AUTHOR INFORMATION
Accepted for publication August 7, 2001.
The Health, Aging and Body Composition study was funded by grants N01-AG-6-2102,
N01-AG-6-2103, and N01-AG-6-2106 from the National Institute on Aging, National
Institutes of Health, Bethesda, Md. This work was also supported by pilot
investigator grant 556198 to the Center for Aging in Diverse Communities,
University of California, San Francisco, funded by the National Institute
on Aging, National Institute of Nursing Research, and Office of Research on
Minority Health, National Institutes of Health. It was also supported by faculty
development in general internal medicine grant 1D08PE50109-01 from the US
Department of Health and Human Services, Washington, DC (Dr Kanaya).
We acknowledge and thank the reviewers of the Health, Aging and Body
Composition study publications committee for their valuable contributions
to manuscript revision.
Corresponding author: Alka M. Kanaya, MD, Division of General Internal
Medicine, Department of Medicine, University of California, San Francisco,
1701 Divisadero St, Suite 554, San Francisco, CA 94143-1732 (e-mail: alkak{at}itsa.ucsf.edu).
From the Division of General Internal Medicine, Department of Medicine
(Drs Kanaya, Pérez-Stable, and Bauer), Prevention Sciences Group (Ms
F. Harris and Dr Bauer), Center for Aging in Diverse Communities (Drs Kanaya
and Pérez-Stable), Medical Effectiveness Research Center for Diverse
Populations (Dr Pérez-Stable), University of California, San Francisco;
and the National Institute on Aging, National Institutes of Health, Bethesda,
Md (Drs Volpato and T. Harris).
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