Power Spectral Analysis of Heart Rate Variability In Hypothyroidism

iodothyronine. These hormones play vital role
in metabolism, profound effect on sensitivity of
tissues to catecholamine , stimulates O2
consumption of most of the cells in the body,
Power Spectral Analysis of Heart Rate Variability In
Hypothyroidism
MatiaAhmed1
, Noorzahan Begum2
, Sultana Ferdousi3
,Shelina Begum4
,TaskinaAli5
Abstract
Background: Hypothyroidism is one of the common clinical problems with high incidence in women
particularly in older age. It has profound effect on metabolic processes in almost all tissue of the body
and heart is particularly sensitive to thyroid hormone action. Power spectral analysis (PSA)is useful
to determine the cardiac autonomic regulation by assessing sympathovagal balance. Objectives: To
assess the cardiac autonomic nervous activity by power spectral analysis of heart rate variability in
patients with hypothyroidism.Method: This cross sectional study was carried out in the Department
of Physiology, Banghabandhu Sheikh Mujib Medical University from 1st July 2008 to 30th June 2009
on 60 female patients of 30-50 years with hypothyroidism (group B). For comparison, 30 age sex
matched apparently healthy euthyroid subjects (group A) were also studied. Based on treatment,
hypothyroid patients were divided into B1 (patients on their 1st day of diagnosis, before starting of
treatment) and B2 (patients with medication for 12-18 months). The patients were selected from the
Out Patients Department of Endocrinology, BSMMU. Serum TSH and FT4 levels of all subjects were
measured by AxSYM system. To perform power spectral analysis of HRV Total Power(TP), Low
Frequency(LF),High Frequency(HF),LF norms (nu),HF norms (nu) and LF/HF ratio were assessed by
a Polyrite. For statistical analysis of data, Independent Sample t test, One WayANOVAand Pearson’s
correlation coefficient test were done as applicable.Results: The mean serum TSH(38.16 vs2.18;2.04)
was significantly higher and FT4(5.12vs 15.13; 15.08) was significantly lower(p<0.001) in group B1
than those of group A and B2.The mean values of all the spectral parameters of HRV in euthyroid
subjects were within normal range. In untreated hypothyroid patients the mean values of TP (
77.11vs2702.87;2683.73), HF power(56.68 vs 342.02; 437.34) HF nu( 24.79 vs 36.35;34.55) were found
significantly (p<0.001) lower LF/HF(1.7±0.3,3.16±.84 and 1.93±.58) and LF nu(75.32 vs 63.71, and
65.90) were significantly (p<0.001) higher and in comparison to both euthyroids and treated patients.
Conclusion: Alteration in cardiac autonomic nervous activity characterized by reduced vagal
modulation and higher sympathetic activity may occur in the hypothyroid patients..
Keywords:HRV,LFHFLF/HFHypothyroidism
J Bangladesh Soc Physiol. 2010 December; 5(2): 53-59
For author affiliations, see end of text.
http://www.banglajol.info/index.php/JBSP
Introduction
he thyroid gland is the largest
endocrine gland of our body, produces
two major hormones, thyroxin and tri-T necessary for normal growth and maturation and
therefore affects every organ and organ
system.1,2
Hypothyroidism is one of the common endocrine
disorder in our country.About 3% of the general
population are hypothyroids. The incidence is
Article
53 J Bangladesh Soc Physiol. 2010 December; 5(2): 53-59

more in women particularly in older age but men
are also affected.3
Hypothyroidism may be associated with different
cardiovascular and metabolic disorders including
autonomic nerve function disorders which are
evident by the changes in cardiac autonomic
nervous activities (CANA). Increased
sympathetic and decreased parasympathetic
activities are the usual findings in hypothyroids.
Increase in sympathetic activity possibly be due
to TRH which directly stimulates sympathetic
outflow within the CNS.4 Dysfunction of ANS
may also affect various other organs.
Heart rate variability is the result of intrinsic
cardiac automaticity and an indicator of the
interaction between cardiac sympathetic and
parasympathetic activity5.It has been reported
that increase in sympathetic and decrease in
parasympathetic activity may reduce beat to beat
variability6 .
Power spectral analysis of HRV can demonstrate
the definite impairment in cardiac autonomic
control, by its ability to detect sympatho-vagal
imbalance5,7. Power spectral components for
HRV analysis includes Total power, VLF power,
LF power, HF power, HF norm, LF norm and LF/
HF ratio5.
Several studies reported Lower values of total
power, HF power and LF power in untreated
hypothyroids than those of euthyroids8-11.
Again, higher LF norm and lower HF norm in
untreated hypothyroid patients in comparison
to healthy control was also reported by some
investigators 8,9,12. Cacciatori et al. Galetta et al.
observed higher LF/HF ratio in untreated
hypothyroids than those of euthyroids and also
treated patients9-10 .
Though most of the time they remain unnoticed,
it is possible to prevent the development of
various complications related to autonomic
dysfunction in thyroid disfunction. In our
country, heart rate variability by power spectral
analysis was investigated in hyperthyroids13. In
addition, several studies on autonomic nerve
function in some clinical condition and health
such as diabetes mellitus, aging process, obesity
and post-menopausal women were done by
conventional method14-17 However, no published
data is yet available on hypothyroidism by power
spectral analysis.
Therefore, the present study was undertaken to
assess the autonomic nerve function status in
hypothyroid patients by power spectral analysis
of HRV.This study may be useful in screening of
autonomic nerve function status in
hypothyroidism in order to minimize hypothyroid
related complications.
Methods
This cross sectional study was carried out to
observe the HRV by time domain method in 60
hypothyroid female patients (group B) with age
ranged from 30-50 years in the Department of
Physiology , Bangabandhu Sheik Mujib Medical
University from 1st July 2008 to 30th June 2009 .
For comparison 30 age and sex matched
apparently healthy euthyroid subjects ( group-
A) were also studied Hypothyroid patients were
further subdivided according to treatment
received into B1, i.e. untreated hypothyroid
patients and B2 i.e. treated patients achieving
euthyroid status. The patients were selected from
the Out Patient Department of Endocrinology,
BSMMU and the controls were selected from
the same area near to hospital by personal
contact. All the subjects were free from heart
disease, hypertension, diabetes mellitus. chronic
renal failure, psychic disorders, and smoking .
After selection, the subject was thoroughly
informed about the aim, objectives and detail
procedure of the study before examination and
collection of blood sample. They were
encouraged for voluntary participation and were
allowed freedom to withdraw from the study
whenever she liked even after participation. If
she agreed to enroll to the study, informed written
consent was taken from them. For examination
the subjects were advised to have their meal by
9:00 pm on previous night, to remain free from
any physical or mental stress, not to take any
drugs affecting central nervous system and to
have a good sleep at night before the examination
HIV in Hypothyroidism Article
J Bangladesh Soc Physiol. 2010 December; 5(2): 53-59 54

day. The subject were advised to avoid tea or
coffee at breakfast and to attend the Autonomic
Nerve Function Test Laboratory
Between 9:00 to 11:00 a.m. on the day of
examination. Then the subject was interviewed
and detail history regarding personal history, drug
history, past medical history was taken to exclude
exclusion criteria. Then thorough physical
examinations and anthropometric parameters like
height, weight, BMI, were taken and the
information were recorded in a prefixed
questionnaire. Then the subjects were kept under
complete bed rest in supine position for 20
minutes in a cool and calm environment. During
this period she was advised not to talk, eat or
drink and also not to perform physical or any
mental activity, even sleep. Then all preparations
for recording of the Heart Rate Variability
parameters were made by connecting the
channels to a transducer for ECG to a
computerized polygraph and 5 minutes recording
was taken in resting supine position. Data were
obtained by software analysis of the power
spectral band of the HRV5. After the recording is
over, 5ml of venous blood was drawn from the
subject .The serum level of thyroid hormone was
measured by Microparticle enzyme Immuno
Assay(MEIA) method. Data were expressed as
mean ± SD. For statistical analysis, Independent
Sample t test and one wayANOVAwere used as
applicable.
Results
All the subjects were matched for age and BMI
and no statistically significant differences were
observed among them. (Figure 1)
The mean serum TSH was significantly higher
(p<0.001) and FT4(5.12vs 15.13; 15.08) was
significantly lower(p<0.001) in group B1 than
those of group A and B2 but no statistically
significant differences were observed regarding
both FSH and FT4 when compared between group
A and B2 (Table I).
Table I: Serum TSH and FT4 levels in different
groups (n=90)
Groups TSH(mIU/L) FT4(pmol/L)
A(n=30) 2.18±1.04 15.13±4.21
B1(n=30) 38.16±30.50 5.12±1.89
B2(n=30) 2.04±1.01 15.08±3.95
Statistical analysis:
Groups P values
A vs B1 vs B2 0.000*** 0.000***
A vs B1 0.000*** 0.000***
A vs B2 0.608ns 0.964ns
B1 vs B2 0.000*** 0.000***
Datawereexpressedasmean±SD. ***=P<0.001.
ns =. P>0.05
.The mean values of all the spectral parameters
of HRV in euthyroid subjects were within normal
range. In untreated hypothyroid patients the
mean values of TP, HF power, HF nu were found
significantly (p<0.001) lower but LF/HF ratio and
LF nu were significantly (p<0.001) higher and in
comparison to both euthyroids and treated
patients. Again, no statistically significant
differences were observed when all these
parameters were compared between groupAand
B2. (Table II &Table III)
Figigure 1: Age and BMI in different groups
(n=90)
Article HIV in Hypothyroidism

Table II: Power Spectral HRVMeasures in Different groups(n=90)
Groups Total Power(ms2) LF%(ms2) HF power(ms2)
A(n=30) 2702.87±1297.89 14.65±10.7 342.02±362.15
B1(n=30) 1177.11±584.04 17.87±11.5 56.68±50.76
B2(n=30) 2683.73±1275.87 14.8±12.3 437.34±410.47
Groups
A vs. B1 vs. B2
a 0.000*** 0.825ns 0.000***
A vs. B1
b 0.000*** 0.844ns 0.000***
A vs. B2
b 0.954ns 0.698ns 0.344 ns
B1 vs. B2
b 0.000*** 0.576ns 0.000***
Data were reported as mean ± SD.Statistical analysis were done by One wayANOVAa and Independent sample
‘t’ test b.LF = Low frequency, HF = High frequency, ms2 = squared millisecond.
Table III: Power Spectral HRV Measures in Different groups(n=90)
Groups LFnu HFnu LF/HF
A(n=30) 63.71±4.07 36.35±4.12 1.7±0.3
B1(n=30) 75.32±4.85 24.79±4.89 3.16±.84
B2(n=30) 65.90±5.23 34.55±4.67 1.93±.58
Groups
A vs. B1 vs. B2
a 0.000*** 0.000*** 0.000***
A vs. B1
b 0.000*** 0.000*** 0.000***
A vs. B2
b 0.161ns 0.105ns 0.982ns
B1 vs. B2
b 0.000*** 0.000*** 0.000***
Data were reported as mean.±SD. Statistical analysis were done by One wayANOVAa and Independent sample
‘t’ test b.
LFnorm.= LF power in normalized unit,[{LF/total power-VLF}×100] HF norm.=HF power in normalized
unit,[{LF/total power-VLF}×100] LF/HF= ratio of low frequency and high frequency.
Group A: Apparently healthy Euthyroids.(control)
Group B : Hypothyroid (study group)
B1 : Untreated .
B2 : Treated.
Discussion
In last decade several investigators had assessed
cardiac autonomic activity by power spectral
analysis of HRV from the ECG recoding in
Hypothyroid patients8,-11,18-20.
Different power spectral components of HRV has
been used as marker of cardiac autonomic activity.
The task force guideline for HRV analysis have
demonstrated the interpretation of these
parameters to understand the status, behavior

and the balance between sympathetic and
parasympathetic due to their continuous
interaction5. The total power represents the total
variability of R-R interval and is the result of the
total cardiac autonomic nervous activities and
hormonal activities on heart. Therefore its lower
value indicates lower modulation of the cardiac
autonomic nervous activities on heart5.
Twomajorcomponentofspectralband,HFpower
and HF norm reflect the parasympathetic
modulation on the heart5 and LF power represents
the sympathetic activity on the heart; though,
some investigators claimed that it is also under
some parasympathetic contribution whereas LF
norm emphasizes the controlled & balanced
behavior of the sympathetic nervous system5.
The cardiac sympathetic and parasympathetic
activities are always in interaction. The LF/HF
ratio can be considered as a marker of their
sympatho vagal balance.
The values of spectral components of HRV in
euthyroid subjects in this study were comparable
to those reported by the various investigators
from different countries .18,21,23,24 and also from
our country. 13-17
In this study no significant differences were
observed in all these parameters when compared
between euthyroid and treated patients with their
almost euthyroid status.
In the present study, significantly lower total
power in untreated hypothyroid is consistent
to the findings of some other studies9-11
Again, significantly lower values of HF power
and HF norm in untreated hypothyroid indicating
theirreducedvagalmodulationofheart compared
to euthyroid and treated hypothyroid patients
who obtained euthyroid status. Similar
observations were also made by some
researchers8-10,12.
A conflicting picture regarding LF power and LF
norm in hypothyroidism is available from various
studies. Some showed an increase, some showed
a decrease and some others found no change in
LF in hypothyroid compared to euthyroid
subjects.
In the present study though higher but
statistically not significant value of LF and
significantly higher values of LF norm in
untreated in hypothyroid is in contrast to those
researchers8,12, who found significantly lower
LF and LF norm in hypothyroids but LF norm is
comparable to some studies9.
LF/HF ratio was found significantly higher in
hypothyroids compared to control and euthyroid
treated hypothyroid patients in this study. This
finding is consistent to the observations made
by some researchers9,10,25 but contradicts to
other investigator’s findings8,12.
Findings from research articles analyzing HRV
showed that various authors have proposed
several mechanisms with physiological
background for the observed changes of power
spectral components in the deficient state of
thyroid hormone in human or in animal model
experiment.
It has been suggested that increased sympathetic
and decreased vagal modulation might occur in
hypothyroidism9,10,26.But decreased HR is one
of the important feature of hypothyroid which is
attributed to desensitization of receptor and also
due to cardiac chronotropic response to
adrenergic stimulation despite evidence of
sympathetic overactivity. It has also been
suggested that decreased binding of
catecholamine with alpha and Beta receptors in
cardiac myocites might be responsible for
cardiovascular changes in Hypothyroids10.
Several investigator suggested that though
plasma catecholamines are increased in
hypothyroidism but overall depression of
adrenergic responses at cardiac and peripheral
level indicates desensitization both at the
receptor or post receptor site9,10,21,23,24, 26.
In this study, higher values of LF power, LF norm
are suggestive of presence of sympathetic over

activity in the hypothyroid patients of the
present series. However, decreased values of total
power, HF power and HF norm revealed reduced
vagal tone in these groups of patients. Moreover,
shifting of sympatho vagal balance towards
sympathetic predominance in this study is also
in favor of sympathetic over activity in
hypothyroid patients.
Conclusion
Alteration in cardiac autonomic nervous activity
characterized by reduced vagal modulation and
higher sympathetic activity may occur in the
hypothyroid patients.
Author affiliations
1. *Matia Ahmed, Assistant professor, Department of
Physiology, Uttara Adhunik Medical College,
Uttara.Email: matia.arman@gmail.com
2. Noorzahan Begum, Professor Department of
Physiology, Bangabandhu Sheikh Mujib Medical
University(BSMMU), Bangladesh, Email:
noorzahanbeg@yahoo.com
3. Sultana Ferdousi, Assistant professor, Department
of Physiology, Bangabandhu Sheikh Mujib Medical
University(BSMMU), Bangladesh,
Email:sferdousiratna@ gmail.com
4. Shelina Begum. Professor, Chairman, Department
of Physiology, Bangabandhu Sheikh Mujib Medical
University(BSMMU), Bangladesh.
5. Taskina Ali, Assistant professor, Department of
Physiology, Bangabandhu Sheikh Mujib Medical
University(BSMMU), Bangladesh, Email:taskinadr@
gmail.com
*for correspondence
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