Kathryn Jean Lucas, MD
611 N 35th St
Morehead City, North Carolina 28557
inside health
Use of Thyroid
Ultrasound Volume in Calculating Radioactive
Iodine Dose in Hyperthyroidism
By: K. Jean Lucas,
M.D., FACP, FACE
One hundred
and twenty-one patients treated with I131 had
a thyroid ultrasound to precisely measure thyroid
volume. This volume measurement was used to
determine the radioactive iodine dose. The average size
(± SEM) of the thyroid glands measured in this manner
was 39.7cc ± 1.9cc. A significant correlation was found
in the estimated size of the gland by the
endocrinologists and the ultrasound volume. Of the 121
patients, 89 patients had the same I131 uCi/g
tissue factor to determine the radioactive iodine dose.
This group of patients was further evaluated in this
study. The average I131 dose (± SEM)
given was 13.2mCi ± 0.5 mCi. The average time until
hypothyroidism was 2.85 ± 0.14 months.
Ultrasound provides a safe and precise way to determine
actual thyroid size when calculating I131
doses.
Introduction
Radioactive iodine (1,2) has been used for over
40 years to treat hyperthyroidism caused by Graves
disease, hyperfunctioning nodules, and toxic multinodular
goiters. The success of the treatment depends on
the dose of radioactive iodine entering the cells of the
thyroid. The dose needed for therapy is directly
proportional to the size of the gland in grams and
inversely proportional to the iodine uptake. The
amount of radioactive iodine is commonly calculated by
estimating size in grams by palpation of the gland and
measuring iodine uptake by the use of I123.Currently,
gamma cameras used for thyroid scans, because of their
two dimensional character and scatter of the image, are
inaccurate in determining thyroid volume. In this
study, ultrasonography of the thyroid was used to
precisely measure thyroid volume. Volume
measurements were obtained in each patient prior to
treatment with radioactive iodine. Each patient in
the analysis for the study was followed for at least 6
months in order to determine the efficacy of this
therapy.
Materials and Methods Ultrasound
Volume and Iodine Uptake Determinations
Radioactive iodine treatments were started in the
author’s office in March of 1994.All patients who had
not had a previous thyroid uptake and scan were scheduled
to have 6 hour and 24 hour iodine uptakes and an
ultrasound prior to being treated with the radioactive
iodine. The ultrasound volume was measured in each
patient using Brunn’s (3) formula:
Volume = 0.52 (length
x width x depth)
The value 0.52 represents pi/6. Length was the
longitudinal diameter. Width was measured as the
transverse or lateral diameter. Depth was the AP
diameter. The final volume was calculated by adding the
volumes of the two lobes together. The uptake was
determined by dosing 100uCi of I123,
measuring the six and twenty-four hour uptakes by use of
a Capintec 2000 uptake device, and calculating the
fraction of the radioactivity absorbed by the thyroid.
Treatment
Dose Calculation
The volume in cubic centimeters determined by ultrasound
was equated to the weight in grams by a 1:1 ratio. (4)
The treatment dose was calculated by multiplying the
grams of thyroid by the factor 200uCi per gram of tissue
and dividing by the uptake over 100 (using the uptake at
24 hours). This factor was used in the majority of the
patients (73.6%). These patients will form the
basis of the further evaluation and discussion.
Patients
Eighty-nine of 121 consecutive patients who had a
thyroid ultrasound and were treated by the author from
March 1994 until December 1996 were included in the
analysis. 15 (16.8%) patients were not able to be
followed for 6 months and were omitted from
evaluation. Follow-up time was at least 6
months in all the remaining patients.
Seventy-two females and seventeen males were entered into
the study.Their
ages ranged from 19 to 80 yrs.; mean ± SEM was 40.7 ±
1.3yrs. All patient information regarding
demographics, clinical presentation, antithyroid antibody
titers was placed in a database program, Access by
Microsoft.
Follow-up
The primary goals of the radioactive iodine therapy in
our office were to treat hyperthyroidism and prevent
recurrence by the induction of hypothyroidism.
Because of previous hyperthyroidism, the TSH
(thyroid stimulating hormone) often stays suppressed for
months after treatment. Therefore, in this study,
the TSH was not the only parameter used to determine when
a patient became hypothyroid. The free T4
value was used when it dropped below normal, the patient
responded to thyroid hormone replacement,and he or she
remained on replacement therapy with subsequently normal
thyroid hormone and TSH values. The free T4 and TSH
were determined in patients 4 to 8 weeks after treatment
and every 2-3 months until hypothyroidism occurred.Criteria for
diagnosing hypothyroidism were: 1) elevated TSH or 2) low
free T4 with the initiation of thyroid hormone
therapy and subsequent continuation of this therapy with
a normal TSH.
Results
Volume measurements in the thyroid ranged from 8.0 cc to
89.05 cc , (mean ±SEM 39.7 ± 0.14 cc). (see figure 1)
Estimation of the thyroid size by the endocrinologists
using palpation correlated strongly with the volume
measured by ultrasound (r = 0.82, see figure 2) The doses of radioactive iodine ranged from
7.16 mCi to 29.9 mCi; the mean ± SEM dose was
13.19mCi ± 0.5 mCi.Side-effects of the radioactive iodine were minimal.
Radiation thyroiditis did not occur in any of the treated
patients. One patient developed an exacerbation of
congestive heart failure two weeks after the treatment.
Out of the 74 patients who had adequate follow-up for at
least 6 months, 98.6% were successfully treated for
hyperthyroidism. 82.4% (#61) became hypothyroid and
16.2% (#13) became euthyroid. One patient had a
recurrence during the 6 month follow-up period and was
retreated with success. The average time to
hypothyroidism was 2.37 ± 0.14 months. (see figure 3)
Discussion
This study used thyroid ultrasonography to accurately
determine thyroid volume necessary for calculating the
radioactive iodine dose used to treat hyperthyroidism.
Physical examination proved to be reliable in correctly
estimating thyroid size in this study. However,
other studies have found a poorer correlation between
palpated thyroid size and ultrasound volume. Thyroid
palpation was found to overestimate the size of the
thyroid in screening adults and children for thyroid
disease (5,6) In another study, clinicians
underestimated goiters over 40cc. (7) although they did
well in estimating volumes under 40cc. In a study
by Langer (8) in which thyroid glands measured during
autopsies were compared with ultrasound and palpation
results. The results by palpation were the most
variable and least accurate. Training in the
palpation of the thyroid may improve the accuracy.
Ultrasound comparison to palpation was highly correlated
when the clinician was an endocrinologist or a resident
trained in thyroid palpation (9) Palpation may also
underestimate the number of nodules in the thyroid.
(10,11,12)
FIGURE 1: Thyroid Volumes by Ultrasound
FIGURE 2: Comparison of the size of the gland
estimated by the author versus the ultrasound volume.
In this study in which a thyroid scan was not done,
the ultrasound examination was also necessary not only
for the volume measurement but also for the detection of
non-palpable nodules to determine the etiology of the
hyperthyroidism as well as to alert the clinician to the
necessity of fine needle aspirate to determine the type
of nodule present. By using ultrasound
volume, successful results were obtained in 98.6 % of
patients and only 1 (1.4%) needed a second
treatment. The formula used in this study was originally
proposed by Brunn(3) using an ellipse as the
model for the lobe of the thyroid.Because the isthmus is
not included in the measurement, the volume formula
underestimates the total size by a small percentage.
Several studies have used ultrasonography
volume to measure the success of therapy for goiters and
nodules. Yamaguchi et al. (13) evaluated the effect of
antithyroid drugs on goiter size in
hyperthyroidism. Others (14) have correlated the
amount of thyroid stimulating antibodies with the size of
the goiter in Graves disease.
FIGURE 3: Percentage of Patients Becoming Hypothyroid
Over Time
Nygaard (15) followed the change in thyroid volume
after I131 treatment in both toxic and
nontoxic goiters and showed a decrease in size after
treatment over time.Studies
measuring nodule volume have evaluated the efficacy of
suppression therapy (16) by thyroxine and improvement of
thyroid cysts after ethanol or saline injection.
(17,18) These studies show a significant decrease
in size with the use of ethanol injection.
Studies (19,20) looking at normal thyroid volume have
shown a relationship between body weight and thyroid
size. The larger volume in males is related to
their larger body weight, therefore, accounting for the
sex difference in thyroid volume seen in the population.Another study (21)
surveyed the thyroid volumes of younger individuals and
also noted this sex difference.
Other ways of determining thyroid volume by using
ultrasonography have been proposed.Szebeni (22)
discovered that the most common formula (the one used in
this study ) using the ellipse model overestimated
thyroid volume by 20% compared to an integrated method
using a corrected ellipse formula. A more
complicated formula using ultrasound to measure thyroid
volume was used by Rasmussen and Hjorth. (23) The weight
of the thyroid has been correlated with autopsy studies
with cubic centimeters of volume compared to grams of
tissue as 1:1 ratio.(4)
Emrich (24) used ultrasound to measure the area of
hyperfunction in the thyroid by comparing the ultrasound
with thyroid scanning. In a similar study to this
one, Tsuruta (25) measured ultrasound volume in 13
patients prior to radioactive iodine treatment. The
follow-up interval was up to 3 years. Overall 60%
became euthyroid and 40% became hypothyroid. The
goals of treating hyperthyroidism vary from aiming
towards euthyroid status and repeating the radioactive
iodine treatment when relapse occurs versus
administrating a higher dose which leads to
hypothyroidism with no recurrence. Since thyroid
stimulating antibodies may cause the relapse of
hyperthyroidism if the majority of thyroid cells are not
destroyed, the philosophy of treating patients in this
article was the goal of causing hypothyroidism.
In this study of 89 patients, 82.4% became hypothyroid
after the 6-8 months of follow-up. Some
authors (26) have rejected the use of a formula to
determine the radioactive iodine dose. These
physicians opt for the same dose in all patients.
In patients with small glands, this dose leads a high
treatment success rate but larger glands usually are not
treated adequately. Others (27) have examined both
approaches and conclude that volume is the most crucial
factor in glands larger than 30 grams and should be taken
into consideration in determining thyroid radioiodine
dose. Most endocrinologists continue to take volume
into consideration when calculating radioactive iodine
dose.
In summary, this article has shown that thyroid
ultrasound volume measurement is an accurate and useful
way to determine thyroid volume prior to radioactive
iodine treatment. The formula: volume = pi/6 x
length times width x depth is a very simple, reliable way
to calculate thyroid volume and leads to excellent
results with radioactive iodine therapy.
Acknowledgements:
The author wishes to thank Tabitha Sapp for her
assistance in helping with the data collection and
analysis
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