RADIOIODINE UPTAKE BY THE THYROID GLAND

RADIOIODINE UPTAKE

The thyroid gland produces a variety of amino

acids all of which contain iodine. A small amount is

released by the thyroid each day for use by the body's

tissues but the bulk is stored in the gland. A very

great part of the total amount of iodine in the human

body is therefore contained in the thyroid. Each day

the thyroid gland accumulates from the blood about

that amount of iodine that is required to replace what

it releases, incorporated in amino acids (thyroid hor-

mones). If a small amount of radioactive iodine is

administered to a patient whose thyroid function the

physician wishes to study, it follows the same path as

ordinary iodine taken in with food and water; that is,

a part of the administered dose is accumulated by the

gland, another part is excreted via the kidneys and a

very small amount goes to other parts of the body.

Determination of that fraction of a radioiodine dose

which is taken up by the gland within a given time,

therefore, allows the physician to gain some insight

into the daily consumption of iodine by the gland and

hence its daily production of thyroid hormones.

The two most important, although not most com-

mon, diseases of the thyroid are associated with ei-

ther a higher than normal rate of hormone production

(hyperthyroidism or thyrotoxicosis) or a lower than

normal production (hypothyroidism or myxoedema).

Consequently, a high radioiodine uptake by the gland

is usually found in the former and a low uptake in the

latter condition. There are, of course,various other

means at the disposal of the physician to make a diag-

nosis of thyroid malfunction - e. g. observation of

clinical signs and symptoms, determination of the

basal metabolic rate and other tests - but the meas-

urement of thyroid radioiodine uptake provides the

most direct indication of abnormal hormone produc-

tion, particularly if it is combined with other radio-

iodine tests such as determination of the concentration

of hormonal radioiodine in the blood. The radioiodine

uptake is also usually found to be high in the most

common thyroid disorder, namely endemic goiter,

where the gland is frequently greatly enlarged without

increase in hormone production. Thus, the thyroid

radioiodine uptake test is now one of the most widely

practised medical applications of radioisotopes and,

because of its practical value, is usually the first

technique adopted by a newly established hospital

isotope laboratory.

Principle and Methods

The principle of uptake measurement is rela-

tively simple. The amount of gamma-radiation given

off by radioiodine which has been accumulated by the

thyroid at a certain time after its administration to

the patient is compared with the amount of gamma-

radiation emitted by the total dose of radioiodine con-

tained in a vessel known as the "standard". In the

BY THE THYROID GLAND

ideal case, both measurements should be done under

identical conditions, i.e. the relationship between the

radiation detector on the one hand and the thyroid

gland and the "standard" on the other hand should be

the same. However, complete duplication of these

conditions is impossible due to the complexity of the

relationship of the thyroid gland to the neck tissues

which surround it and because of individual differences

in this relationship between one patient and another.

Therefore, the main problem is to carry out the meas-

urement in such away as to reduce to a minimum the

errors resulting from the differences in the relation-

ship between the two sources and the detector and the

effects of individual variations between patients.

Since the first measurements of thyroid radio-

iodine uptake were carried out nearly twenty years

ago,

many laboratories have had their own ideas on

how the measurements should be made and have de-

veloped their own "house" methods. Nowadays, the

techniques vary widely from one country to another

and there does not appear to be a standard method

which would be acceptable to all workers in this field.

Consequently, it is difficult to compare the results

obtained and their value is often doubtful. In fact,

many laboratories have expressed doubt about the ac-

curacy of their own results due to lack of suitable

equipment with which their method could be calibrated.

In order to assist its Member States in the cal-

ibration and standardization of such measurements,

the International Atomic Energy Agency has started a

project under which amember of the Agency's scien-

tific

staff,

who has specialized in this work, is about

to begin a series of visits to different Member States

at their request. Using as calibration equipment a

dummy figure containing known amounts of "mock"

radioactive iodine (i. e. a radioactive substance with

radiation characteristics nearly identical to that of

1-131 but with a much longer half-life), this expert

will calibrate existing local apparatus for the meas-

urements, calculate correction factors where appro-

priate and suggest - if necessary - a standardized

method of measurement so as to ensure that the re-

sults obtained are comparable with those reached at

medical institutions in other countries.

Experts' Recommendations

Before embarking on this project, the Agency

sought the advice of a number of well-known special-

ists*

in this field on a suitable method of measure-

ment that could be accepted as a standard procedure.

* The experts were: Dr. G.F. Barnaby (UK), Dr. R. Hofer (Austria),

Dr.

Wolfgang

Horst (Federal Republic of

Germany),

Dr. L-G. Larsson

(Sweden),

Dr. D.A. Rose (VSA), Dr. W.K. Sinclair (USA), Dr. Ir. Ckr.

Sybesma(NetherlandaJ,Dr.N.G. Trott (UK) and

Dr.

TubianafFrance

From the Agency's Secretariat, Dr. a. Vetter served as

moderator and

Dr. G. G6mez-Crespo as secretary.

The experts met in Vienna in November 1960 and drew

up recommendations primarily with the object of

assisting the Agency in carrying out its calibration

project. They also expressed the hope that these

recommendations would be useful to both advanced and

developing countries since they would help in assess-

ing the suitability of existing equipment and proce-

dures and provide a guide to newcomers in the field

on at least one good method of measuring thyroid up-

take. The experts were, however, aware that several

other methods, if carefully employed, could produce

equally satisfactory results. Their object in recom-

mending a particular procedure was to establish a

relatively simple but reasonably accurate standard

method which could be used even under rather prim-

itive conditions. If inter-comparison of results could

be facilitated by the adoption of such a standard meth-

od,

there would be greater confidence in the published

results and a sounder basis would be found for com-

paring geographical variations in thyroid function.

The recommendations which are being published in

specialized scientific journals are briefly summar-

ized below.

To start with, it is stated that as a general prin-

ciple the amount of radioiodine administered to the

patient should not be larger than necessary, particu-

larly with children. With good equipment and standard

procedures, not more than ten microcuries of iodine-

131 need be given. Higher doses, however, may be

necessary in special circumstances.

The amount of radioiodine used in the "standard"

should be the same as the amount given to the patient,

and the volume of the "standard" should be comparable

to the volume of an average-sized thyroid gland. The

use of a neck phantom is also recommended, in order

to simulate as closely as possible the effects due to

the presence of neck tissues, other than the thyroid

gland, which are within the detector's field of vision.

This phantom should be aplastic cylinder of 15 cm in

diameter and 15 cm in height, with a hole to accept

the standard vessel. As for the detector, the use of

a scintillation counter is recommended but it is stated

that a Geiger-Mueller counter can also be used under

certain conditions.

The recommendations on operational procedure

relate to the optimum distance between the detector

and the source, the optimum time, the position of the

patient, the effects of background and scattered ra-

diation, and counting statistics. It is pointed out that

the optimum distance between the source and the

counter will depend on several variable factors, but

in most cases a distance of between 20 and 30 cm

would appear satisfactory, provided it is kept exactly

the same in both sets of measurements. The thyroid

uptake should preferably be determined after 24 hours

and in any case not within two hours of the administra-

tion of the tracer dose.

In order to minimize the contribution from back-

ground radiation and from extrathyroidal neck radio-

activity the group laid down detailed specifications for

shielding and collimation of the detector. Maximum

values for count rates, expressed as a percentage of

Dr. Gomez - Crespo, a member of the Agency's

scientific staff, who will visit a number of countries

for the standardization of thyroid uptake measure-

ments,

measuring the distance between the mock

thyroid of a dummy figure and the crystal of a

scintillation detector

the maximum count rate along the central axis, are

given for various points outside the field of vision of

the detector. It was recommended that scattered

radiation, i. e. all gamma-radiation from the source

which does not directly reach the detector, should be

excluded either by placing a lead filter of 1.5 mm

thickness in front of the detector or by properly set-

ting the electrical threshold.

Calibration Equipment

The calibration equipment which the Agency's

expert takes with him was put together in the Agency's

laboratories in Vienna. Its principal features follow

those first devised by the Medical Division of the Oak

Ridge Institute of Nuclear Studies in the United States

several years ago, with some important modifications.

A dummy figure of the upper part of a human body was

constructed using fiber glass, and the dummy was

filled with a small amount of "mock" radioactive io-

dine in order to simulate "body background", i. e. the

radiation which, under "live" conditions, is given off

by that part of the administered dose of 1-131 which,

at the time of measurement, has neither been accu-

mulated by the thyroid gland nor been excreted by the

kidneys but is present in other parts of the body or in

the circulating blood. In the place of the thyroid gland

there is space left in the neck of the dummy figure to

insert plastic containers. There are several such

containers which vary in size to simulate the sizes of

normal and enlarged thyroid glands and also vary with

respect to the amount of "mock" iodine they contain so

as to represent three different situations which might

be observed inhuman subjects: normal, high and low

The dummy figure placed in a specially constructed

suit-case in which it will be carried to different

countries. The travelling test kit also includes

interchangeable mock thyroids and a set of

" standards *

radioiodine uptake- Further, there are a number of

"standard" vessels of various sizes and shapes con-

taining amounts of "mock" iodine representing the

total dose administered to the "patient". Finally, a

plastic neck phantom and various filters are included

in the calibration set.

The actual amounts of "mock" iodine contained

in the various models of the thyroid and in the various

standard vessels were measured with great precision

in the standardization section of the Agency's labora-

tory, so that the ratio between the activity of the

"gland" and that of the "standard" is accurately known.

This allows each laboratory to test the accuracy of

their technique by carrying out an uptake measurement

on the dummy figure. If their technique is satisfac-

tory the results should correspond to the known ratio;

if small differences are found, appropriate correction

Dr. Gomez-Crespo demonstrating the interchange-

able mock thyroid in the dummy figure

factors can be calculated without modifying the tech-

nique substantially. If, however, the currently used

method proves to be unsatisfactory, the expert will

suggest the adoption of the standard method recom-

mended by the group mentioned above.

It is expected that the expert will stay in each

country for about one to two weeks, depending on the

number of laboratories he is asked to visit. So far,

nineteen countries have formally requested the

Agency's services and several others have expressed

interest. Visits of the expert will be arranged to

groups of countries in a particular region of the world

and the first tour has been scheduled for a small group

of European countries to allow the expert to return to

Vienna if experience should indicate the need for any

modifications of the equipment to be done before coun-

tries outside Europe are visited.