Biological Half life of Iodine in Normal and Athyroidic persons.


Gary H. Kramer, Barry M. Hauck and Michael C Chamberlain


†Human Monitoring Laboratory, Radiation Protection Bureau, 775 Brookfield Road, Ottawa, Ontario K1A 1C1, Canada (,


‡Division of Nuclear Medicine, Ottawa Civic Hospital, 1053 Carling Avenue, Ottawa, Ontario K1Y 4E9, Canada.



The biological half-life of iodine in the human thyroid might be expected to be, on average, an invarying quantity.  In 1959 the International Commission on Radiological Protection (ICRP) recommended that the biological half-life of iodine should be138 days (ICRP 1960).  This was revised downwards in 1978 to a value of 120 days (ICRP 1979) and yet again in 1989 to a value of 80 days (ICRP 1989).  Similarly, the amount of iodine retained by the thyroid has not changed from 0.3 over this period.

            The metabolism of iodine by an adult according to the ICRP (ICRP 1989) is that 0.3 of the initial intake is taken up by the thyroid and 20% goes to faecal excretion, the biological half times are: blood, 0.25 d; thyroid, 80 d; rest of the body, 12 d.  The recycling of iodine can be best be described by a two compartment model but this is not seen when using 131I as the short radiological half-life precludes the resolution of the two compartments.

            According to this model, person without a thyroid gland would be expected to excrete all iodine immediately, or at least within a very short period of time as there is some iodine retention by the salivary glands.  There are two main types of salivary glands - the major salivary glands and minor salivary glands. The three types of major salivary glands are the parotid glands, submandibular glands, and sublingual glands. There are two of each type - one on the left side and the other on the right. The parotid glands are the largest salivary glands and are found on each side of the face, just in front of the ears. They overlie the jaw joint and would not contribute many photons to a detector placed in front of the thyroid gland.

            The submandibular glands are the next largest salivary glands and are found on either side of the neck, under the chin and tongue area. The sublingual glands are found deeper in the neck than the submandibular glands, under either side of the tongue. There are about 600-1,000 minor salivary glands, which are too small to see without a microscope. These minor salivary glands are located beneath the lining of the lips, tongue, hard and soft palate, inside the cheeks, nose, sinuses, and voicebox.  These glands are closer to a detector placed in front of the thyroid gland, especially if it has a large diameter, and could contribute photons.  However, as the biological half life of the salivary gland is approximately 10 hours (Nishizawa et al. 1985) one would expect the athyroidic subjects’ retention to be governed by this pathway

             In 1996 the Human Monitoring Laboratory (HML) and the Ottawa Civic Hospital collaborated to measure the biological retention of iodine in normal and athyroidic patient by sequential measurements of the thyroid or whole body retention of 131I.  The advantages of the two organisations cooperating to perform this study was that no person received an unnecessary exposure to radioactive materials as all the participants in this study would have received the diagnosis or therapeutic doses of 131I regardless of their participation.  The disadvantages were that the age/gender mix of the subject pool could not be pre-determined.

            The study commenced in March 97 and finished in December 1999.  The results have been compared to the ICRP recommendations and the ICRP metabolic models.



            Volunteers: The subjects participating in this study were solicited at the Ottawa Civic Hospital by the nursing staff following either a diagnostic or therapeutic administration of 131I.  Each volunteer came to the HML for up to six counts spanning up to six weeks.   Volunteers were compensated for travelling expenses for each visit to the HML and each completed a consent form prior to the first count.

            Low Background Counting Chamber:  The low background counting chamber which houses the whole body/thyroid counter was constructed in 1959 by the  Dominion Bridge Company using material supplied by the Steel Company of Canada. Prior to construction, samples of steel were sent to the University of Toronto, Physics Department, to test for radioactive contamination. Evidence of some contamination (mostly 137Cs and 60Co) was found that was attributed to radioactive fallout from atomic bomb testing in the 1940's and 50's. The chamber was installed in the Radiation Protection Bureau in 1960 and have been used in the Human Monitoring Program of the Health Canada for 40 years.

            The thickness of each chamber wall, floor, and ceiling is 0.2 m, and the approximate weight of the chamber is 100 tons. The wall thickness is sufficient to reduce the gamma rays from naturally occurring radioactivity in the surrounding building materials by a factor of about 1000, and the cosmic rays to about 60% of their unshielded intensity. The inner surfaces of the rooms are covered by 6.3 mm of lead which reduces the background, below 0.1 MeV, by a factor of two.

        The inside dimensions of the chamber is 1.52m by 2.13m by 2.13m. The chamber is equipped with double doors operated by electric motors controlled from the laboratory. There is a second control in the chamber which can be used to open the doors from inside in case of emergency. An intercom is also provided for communication between subject and operator, as well as music to relieve the tedium of lengthy counting periods. Subjects may also be viewed through a large water filled window of dimensions 0.3m by 0.46m by 0.6m wide.

            Detector systems:  The whole body counter is equipped with six NaI(Tl) detectors combined in two triangular arrays.  The upper array consists of three detectors scan above and the lower array consists of three detectors scan below the subject.  The upper array is on a moveable arm that can be raised from the bed surface to the roof of the counting chamber.  The lower array is in a fixed geometry 12 cm below the bed.

            Each detector array is powered by an independent high voltage supply.  The signal from each detector is processed by a preamplifier.  The three for the upper array are connected to a dual sum invert amplifier and the other three for the lower array to another.   These modules sum the incoming signals into single signals.  The signals are then processed by two separate amplifiers that are connected to a multichannel buffer.  Spectral analysis is performed on a computer using using EG&G’s GDR software custom modified for the HML

            Each detector in the upper and lower arrays is a cylindrical NaI(Tl) crystal that is nominally 12.7 cm in diameter and 10.2 cm high.  The crystal is optically coupled to a low background photomuliplier tube.  The outer casing of the detector is stainless steel 304 (Fe, 70%; Cr, 19%; Ni,11%; specific gravity, 8.02) which is 0.635 mm thick.  The transmission of photons through the outer casing at 100 keV 83% rising to 97% at 1000 keV.

            Only one detector of the upper array is used for thyroid counting, the other two being removed from the array by disconnecting the signal cables.  Normally the detector is placed centred over the supine subject’s thyroid gland at a distance of 14 cm.  However, some of the subjects had such high activities that requiring the detector had to be raised to reduce the dead time to manageable levels.

            The detector arrays scan the subject for normal whole body counting, but the scanning detector geometry can also identify the location of a radionuclide that is not homogeneously distributed in a person (or phantom) using the Multi Channel Scaling (MCS) mode of the Whole Body Counter. The latter mode was used in conjunction with the thyroid count mode.

            Counting Efficiency: The counting efficiency for 131I thyroid counting was determined using the BRMD thyroid phantom (Kramer et al 1996a, 1996b) placed as the neck section of a Reference Man BOMAB phantom (Kramer et al 1991) using 133Ba as a surrogate for 131I.  The thyroid counter was also calibrated using a BOMAB phantom containing 133Ba distributed homogeneously throughout the phantom.  The efficiency was determined at the normal counting distance (14 cm) and at larger distances.

            Counting Protocol: All subjects were measured in the whole body counter in a supine position. The first count was a whole body count with the detectors scanning over the subject.  The counts were acquired in multi channel scaling mode simultaneously with the normal acquisition.  The second count was a thyroid count where the subject remained supine but also extended the neck to raise the thyroid gland above the collar bones. 

            The counting regime for diagnostic patients was begun within a week of the initial administration of 131I with repeat counts being performed at approximately 7 day intervals; however, the regime for therapeutic patients had to commence as quickly as possible to measure the rapidly excreted iodine.  Repeat counts were daily (where possible) for the first three to four and the last counts were at a weekly interval.  The detector array was raised for these patients.

            Half-Life Determination: The effective half life is given by:




where 8eff is the effective decay rate (d-1), 8rad is the radioactive decay rate (d-1), and 8biol is the biological decay rate (d-1).  8eff is obtained from the linear regression of Ln(thyroid activity) as a function of time,  8rad is 8.03 days (reference) and so 8biol can be obtained from Eqn. 1.  The biological half-life is then obtained from:





where T½is the half-life (d) and 8 is the decay rate (d-1)



            Volunteers: Diagnostic patients were given about 400 kBq.  The therapeutic patients are in two groups: the higher amount of 131I is for treatment (150 GBq), the lower amount (150 Mbq) of 131I is for confirmation treatment was successful.

            Biological Half-Life: The 131I retention as a function of time can be described by a single compartment and this was observed by all the normal subjects measured at the HML.  By contrast, the athyroidic patients showed a different pattern of retention.  This is surprising on two counts, first that there was any retention at all, and secondly because this retention is described by a two compartment model.

            The data shows that the biological half-lives of the normal patients varied from 11.4 to over 4,000 days.  Similarly the uptake values vary from 3% to 63%.  Despite the fact that all these subjects received 131I as a part of a medical diagnosis subsequent evaluation of their condition showed them all to be normal.  Therefore, they are considered to representative of a normal healthy North American population.

            The athyroidic patients fall into two distinct uptake groups.  These correspond to the subjects who received sufficient 131I to ablate the thyroid gland and the other group, with the slightly higher uptake values, those subjects who received a confirmatory amount of 131I that the ablation (previously done) was successful.  The short term retention is probably due to salivary gland retention, but the mechanism of the longer term compartment is unknown.

            Table 1 summarises all the data.  Subject MBH   who has an unusually long biological half-life for iodine has been eliminated from the data set as an outlier and is not considered in the subsequent statistical analysis.

            Table 1shows that the average biological half life is 57.3 " 5.4 days.  Direct comparison with the ICRP value of 80 days is difficult as there are no uncertainties associated with this value.  A comparison can be made if it is assumed that the uncertainty on the ICRP value is similar to the one obtained in this study.  Performing a t-test, to test the null hypothesis that there is no difference between the two values, one obtains a t-value of 10.86 (t-crit = 2.33, P = 0.000) strongly suggesting that the null hypothesis be rejected.  It is likely that the decrease in biological half-life of the North American group that participated in this study is a continuation of the trend identified above and is likely due to changes in food and its additives.  Iodine is now plentiful in the diet and the body has no reason the retain the material for long periods of time.  Similarly the average fractional uptake of the thyroid seems to have declined from the ICRP recommended value of 0.3 to 0.22 " 0.02.

            Retention model: Based on the findings of this study the retention model of iodine can be written as:







            This study has shown that the biological half life of iodine seems to have decreased since the ICRP recommended 80 days.  Changing the half life to the value of 57 days and the uptake to 0.22 will not have much effect on the dose delivered by 131I as the dominant removal pathway is the radioactive decay (8.04 day half-life); however, for 125I and other long lived isotopes it may be a dose reduction consideration.



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International Commission on Radiological Protection.  Limits for intakes of radionuclides by workers.  Oxford:  Pergammon Press; ICRP Publication No. 30, Part I; 1979.


International Commission on Radiological Protection.  Age dependent doses to members of the public from intakes of radionuclides: Part 2 ingestion dose coefficients.  Oxford:  Pergammon Press; ICRP Publication No. 67, Part 2; 1993.


Kramer G.H., Noel L. and Burns L.C.  The BRMD BOMAB Family.   Health Physics, 61(6): 895-902; 1991.


Kramer, G.H., Olender G., Vlahovich S., Hauck B.M., Meyerhof D.P.   Comparison of the ANSI, RSD, KKH and BRMD Thyroid-Neck Phantoms for 125I thyroid monitoring.  Health Physics 70(3):425-429; 1996a.


Kramer, G.H., Gamarnik, K.,Noël, L., Burns, L.C.,Meyerhof, D.  The BRMD Thyroid-Neck Phantom:  Design and Construction.  Health Physics 71(2): 211-214; 1996b.


Nishizawa, K.; Hamada, N.; Sadayuki, S.  In Vitro monitoring of salivary 125I.  Health Phys. 49(2): 290-295; 1985.



Table 1 Statistical summary of the biological half lives for all subjects


Normal T½

Athyroidic short T½

Athyroidic long T½