Abstract
Although radioactive iodide uptake (RAIU) is one of the reliable diagnostic methods for thyroid function in adult humans, especially in the diagnosis of thyrotoxicosis, there are limited data for RAIU during pregnancy and lactation in humans and animals. Therefore, we proposed to validate RAIU for the lactating rhesus monkey to further human model studies in thyroid disease. RAIU was performed at 6 and 24 h using 100 µCi of 123I orally in four lactating monkeys. The thyroid and thigh were counted using a scintillation probe and multichannel analyser. A dose/standard ratio of counts/minute was calculated to compensate for background, utilizing differences in the activity between the dose administered and a standard. Thyroidal RAIU varied significantly among monkeys: 6.71 ± 2.40% for the 6 h uptake and 15.44 ± 7.71% for the 24 h uptake. These data showed that the RAIU test may allow a rational clinical approach to thyroid function testing for lactating rhesus monkeys. Additional studies are needed for assessing thyroid function in rhesus monkeys of varying ages and gender with clinical abnormalities.
Radioactive iodide uptake (RAIU) is a test that directly assesses the level of gland activity and integrity of hormone biosynthesis and is commonly used for the evaluation of thyroid function and the diagnosis of thyroid diseases in adult humans especially for the differential diagnosis of thyrotoxicosis. It is well known that the percentage of iodide uptake varies according to iodine status. The standard 123I RAIU test is usually performed at 24 h after oral administration. An early uptake (2–6 h) value has also been found useful in the evaluation of human patients with increased hormone turnover in whom the 24 h uptake may be normal, but early uptake is elevated. 1
Rhesus monkeys (Macaca mulatta) have been used extensively as a non-human primate model to study many aspects of pregnancy, lactation and postnatal growth period including controlled alterations to the maternal environment. 2,3 Rhesus monkeys are considered one of the best animal models for thyroid function assessment, particularly due to similarities between human infants and monkeys. 2
The RAIU test measures how much radioactive iodine is accumulated by the thyroid gland after various intervals of time using a gamma scintillation counter. Two radioisotopes of iodine, 123I and 131I, are commonly used for diagnosis of thyroid disease. For diagnostic purposes, 123I (13 h half-life and 159 keV gamma emission) is the radionuclide of choice because it delivers significantly less radiation dose to the thyroid than 131I. 123I is available as a capsule or solution with high specific activity and is almost carrier free. The measurement of radioactive uptake in the thyroid is the most widely used test in human clinical nuclear medicine laboratories.
For the evaluation of thyroid anatomy and function, 123I scan is regarded as more reliable than 99 mTcO4 −. 4 The attributes of 123I should make its use especially attractive in pregnant and breastfeeding women, newborns, and infants for thyroid scan and uptake. Thyroid scan (morphology) and uptake (function) are both accomplished after a single dose of 123I for better diagnostic accuracy. Radionuclide imaging in the investigation of congenital hypothyroidism is of value in determining the presence and location of functioning thyroid tissue. 5 Even the relatively simple technique of neck counting after radioactive iodide administration is capable of showing that non-goitrous, non-endemic hypothyroid infants and children have residual thyroid tissue. 6 Recently, radioiodine uptake has been used as one of the main prognostic factors for success of radioiodine therapy in patients with differentiated thyroid cancer. 7
123I is generally administered orally because the iodide ion is readily absorbed from the stomach. By 24 h, about 75% of an intravenous injected dose is excreted, and 15% is taken up by the thyroid. In hypothyroidism, the thyroid takes up little (sometimes less than 1% to 2%) of the iodide with excretion often greater than 95%. 8
Although RAIU is one of the most important diagnostic methods for thyroid function in adult humans today, 9 there are limited data for RAIU in laboratory animals especially for breastfeeding females. Therefore, we proposed to validate the RAIU for the lactating rhesus monkey in order to further human model studies.
Materials and methods
Materials
Sodium iodide (Na 123I) for diagnostic use was supplied in capsules for oral administration in the dosage of 100 μCi/capsule (Cardinal Health, Sacramento, CA, USA).
Subjects
Subjects were four lactating rhesus monkeys (Macaca mulatta), aged 12.5 ± 1.4 years old and 7.58 ± 0.95 kg in weight. The rhesus monkeys were born and raised at the California National Primate Research Center of the University of California. Rhesus monkeys were socially housed indoors in individual cages under a 12 h light:12 h dark cycle, and temperature range of 21–25°C. They were fed monkey pellets (Ralston-Purina, St Louis, MO, USA), available ad libitum, and fresh fruit once daily, and had continuous free access to water. All monkeys used in the study were initially examined by a veterinarian. All experiments were performed according to a protocol approved by the Institutional Animal Care and Use Committee in compliance with all NIH standards.
Thyroid RAIU measurement
Before RAIU measurement in the monkeys, we validated the counting method. Measurements for 57Co and 123I were made at various crystal-to-phantom distances to determine the effect of these variables on measurement of the source activity. Also, the effect of the spectrometer window setting was tested by counting a source both in air and in a neck phantom. A range of energies was tested. Various combinations of window widths were tested for the standard and background to maximize sensitivity for a given crystal geometry. We determined the proper window setting on the spectrometer so that as many acceptable pulses as possible would be received and counted by the analyser system in order to reduce the background and increase count rate. The range of 0.2 MeV counting windows was optimized to provide maximum net counts of 123I. Net standard counts were determined by placing the 123I dose in a neck phantom prior to administration. The animal was placed on the table, and a scintillation probe with collimator was placed over the thyroid, 4.5 cm from the surface in order to measure and record the cpm over the thyroid. The sensitivity of the counting system for 123I was optimized for using a set distance from the thyroid gland. The counting geometry was consistent for every measurement on every monkey. The thyroid and thigh were counted for one minute each in duplicate. The cpm over the thyroid was adjusted for radioactive decay of the ingested 123I since time zero. Body background activity was determined by counting the thigh.
Calculation:
RAIU were performed at 6 and 24 h using a scintillation probe with a 5 cm × 5 cm NaI (TI) crystal. RAIU was conducted under light intramuscular ketamine (5 mg/kg) and medetomidine (30 μg/kg intramuscularly) sedation. While sedated, a nasogastric tube was passed into the stomach of each monkey. The content of an 123I capsule was dissolved in 2 mL distilled water to produce a solution having specific activity of approximately 100 μCi/2 mL which was loaded into a syringe. After administration of the initial bolus, the tube was flushed three times with 2 mL of water which was also administered. Background radiation was counted, and the multichannel analyser (Picker Spectroscaler 4R) was calibrated (using a 57Co rod source) for one minute, which was followed by a standardizing count of the same volume and activity as the dose given to the monkey. Oral 123I administration was performed at 09:00 h. RAIU was measured at 15:00 h on the same day of 123I application and at 09:00 h on the following morning. Examples of the background and standard counting rates were 384 and 948,047 cpm, respectively. The background was insignificant because background was smaller than the square root of 948,047 (=973). 10
Thyroid hormone analysis
The following thyroid function tests were validated and performed for each monkey: serum thyroxine (T4), triiodothyronine (T3) and thyrotropin (TSH). These serum hormones were measured using competitive chemiluminescent assays. The TSH assay was a two-site sandwich chemiluminescent immunoassay using direct chemiluminometric technology. All assays utilized the Bayer ACS-180 platform (Bayer Healthcare, Tarrytown, NY, USA). All samples were analysed in duplicate. Inter-assay and intra-assay coefficients of variance were the following: T4: 3.2% and 4.4%; T3: 2.5% and 5.8%; and TSH: 3.6% and 4.3%, respectively. All analyses were conducted by the Endocrine Core Laboratory at the CNPRC.
Data analysis
Results are expressed as means and standard deviations (SD).
Results
The mean thyroid radioiodine uptake for the monkeys was 4.0–9.7% for the 6 h uptake and 8.7–25.5% for the 24 h uptake. Table 1 gives detailed information for the 6 and 24 h uptakes. Baseline thyroid function tests, T3, T4 and TSH hormone levels, were in the reference range of rhesus monkeys (Table 2).
RAIU (%) in healthy lactating rhesus monkeys at the CNPRC
RAIU = radioactive iodide uptake
Serum TSH, T3 and T4 concentrations in healthy lactating rhesus monkeys at the CNPRC
T4 = serum thyroxine; T3 = triiodothyronine; TSH = thyrotropin
Discussion
RAIU from different references and different species vary considerably, and variations in normal thyroid function have been reported in animal species and humans 11–16 (Table 3). RAIU is affected by the method of application of radioiodine. This variation is especially marked in children and animals because of variation in the size of the thyroid gland. The net count rate obtained from a radioactive source is governed by many factors including the probe voltage, resolving time, geometry, scatter, energy resolution, absorption, background, efficiency of counting and collimation. 10,17–19 Thyroidal radioiodine uptake at baseline varied among lactating monkeys between 4.0% and 9.68% for the 6 h uptake and between 8.65% and 25.52% for the 24 h uptake (Table 1).
% RAIU measurement in human and animals
NE = not explained; RAIU = radioactive iodide uptake
*Four hours
†Eight hours
‡SE
Generally, for domestic animals, % RAIU is 4–19 at 24 h, 8 and for humans is 7–35 at 24 h. 10 Our results are in agreement with previous reports in adult humans, 4,14,15 children, 20 domestic animals 8 and rhesus monkey. 16 In the RAIU of a study in children, which was validated with 123I using a multichannel analyser, 17 the crystal distance from the surface was 5 cm which is similar to the distance in the current study (4.5 cm). Usually, this distance is 18–25 cm in the adult human. In the current study, RAIU was not significantly different from a previous rhesus monkey study. 16 However, 123I was given intravenously in that study versus orally in the current study, and the female rhesus monkeys in that study were not lactating, whereas the monkeys in the current study were lactating. Furthermore, the counting technique in that study was not disclosed.
Although 123I RAIU has been used for many years 7,21,22 without evidence of risk for thyroid cancer, today some clinicians and veterinarians still hesitate to recommend RAIU for neonates, children and animals because of concern about radioisotope exposure. The radioactivity exposure to thyroid gland which is generated by 123I is only 1% of 131I 23 and its half-life is 13.5 h. As recommended by Leonard Berlin, 24 we also believe that RAIU is safe and important for the diagnosis of thyroid diseases. It uses the same dose of radioactive iodine as scintigraphy, 25 but has the advantage of indicating the extent to which the thyroid is functioning. Especially, 123I RAIU may be attractive for neonate and children. In addition, 123I RAIU may help define underlying genetic factors and mechanisms of thyroid development and differentiation.
Despite data limitations for using RAIU in children and domestic animals, 123I thyroid uptake is a clinically important procedure which may maximize the information available for the non-human primate model in further study of many aspects of pregnancy, lactation and growth including controlled alterations to the maternal environment. 2,3 RAIU may also provide insight into clinical and genetic correlation of thyroid diseases in laboratory animals as it does for adult humans. RAIU may also be useful for newborn monkeys, as several mental retardation and growth failure syndromes can be prevented, since the monkey is considered the best model of thyroid function assessment, 2,26–28 fetal brain development 29 due to similarities between human infants and monkeys during the course of thyroid gland and brain development and behavioural maturation. RAIU can also serve in the differential diagnosis of hyperthyroidism and for estimation of therapeutic doses in animals.
In conclusion, these data showed that the RAIU test may allow a rational clinical approach to thyroid function testing for lactating rhesus monkeys. Additional studies are needed for assessing thyroid function in lactating rhesus monkeys of varying ages with clinical abnormalities.
Footnotes
Acknowledgements
The authors would like to thank Sarah Davis and Toni Traill for their help in handling the primates and Marjorie Ahl for her production assistance. This study was funded by the Western Institute for Food Safety and Security (WIFSS).