Reference values for thyroid uptake of technetium-99m pertechnetate for the Namibian population

Abstract

Thyroid physiology and structure can be evaluated by scintigraphic imaging. Iodine-131, discovered in the late thirties was the first radioisotope to be used for thyroid uptake assessment, since then thyroid uptake and imaging continues to play a vital role in different thyroid related clinical situations. Because Iodine-131 has serious disadvantages related to high radiation dose, it has been limited to staging and follow up of patients with thyroid carcinomas. Its substitute Iodine -123 has proved to be suitable in terms of shorter half-life and better imaging energy. Technetium-99m pertechnetate, because of its availability is also used for thyroid imaging and uptake assessment. The similarity of the Technetium-99m pertechnetate ions and iodide is the explanation for the thyroid glands ability to absorb it. Historically, chronological changes in the normal values of thyroid uptake normal values have been reported in different geographical areas. These changes have been linked to geographical and chronological fluctuations in dietary iodine intake in different populations. Technetium-99m pertechnetate, Iodine-131 and Iodine-123, all reflect thyroid iodide accumulation. In areas where there is iodine deprivation there will be elevated radioisotope uptake in euthyroid persons, and radiotracer uptake is even higher in areas of Iodine deficiency. Conversely in areas where a population is exposed to stable iodine and abundant amount of iodine intake from other sources, radiotracer uptake will be decreased. Namibia is a country with mixed ethnicity consisting mainly of Africans (Hereros, Ovahimba, Kavangos, Caprivians, Ovambo, Damaras, Namas, Bushmen, Coloureds and Caucasians). The population has access to dietary iodine and all ethnic groups consume reasonable amounts of food containing iodine such as bread, dairy products, table salt and sea food. Similar to other Nuclear Medicine departments worldwide, Namibia makes use of Technetium-99m pertechnetate for thyroid uptake and imaging. However thyroid uptake quantification needs to be related to normal values as a point of reference. Despite reports of thyroid uptake fluctuations of normal values and reports emphasising the importance of periodic checks on the thyroid uptake normal values, Namibia has never determined these reference values. This study examines the fundamental statement posed in the hypothesis that the thyroid uptake reference values for Technetium-99m pertechnetate in a Namibian population deviate from available International normal reference values. Eighty three participants considered to be euthyroid from Windhoek, Namibia received Technetium-99 m pertechnetate as part of their evaluation. The euthyroid state was based on a combined evaluation of clinical history, palpation of the thyroid gland and assessment of thyroid hormones (TSH, T3 and T4).

The objectives of the study were:  To establish normal reference values of Technetium-99m pertechnetate uptake in euthyroid persons in Namibia.  To compare the outcome reference values to current available existing International normal reference values.  To determine possible factors contributing to the deviation of thyroid uptake reference values in the Namibian population.  To recommend reference values for Namibia. This was achieved by studying the unmedicated 83 participants with their iodine diet, rather than patients with thyroid or other diseases. The participants reported to the Nuclear Medicine department. The participants completed a questionnaire designed to reveal the presence of thyroid pathologies or any other pathology. The completed questionnaire was reviewed and if the participant fulfilled the requirements for the study, blood was drawn for thyroid hormone assessment. The participants were then given 100MBq Technetium-99m pertechnetate intravenously. Using a low energy high resolution dual head gamma camera, the thyroid uptake was recorded at 20 minutes. There was a marked difference between the International normal values (0.75% to 4%) used in the Nuclear Medicine department and those found in this study. The results of seven subjects were excluded due to abnormal blood results, and technical errors, bringing the actual sample size to 76 euthyroid participants. The age range was 39 to 81 years, and there were 58 females and 18 males. The mean uptake for the euthyroid group was found to be 0.78%± 0.45%. The Kolmogorov-SmirnovaLilliefors Significance parametric test presented us with results that state that our population has a non-normal distribution, and therefore the standard deviation should not be applied to determine the normal values. The statistical test for skewness and kurtosis was adopted in this study to correct for skewness and the normal values determined were 0.35% to 1.22% (95.5% CI) of Technetium-99m pertechnetate at 20 minutes. An alternative method used to determine the normal reference range for Technetium-99m pertechnetate in a study sample obtained from a non- normal distribution was determined using the 5th and the 95th percentiles .The normal reference range using the 5th and the 95th percentile was 0.17% - 1.7%. This study provides new evidence supporting the importance of periodic checks on normal reference values. The evidence is provided by the analysis of the empirical data obtained in this study of a population that has sufficient daily intake of stable iodine. The results obtained from this study and other studies proving the difference in thyroid uptake between different populations and geographical regions makes it incumbent upon laboratories to establish a normal reference range for a particular geographical location. Furthermore the study attests to the importance of re-evaluating normal values for thyroid uptake as part of a quality assurance programme.