Parathyroid Changes After RAI in Patients With Differentiated Thyroid Carcinoma.

Objective: The purpose of this study was to investigate parathyroid hormone (PTH), serum calcium, phosphorus, and 25-hydroxyvitamin D (25-OH-VD) changes before and after radioactive iodine (RAI) in differentiated thyroid carcinoma (DTC) patients at different time points. Methods: A total of 259 DTC patients who received RAI were prospectively enrolled. We evaluated PTH, serum calcium, phosphorus, and 25-OH-VD levels at baseline pre-RAI, five days, six weeks, and six months post-RAI, respectively. We analyzed the risk factors of hypocalcemia at five days post-RAI. Results: The mean PTH, serum calcium and phosphorus values decreased five days post-RAI compared with pre-RAI (PTH 4.18 ± 1.23 pmol/L vs. 3.95 ± 1.41 pmol/L; calcium 2.27 ± 0.09 mmol/L vs. 2.20 ± 0.11 mmol/L; phosphorus 1.25 ± 0.17 vs. 0.98 ± 0.20 mmol/L, P < 0.05), and the differences were statistically significant. The mean 25-OH-VD levels did not significantly decrease at five days post-RAI. 21.2% (55/259) of patients had hypocalcemia at five days post-RAI, and all of them were given oral calcium supplements. At six weeks post-RAI, all of the above parameters were higher than those at five days post-RAI. Multivariate regression analysis showed that baseline pre-RAI serum calcium < 2.27 mmol/L, PTH < 4.18 pmol/L and negative 99mTcO4- thyroid imaging were risk factors for hypocalcemia at five days post-RAI. Conclusion: For DTC patients with normal PTH and serum calcium levels at pre-RAI, their PTH, serum calcium, and phosphorus levels decreased at five days post-RAI. About one-fifth of patients could have hypocalcemia at five days post-RAI. Lower baseline pre-RAI serum calcium and PTH levels and negative 99mTcO4- thyroid imaging were risk factors for hypocalcemia five days post-RAI.

Clinical Study on Safety and Efficacy of Microwave Ablation for Primary Hyperparathyroidism.

OBJECTIVE: To evaluate the safety, feasibility, and efficacy of microwave ablation (MWA) for the treatment of primary hyperparathyroidism (PHPT). MATERIALS AND METHODS: This study enrolled 67 PHPT patients (22 men, 45 women; mean age, 56.0 ± 16.3 years; range, 18-83 years) from January 2015 to December 2018. The laboratory data, including the serum intact parathyroid hormone (iPTH), calcium, phosphorus, and alkaline phosphatase (ALP) levels, were evaluated before MWA and again 2 hours, 1 day, 7 days, 1 month, 3 months, 6 months, 12 months, 18 months, and 24 months after. RESULTS: Complete ablation was achieved with all 72 hyperplastic parathyroid glands found on the 67 patients enrolled, 64 of whom were treated in one session and 3 were treated over two sessions. The technical success rate was 100%. The median follow-up time was 13.6 months (range, 10.0-31.1 months). The clinical success rate was 89.4%. The volume reduction rate was 79.4% at 6 months. Compared to pre-MWA, the serum iPTH, calcium, phosphorus, and ALP levels had significantly improved 6 months post-MWA (iPTH, 157.3 pg/mL vs. 39.2 pg/mL; calcium, 2.75 ± 0.25 mmol/L vs. 2.34 ± 0.15 mmol/L; phosphorus, 0.86 ± 0.20 mmol/L vs. 1.12 ± 0.22 mmol/L; ALP, 79 U/L vs. 54 U/L, respectively; all, p < 0.01). Hoarseness was a major complication in 4 patients (6.0%), but it improved spontaneously within 2-3 months. CONCLUSION: MWA is safe, feasible, and effective for the treatment of PHPT.

Near-infrared irradiation of the thyroid area: effects on weight development and thyroid and parathyroid secretory patterns.

This study evaluates possible changes in weight and the secretory patterns of the thyroid and parathyroid glands irradiated with a 780-nm light-emitting diode (LED) source under conditions allowing their identification in an animal model. The use of near-infrared light sources to detect the parathyroid glands has been a subject of research due to the benefit provided to patients undergoing thyroid and parathyroid surgery. However, few studies have explored possible changes in weight and the secretory patterns of the glands when subjected to light stimulation. Thirty-two male Wistar rats were divided into two groups and subjected to cervical dissection and irradiation of the thyroid-parathyroid region under general anesthesia. The animals in group 1 (irradiated) were exposed to a 780-nm LED light beam for 180 s (dose 1.37 J/cm2), whereas the animals in group 2 (control) were not irradiated. Blood samples were collected pre-exposure, 7 min after exposure, and 30 and 60 days after exposure to measure calcium, parathyroid hormone (PTH), triiodothyronine (T3), thyroxine (T4), and thyroid-stimulating hormone (TSH) levels in both groups. Weight variations between the evaluation periods were also analyzed. Parametric statistics were used (Student's t test and ANOVA) after assuming normal distribution of continuous variables with the Shapiro-Wilk test. No significant variation was observed in any of the analyzed parameters pre- and postexposure. A significant increase in the calcium level was observed at 30 days in the irradiated group compared with that in the control group (11.156 ± 1.31 mg/dl vs 10.300 ± 0.30 mg/dl; df = 30 p < 0.03) but this change was no longer observed at 60 days (9.925 ± 0.23 mg/dl vs 10.014 ± 0.18 mg/dl; df = 14 p = 0.21). Irradiated rats gained less weight in the first 30 days after surgery (39.647 ± 32.39 vs 55.917 ± 22.61; df = 30 p = 0.146) and at 60 days (84.000 ± 27.36 vs 84.571 ± 5.62; df = 14 p = 0.957), no differences were observed between the two groups. No significant variations in weight development or changes in the secretory patterns of the thyroid and parathyroid glands exposed to near-infrared stimulation were observed, confirming the safety of this light source in the identification of the parathyroid glands.

Microwave ablation of hyperplastic parathyroid glands is a treatment option for end-stage renal disease patients ineligible for surgical resection.

BACKGROUND: Secondary hyperparathyroidism (SHPT) is a frequently encountered problem in patients with end-stage renal disease (ESRD). Some patients with severe SHPT could not be managed by medical treatment and are ineligible for surgical resection. PURPOSE: Our objective was to evaluate the efficacy, safety of microwave ablation (MWA) on these patients. MATERIALS AND METHODS: Between 1 April 2015 and 28 February 2017, 35 patients (M/F 19/16, age 49.8 ± 12.9 years) were enrolled. All patients were treated with MWA. Levels of intact parathyroid hormone (iPTH) and of serum calcium and phosphorus were compared pre- and post-ablation. Repeated-measures ANOVA was used to compare treatment outcomes pre- and post-ablation. RESULTS: Complete ablation was achieved in all 63 glands in the 35 patients with SHPT. The mean follow-up time was 15.9 ± 2.2 months. The maximum gland diameter was 6-31 mm (mean, 14.9 ± 5.5 mm). The trends of the changes in iPTH and calcium levels showed a curve: the level of iPTH and calcium at 6 months post-ablation were lower than those pre-ablation (both p < .0001); after then iPTH remained relatively stable and the end of follow up, with no rebound (p < .0001), while instead of calcium at the end of follow up was not significantly lower than pre-ablation (p = .462). The trend in the change in phosphate levels showed a straight line; the level of phosphate at 6 months post-ablation and at the end of follow up both were significantly lower than pre-MWA (p < .001). There was no major complication. CONCLUSIONS: In this series, MWA was used successfully to treat SHPT patients who are ineligible for surgical resection.

Pilot study of parathyroid glands in adult and pediatric subjects exposed to ionizing radiation after the ChNPP accident, methodology of parathyroid diagnostic ultrasound. / PILOTNE DOSLIDZhENNIa STANU PRYShchYTOPODIBNYKh ZALOZ OSIB, OPROMINENYKh VNASLIDOK AVARIÏ NA ChAES DOROSLOGO TA DYTIaChOGO VIKU, METODOLOGIIa ÏKh UL'TRAZVUKOVOGO DOSLIDZhENNIa.

OBJECTIVE: Estimation of the parathyroid hyperplasia prevalence after the ChNPP accident in adults exposed to ion izing radiation and their descendants using the diagnostic ultrasound and its methodology elaboration. MATERIALS AND METHODS: The pilot prospective study of the prevalence of parathyroid hyperplasia among the Chornobyl Nuclear Power Plant (ChNPP) accident adult survivors (n=686) and their descendants (54 children) was performed using diagnostic ultrasound examination of thyroid and parathyroids. Among the study subjects there were 339 ChNPP accident clean up workers (ACUW), 32 persons were evacuated from the 30 km exclusion zone and 224 ones were included to the control group. Diagnostic ultrasound of thyroid and parathyroids was performed according to the standard method. Additionally, in children with parathyroid hyperplasia an additional assay of 25 hydroxyvitamin D levels in serum was performed. In calculating the statistical significance, its level p < 0.05 was considered statistically significant. RESULTS: Parathyroids are a few small but critically important endocrine glands that synthesize parathyroid hormone, regulating mainly phosphoric calcium metabolism. Insufficient (hypoparathyroidism) or excessive (hyperparathy roidism) function of parathyroids is harmful to the patients affecting the state of nervous and cardiovascular sys tem. Parathyroidss can accumulate isotopes of cesium, strontium and radioactive iodine. The available data testify to an increased incidence of clinically significant hyperplasia of parthyroids (more than 9 mm in adults and more than 5 mm in children) among persons exposed toionizng radiation as a result of the accident at the ChNPP (28.64%) and their descendants (23.8-70.6%). First of all are concerned those adults who live in contaminated areas in comparison with the control group (24.15% in not irradiated). Evacuees from the 30 km exclusion zone being the category of people who were exposed to the absorbed iodine isotopes in the first days of the Chernobyl accident are the another risk group. These data demonstrate sensitivity of parathyroidss to the impact of incorpo rated isotopes (iodine, cesium and strontium), which in the long term exposure create conditions for structural and functional changes in regulation of phosphorous calcium metabolism being the basis for a significant prevalence of osteopenia and osteoporosis in irradiated individuals and their descendants. A number of further studies are required to clarify the findings and to disclose the hormonal mechanisms of radiation effects on parathyroids. CONCLUSIONS: Parathyroid glands are radiosensitive and susceptible to effects of strontium, cesium and iodine iso topes, which cause parathyroid irradiation and subsequent structural and functional changes, being a prerequisite for development of osteopenia and osteoporosis in the ChNPP accident survivors and their descendants. High inci dence of parathyroid hypertrophy is found in the inhabitants of the radiation contaminated territories (long term irradiation by cesium isotopes), as well as in evacuated from the 30 km exclusion zone (irradiation by iodine iso topes in the early days of the accident).

Parathyroid gland function after radioiodine ((131)I) therapy for toxic and non-toxic goitre.

INTRODUCTION: The therapeutic effect of radioactive iodine ((131)I) on benign goitre consists of the emission of tissue-destructive beta-radiation. Since the range of beta (131)I radiation in tissue can reach 2.4 mm, it can affect the adjacent parathyroid glands. The purpose of this paper is to assess parathyroid function in patients with toxic and non-toxic goitres, up to five years following (131)I therapy. MATERIAL AND METHODS: The study sample consisted of 325 patients with benign goitres (220 with toxic nodular goitre (TNG), 25 with non-toxic nodular goitre (NTNG), and 80 with Graves' disease (GD) treated with (131)I. The therapeutic activity of (131)I for each patient was calculated using Marinelli's formula. The serum levels of fT3, fT4, TSH, iPTH and Ca(2+), Ca and phosphates were determined one week before (131)I administration, as well as every two months up to a year following the therapy, and then after three and five years post-treatment. RESULTS: After two months following the administration of (131)I, all the treated patients showed a statistically significant above normal increase in iPTH concentrations (amounting to a value almost twice the norm in patients with TNG), which remained stable up to ten months after treatment, to return to normal level in the following months. In all the patients, Ca(2+), Ca, phosphates concentration remained within normal range throughout the course of the study. The concentrations of fT3 and fT4 quickly returned to normal after (131)I administration, and remained within normal range until the completion of the study. CONCLUSION: Radioiodine treatment of benign thyroid disorders results in transient (up to ten months after (131)I administration) hyperparathyroidism. The condition does not influence the level of calcium and phosphates concentration in any significant way.

Parathyroid changes after high dose radioactive iodine in patients with thyroid cancer.

OBJECTIVE: The study aimed to investigate the effect of high dose radioactive iodine (RAI) on parathyroid function in patients with differentiated thyroid cancer. METHODS: Nineteen patients (8 men/11 women, age 46.5 +/- 13.2 years) undergoing RAI for thyroid remnant ablation were enrolled in the study. The biochemical parameters related to parathyroid function [serum calcium (Ca), phosphate (P), creatinine (Cr), alkaline phosphatase (ALP), intact parathyroid hormone (iPTH), urinary Ca, cAMP concentrations and the maximum tubular capacity for phosphate per unit volume of glomerular filtrate (TmP/GFR)] were evaluated at baseline and at the 1st, 3rd, 6th and 12th months of RAI administration. SPSS 15.0 was used for statistical analysis. RESULTS: For all patients, thyroid-stimulating hormone levels were >30 U/ml at baseline and <0.1 U/ml at the following visits. Serum iPTH levels were decreased significantly at the 6th month and reached basal levels at the 12th month (baseline vs. 6th p = 0.027, 1st vs. 6th p = 0.011, 3rd vs. 6th p = 0.047, 3rd vs. 12th p = 0.014, 6th vs. 12th p = 0.001). At the 6th month, P and TmP/GFR levels were higher (p = 0.036, 0.017, respectively), and urinary cAMP measurements were lower (p = 0.020) compared to those of the 1st month. No difference was detected concerning the other parameters. Serum Ca levels decreased below 2.1 mmol/l in several patients (n = 5 at 1st month, n = 4 at 3rd month, n = 8 at 6th month and n = 3 at 12th month) without clinical symptoms. CONCLUSIONS: The study indicated a transient decline in PTH levels at the 6th month following RAI therapy. Although this decrease did not cause symptoms in any of the present cases, this pattern might be important especially in individuals with diminished parathyroid background.

Long-term parathyroid- and c-cell function after radioiodine for benign thyroid diseases.

The study examined static and dynamic parathyroid hormone and calcitonin secretion after radioiodine, and was a retrospective study of patients having received radioiodine for benign conditions 8-12 years earlier. In one group of patients parathyroid hormone and calcium were measured as single blood tests, in a second group of patients parathyroid hormone and calcitonin secretion capacity were measured during a hypocalcaemic citrate-clamp and a hypercalcaemic stimulation test. Baseline calcium and parathyroid hormone were normal within expected ranges 8-12 years after radioiodine. Stimulated parathyroid hormone or calcitonin secretion did not differ from an age- and gender-matched control group. Radioiodine in doses used for benign thyroid diseases appears safe with regard to parathyroid hormone and calcitonin secretion.

Is the treatment of Hodgkin's disease detrimental to the parathyroid gland?

The thyroid gland is often injured by supradiaphragmatic irradiation for Hodgkin's lymphoma. The aim of the present study was to examine whether the parathyroid gland gets injured by the treatment for Hodgkin's disease. Calcium, phosphorus and parathormone levels of 143 patients with primary treatment for Hodgkin's disease and in complete remission for 2 years were measured as well as the presence of antiparathyroid antibody in patients having antithyroid antibody. Out of the 143 patients studied, 104 received neck irradiation (with or without chemotherapy); among them laboratory alterations were observed in 7 cases. 39 patients received only chemotherapy; 3 of them had alterations. In contrast to the injury of the thyroid gland, no damage to the parathyroid glands associated with the treatment for Hodgkin's disease was noted. It has been concluded that the use of high-dose external radiotherapy does not mean a higher risk as regards the parathyroid gland but further follow-up studies of the patients may result in the revelation of the development of parathyroid lesions.

Increased incidence of parathyroid adenomas following X-ray treatment of benign diseases in the cervical spine in adult patients.

OBJECTIVE: To determine whether or not exposure to ionizing radiation as an adult increases the risk of developing parathyroid adenomas. DESIGN AND PATIENTS: A cohort of 27 415 patients had received X-ray treatment to palliate pain originating from arthrosis and spondylosis from 1950 to 1964. In a subcohort of 8144 patients the cervical spine was the target, or one of the targets for the treatment. With the technique used it could be assumed that, as a rule, the parathyroid glands had been included in the treated volume. Patients with parathyroid adenomas were obtained from the Swedish Cancer Register from 1958 to 1995. The standardized incidence ratio (SIR) was calculated. RESULTS: The number of person-years at risk was 180 492 in the cohort exposed at the cervical spine, and 412 994 in the control cohort irradiated only at other sites. The calculated dose in the parathyroid region in the study cohort was about 1 Gy. The observed number of parathyroid adenomas in the study cohort was 22, and 24 in the control cohort. Expected numbers based on population data were 12.0 and 24.7, respectively, giving an SIR of 1.83 (95% CI 1.14-2.76) and 0.97 (CI 0.62-1.45), respectively. Mean age at exposure was 48.9 years (SD 10.2) and at diagnosis 71.5 years (SD 8.3). The time from exposure to diagnosis varied from 2 to 34 years (median 26 years). The excess relative risk was about 0.8 per Gy. CONCLUSION: Exposure to ionizing radiation to the cervical spine in adult age seems to be associated with an increased risk of developing parathyroid adenomas.

[The correction of the subcellular postradiation changes in the hypothalamus and parathyroid gland by using low-intensity laser radiation (an experimental study)]. / Korrektsiia subkletochnykh postradiatsionnykh izmenenii v gipotalamuse i parashchitovidnoi zheleze pri ispol'zovanii nizkointensivnogo lazernogo izlucheniia (éksperimental'noe issledovanie).

The exposure of the rat adrenals 30 days after radiation (1 Gy) to low-intensity infrared laser radiation arrests development of ultrastructural disorders in the cells of hypothalamus and parathyroid gland, enhances subcellular manifestations of adaptation and rehabilitation processes.

[99mTc-MIBI-scintigraphy before parathyroid surgery?]. / 99mTc-MIBI-Szintigraphie vor Eingriffen wegen Hyperparathyreoidismus?

AIM: In a non prospective study the value of preoperative parathyroid imaging with 99mTc-MIBI-scintigraphy was evaluated. PATIENTS AND METHODS: From August 1, 1987 to December 15, 1997 453 patients were operated on for hyperparathyroidism. Preoperatively 116 patients underwent 99mTc-MIBI-scanning, which had been carried out by several institutes. RESULTS: Primary hyperparathyroidism. Sensitivity of MIBI-scintigraphy in solitary parathyroid adenoma (n = 48) and first-time cervical exploration was 54% (25 true positive scans) and overall sensitivity was 50% (25 true positive scans in 53 patients). In 6 patients with persistence or recurrence of primary hyperparathyroidism MIBI-scintigraphy was true positive (sensitivity 100%). Neither the volume nor the weight of the adenomas influenced the result of MIBI-scanning. There was no correlation between preoperative serum concentrations of calcium and intact parathormone and sensitivity of MIBI-scintigraphy. Renal hyperparathyroidism. Before first-time cervical exploration sensitivity of MIBI-scintigraphy (n = 40) for detecting all abnormal parathyroid glands in the presence of diffuse hyperplasia yielded only 10%. In 5 of 7 patients, who underwent reexplorations of the neck or mediastinum for persistence or recurrence of renal hyperparathyroidism MIBI-scintigraphy was true positive (sensitivity 71%). The sensitivity for detection of hyperplastic parathyroid glands correlated with the weight as well as with the volume of the glands (p < 0.001). The weight of imaged glands ranged from 0.21 to 9.76 g, median 1.27 g and their median volume was 1.12 ml (range: 0.04-15.63 ml). For non imaged glands a median weight of 0.49 g and a median volume of 0.3 ml (ranges: 0.03-10.34 g, 0.009-9.8 ml, respectively) could be estimated. CONCLUSIONS: First-time cervical exploration for hyperparathyroidism can be carried out with a high success rate and without any preoperative localization study. Before recurrent parathyroid surgery we recommend 99mTc-MIBI-scintigraphy because of its > 90% sensitivity.

Dose-response relationships for radiation-induced hyperparathyroidism.

It has been hard to establish with certainty that radiation exposure is a risk factor for developing hyperparathyroidism. In part this is because many cases of hyperparathyroidism remain asymptomatic and escape clinical detection. We present results from a study of 2555 subjects who received external beam radiotherapy to the head and neck area for benign conditions before their 16th birthday between 1939 and 1962. The average length of follow-up was 36.6 yr. There were 36 confirmed cases of hyperparathyroidism. Based on a relative risk model, the excess relative risk increased significantly by 0.11/centigray; however, the confidence interval was wide (95% confidence interval, 0.0-17.2). The hyperparathyroidism rates and dose-response relationships were not affected by gender or age at first radiation treatment. The demonstration of a dose-response relationship within an irradiated cohort supports an association between radiation exposure and hyperparathyroidism and suggests that the calcium levels of individuals irradiated to the head and neck area should be monitored.

Parathyroid hormone secretory reserve in patients submitted to 131-iodine therapy for hyperthyroidism

Twelve euthyroid patients who had treated with I for hyperthyroidism due to Fraves' disease and six normal controls were submitted to an EDTA infusion test. Ionized calcium and parthyroid hormone were measured in serum samples collected every 10 min during the 2-h-test. Basal values for calcium (1.22 ñ 0.03 vs 1.23 ñ 0.03 pmol/l, mean ñ SD, controls vs patients) and parathyroid hormone (3.3 ñ 0.65 vs 5.5 ñ 2.32 pmol/l_ as well as maximum response during infusion (1.01 ñ 0.04 vs 1.01 ñ 0.05 for calcium and 12.0 ñ 2.2 vs 13.1 ñ 3.7 for parathyroid hormone) were not significantly different. We conclude that I treatment for hyperthyroidism due to Graves' disease had no effect on the parathyroid gland secretory reserve of the patients studied

Therapeutic radiation and hyperparathyroidism. A case-control study in Rochester, Minn.

A case-control study was conducted among residents of Rochester, Minn, to assess the influence of prior therapeutic radiation on the risk of primary hyperparathyroidism. Fifty-one cases of surgically proven primary hyperparathyroidism diagnosed from 1975 through 1983 were each matched by age and gender to two control subjects, with radiation exposure documented through preexisting medical records. The overall odds ratio for any prior therapeutic radiation therapy was 1.9 (95% confidence interval, 0.9 to 4.4) and it was 2.3 (95% confidence interval, 0.9 to 5.7) when limited to those with prior head and neck radiation. Among women, the figures were 2.9 (95% confidence interval, 1.1 to 7.5) for any prior therapeutic radiation and 3.4 (95% confidence interval, 1.2 to 10.2) for head and neck exposure. This study confirms the association between primary hyperparathyroidism and prior therapeutic radiation exposure, at least for women in this population.

Early changes in parathyroid function after high-dose irradiation of the neck.

Hyperparathyroidism (HPT) will develop years later in a number of patients irradiated to the neck for benign diseases. Twenty-nine patients with malignant diseases were studied for 3 years after irradiation with cancer therapeutic doses to the neck. The serum concentration of parathyroid hormone (iPTH) showed an increasing trend suggesting that HPT also might develop in these patients.

Thyroid, parathyroid, and salivary gland evaluations in patients exposed to multiple fluoroscopic examinations during tuberculosis therapy: a pilot study.

The prevalence of thyroid, parathyroid, and salivary abnormalities was determined in 91 women who received an average of 112 fluoroscopic chest examinations during pneumothorax treatment for tuberculosis more than 40 yr previously and in 72 women treated for tuberculosis by other modalities. Thyroid abnormalities were determined by physical examination, scintiscans, and measurements of serum free T4 index, TSH, and thyroid microsomal antibodies. Thyroid nodules were diagnosed in 7.7% of the exposed and 4.2% of the comparison group (prevalence ratio, 1.8; 90% confidence interval 0.6-5.7). Autoimmune thyroid disease was diagnosed in 15.2% of the exposed and 6.9% of the comparison group (prevalence ratio, 2.2; 95% confidence interval, 0.8-6.2). No salivary tumors were detected. Two exposed women and 1 comparison woman had primary hyperparathyroidism. Although absorbed dose to the thyroid could not be precisely determined, approximately 60 rads would be expected to yield the observed excess of thyroid nodules. While the prevalence ratios were not significantly increased in the exposed group, the results suggest that susceptibility of the thyroid to nodules from cumulative radiation doses of this magnitude could be increased even when the doses are accumulated over years and that such x-ray exposure of the thyroid gland may predispose the patient to the development of autoimmune disease.

Effect of decicurie doses of radioactive iodine 131 on parathyroid function.

Although parathyroid deficiency has been reported after administration of relatively small doses of iodine 131, reports of such deficiency after the much larger doses given in the management of thyroid cancer are notable by their absence in the literature. We observed one such patient, then instituted a prospective study. We observed a 58 percent incidence of apparently diminished parathyroid reserve among 53 patients given high doses of I 131. Susceptibility does not appear to be related to age, sex, extent of thyroid operation, operative technique, the amount of thyroidal I 131 uptake, the use of external radiation to the neck, or to the presence of parathyroid glands or of Hashimoto's disease in the operative specimen; however, susceptibility may be related to the anatomic location of the parathyroid glands in relation to the thyroid gland. The administered dose of I 131 almost reached significant levels (p = 0.1) in the present study. The initial pretreatment serum calcium levels were consistently lower among affected patients (p = 0.01), suggesting the presence of a preexisting latent impairment of parathyroid reserve which was subsequently unmasked after the administration of I 131. Thyroid cancer is compatible with very long-term survival, and therefore, there is a considerable risk of delayed chronic hypocalcemia among patients who have received large doses of I 131, particularly female patients in the premenopausal age group. These patients deserve special management surveillance.