Iodine and thyroxine metabolism in anephric patients receiving chronic peritoneal dialysis

Radiation Protection Considerations for Cancer Patients with End-stage Renal Disease Receiving 131 I Treatment

ABSTRACT

Differentiated thyroid cancer (DTC) is commonly treated first with a partial or complete thyroidectomy, followed by radioiodine (RAI) ablative therapy to eliminate remaining cancer cells. In such treatments, physical decay and urinary excretion are the primary means of 131 I. As such, patients with impaired urinary ability clearance, such as patients with end-stage renal disease (ESRD) whose urinary ability is impaired by dysfunction, can retain abnormally high activities of RAI, posing a concern to both the patient and those with whom the patient interacts. Additionally, ESRD patients are commonly administered dialysis therapy, wherein their blood is externally cycled through a dialyzer (hemodialysis) or filtered by instilling a dialysate fluid into the peritoneum (peritoneal dialysis) to filter uremic toxins from their blood that accumulate due to kidney dysfunction. These factors make determining release and dosing for ESRD patients receiving RAI therapy dependent on a plurality of variables. An evaluation of the current patient release guidelines, as given in US Nuclear Regulatory Commission (US NRC) Regulatory Guide 8.39 Rev. 1 for ESRD patients receiving RAI, has yet to be addressed. In this study, a biokinetic model for 131 I in ESRD patients receiving dialysis has been developed, improving on traditional two-compartment models, reflective of kinetics from multi-compartment models with updated transfer coefficients modified to reflect the different physiological functions of compartments. This updated biokinetic model was integrated with Monte Carlo radiation transport calculations using stylized computational hermaphroditic phantoms to calculate dose rate coefficients in exposure scenarios and compared with those of the point source models of NRC Reg Guide 8.39 Rev. 1 (and the proposed verbiage in Rev. 2). Results demonstrated that the baseline models of Rev. 1 and Rev. 2 overestimated the effective dose rate to an exposed individual for the majority of time post-administration, where both models overestimated the total dose to the maximally exposed individual. However, the application of several patient-specific modifying factors to the Rev. 2 model resulted in an overestimation by only a factor of 1.25, and in general, the results produced with the patient-specific modifications provide improved convergence with the dose rate coefficients computed in this study for ESRD patients.

Evaluation of Thyroid Function in Patients Treated with Continuous Ambulatory Peritoneal Dialysis (CAPD)

Abnormal thyroid function has been described in chronic renal insufficiency, and further changes occur with increasing time on hemodialysis. This study on two groups of patients was done to determine whether treatment with CAPD induces changes in thyroid function. Group A included 16 patients with an average duration of treatment of 2.1 ± 1.1 months; group B included 12 patients with 8.1 ± 2 months. We measured total T4 and T3, free T4, TBG, RT3U, FTI, T4/T3 and basal TSH, and TSH induced by TRH stimulation in plasma, and total and free T4 in dialysate. The results showed a significant decrease in total T4 and T3 in plasma, but no change in free T4 or TSH and no appreciable amounts of the hormones in the dialysate. We believe the decrease in total T4 and T3 is secondary to alterations in transport proteins while thyroid function is maintained by normal free T4 and TSH levels more dependable parameters for the interpretation of thyroid function in chronic diseases.

Thyroidal Status in Uremia - Effects of Hemodialysis and CAPD

Thyroid function was explored in 27 CAPD patients and 25 HD patients. Dialysis was associated with low T3 and T4 levels, increased rT3 concentrations, normal T4: T3 and reduced T4: rT3 ratios, normal FT3 and Thyroglobulin concentrations. TSH levels increased during dialysis but still fell within normal limits. The data rule out a condition of primary hypothyroidism and point out to increased thyroidal dismission associated with the block T4-T3 in peripheral cells. Also the increased FT4 levels in CAPD patients could be due to de novo dismission of thyroidal T4.

TSH Response to TRH in Hemodialysis and CAPD Patients

The thyroid status was studied in 19 uremic patients (11 on HD and 8 on CAPD) and in a group of 17 healthy adult patients. In uremic patients T3 and T4 were significantly reduced (p < 0.001) and rT3 was increased (p < 0.001). The time course TSH response to TRH showed a lower peak response (in 5 patients it did not exceed 5 μU/ml) which on turn was also delayed (the peak was observed at 60 minutes in 63% of them). TSH concentrations in uremics were increased at 0 time (p < 0.02), lower at 20 minutes (p < 0.01) and increased at 120 minutes (p < 0.02). No difference existed between HD and CAPD. The thyroid response to TSH was normal as showed by a normal percent increase over basal values of T3 concentrations at 120 minutes. Uremic patients also showed a peak GH response at 20 minutes which was not observed in controls. The data exclude the existence on a primary form of hypothyroidism and point to the existence of hypothalamic-pituitary abnormalities, which should not be taken as indicative of secondary and/or tertiary hypothyroidism since FT4 values were normal in HD and increased in patients undergoing CAPD (p < 0.05) who on turn showed lower plasma albumin concentrations (p < 0.05).

The effects of dialysis on 131I kinetics and dosimetry in thyroid cancer patients--a pharmacokinetic model

Currently, an accepted post-surgical treatment of patients with thyroid carcinoma is administration of an ablative dose of I. This treatment is well established based on extensive experience and modeling. However, for patients with renal disease, reduced iodine removal rates result in controversial thyroid doses and potentially excessive red bone marrow doses. There are differences of opinion regarding I dose recommendations ranging from a reduction in dose to an increase in dose compared with conventional amounts. Determination of suitable doses must take into account varying dialysis protocols and absorbed dose considerations to the thyroid and sensitive tissues such as red bone marrow. The specific aim of this study was to develop a simple yet comprehensive compartmental model for I kinetics in patients with thyroid carcinoma and end stage renal disease, which accounts for dialysis and provides absorbed dose estimates for the thyroid as well as the red bone marrow. STELLA, a compartmental modeling software program, was used to develop a kinetic model that includes the blood pool, thyroid, gastrointestinal tract, kidneys, bladder, and a conventional dialysis machine. Benchmarking was performed to demonstrate the validity of the model with data obtained from ICRP 30 and MIRD Dose Estimate Report No. 5. Iodine kinetics were simulated for normal patients, thyroid cancer patients, and patients with thyroid cancer and renal failure undergoing two standard types of dialysis, hemodialysis and continuous ambulatory peritoneal dialysis (CAPD). Results in this work show that thyroid doses to patients with thyroid cancer and renal failure on hemodialysis or CAPD are slightly higher than doses to patients with thyroid cancer and normal renal function. These results further indicate that red bone marrow doses to patients with thyroid cancer and renal failure on dialysis can be significantly higher than red bone marrow doses to patients with thyroid cancer and normal renal function, and thus these patients could benefit from a reduction in administered activity. Thyroid doses and red bone marrow doses to patients on standard hemodialysis depend on both dialysis frequency and the time interval between administration and first dialysis. The results in this study provide guidelines on how much activity a patient on dialysis should receive based on thyroid and red bone marrow absorbed dose (Gy MBq) considerations. This study should help to clarify some of the contradictory recommendations regarding I dose for thyroid carcinoma patients with renal failure.

Nuclear medicine studies in the dialysis patient

Nuclear medicine (scintigraphy) studies that are performed in patients being prepared for regular dialysis treatment include the measurement of renal clearance and dynamic studies of renal perfusion and function. Static scintigraphy with 99mTc-DMSA may be used in the evaluation of children at risk of renal damage and further functional deterioration. In patients on peritoneal dialysis, nuclear medicine procedures enable the diagnosis of structural complications such as intra-abdominal herniations and leaks. Diagnosis of infections of the vascular access sites in patients on hemodialysis and of the catheter tunnel in patients on peritoneal dialysis can be made with high diagnostic accuracy using radiolabeled, autologous leukocytes. Scintigraphy is valuable in delineating the extent of deposits of amyloid and parenchymal microcalcifications, and may be helpful in the functional evaluation of organs and tissues involved in the pathophysiology of renal impairment and dialysis. If radioiodine therapy with 131I is performed in patients on hemodialysis with benign or malignant thyroid disease, then pretherapeutic dosimetry is necessary to avoid over- and undertreatment. Radioiodine therapy in the dialysis patient leads to only insignificant contamination of dialysis equipment and marginal exposure to the medical staff.

Thyroid function and serum thyroid binding proteins in prepubertal and pubertal children with chronic renal insufficiency receiving conservative treatment, undergoing hemodialysis, or receiving care after renal transplantation

OBJECTIVE

The abnormalities reported in some thyroid function tests in children with renal disease could be adaptive phenomena, shared by a variety of other nonthyroidal illnesses, or could reflect hypothyroidism.

STUDY DESIGN

To answer this question, we studied thyroid function and serum thyroid binding proteins in 36 prepubertal and 23 pubertal patients with renal disease receiving three different therapies: conservative treatment, hemodialysis, and care after renal transplantation.

RESULTS

During prepuberty, the serum concentration thyroxine binding globulin (mean +/- SE) in the three groups of patients (294 +/- 18, 303 +/- 18, and 323 +/- 16 nmol/L, respectively) was significantly lower than in prepubertal control subjects (451 +/- 71 nmol/L). Only in prepubertal patients after renal transplantation (3583 +/- 573 nmol/L) were serum thyroxine binding prealbumin values lower than in respective control subjects (5999 +/- 908 nmol/L). The serum total thyroxine concentration in the three groups of patients (108 +/- 41.9, 121 +/- 5.7, and 123 +/- 5.5 nmol/L, respectively) was significantly lower than in prepubertal control subjects (149 +/- 10 nmol/L), whereas serum free thyroxine and serum albumin-bound thyroxine concentrations were similar to those in control subjects. The serum total triiodothyronine level in the three groups of patients (2.29 +/- 0.82, 2.13 +/- 0.13, and 2.01 +/- 0.20 nmol/L respectively) was significantly lower than in prepubertal control subjects (3.04 +/- 0.24 nmol/L), whereas serum levels of free triiodothyronine and serum albumin-bound triiodothyronine were similar to those in prepubertal control subjects. During puberty, serum thyroxine binding globulin and serum thyroxine binding prealbumin levels in the three groups of patients were not statistically different from those in pubertal control subjects (309 +/- 47 and 4950 +/- 1230 nmol/L, respectively). Serum levels of total thyroxine, free thyroxine, albumin-bound thyroxine, total triiodothyronine, free triiodothyronine, and albumin-bound triiodothyronine were similar to those in pubertal control subjects except for pubertal patients undergoing hemodialysis. In all clinical groups the basal serum thyrotropin concentration was similar to those in respective control subjects. The frequency of goiter was increased in patients undergoing hemodialysis, probably as a result of iodide washout with dialysis.

CONCLUSION

Children and adolescents with chronic renal insufficiency or endstage renal disease or after renal transplantation do not have a primary abnormality of thyroid function and therefore are not candidates for thyroid hormone treatment.

Colonic excretion of iodide in normal human subjects

Patterns of fecal radioiodine excretion were studied in seven normal young men with intact, unblocked thyroid glands, who received repeated oral daily doses of [125I]iodide in an attempt to achieve isotopic equilibrium. Fecal radioactivity was assumed to have originated either in the circulating inorganic radioiodine (radioiodide) compartment or in the circulating hormonal protein-bound iodine (PBI) compartment. Activity in the PBI compartment was measured directly in serum samples from which radioiodide had been removed by dialysis or resin treatment. Activity in the radioiodide compartment was measured by the difference between total and hormonal radioiodine, and also as a projection from the rate of urinary excretion of radioiodine. These compartments were fitted to the observed sequential fecal radioiodine data in each subject to identify the origins of the fecal radioactivity, using the SAAM modeling program. The fraction of fecal radioactivity attributable to iodide was 0.55 +/- 0.35 (mean +/- SD) (geometric mean 0.44, range 0.25-0.96). In all cases, at least some contribution from the iodide compartment was required for model fit to the observed pattern of fecal radioiodine excretion. These data demonstrate that, despite long-existing opinion to the contrary, iodide is an important component of intestinal iodine excretion in humans. This finding explains the presence of colonic activity in postradioiodide images of athyreotic patients.

Effects of chronic peritoneal dialysis on thyroid function tests

Peritoneal dialysis is associated with large losses of protein. In order to quantify thyroid hormone excretion in the dialysate and to examine the possibility that peritoneal dialysis may result in clinical hypothyroidism, nine endstage renal disease (ESRD) patients undergoing either continuous ambulatory peritoneal dialysis (CAPD) or chronic intermittent peritoneal dialysis (IPD) were studied. Total protein excretion in the peritoneal fluid was 21.5 +/- 2.1 g/24 h and did not vary with the mode of peritoneal dialysis. Thyroid binding globulin (TBG) excretion was 6.4 +/- 1.3 mg/24 h, higher than the values reported in the literature for urinary TBG excretion in patients with the nephrotic syndrome. Despite the higher TBG losses, serum TBG remained in the normal range. Mean peritoneal total T4 and T3 were 8.1 +/- 1.6 micrograms/24 h and 89.5 +/- 14.6 ng/24 h, and there was a significant correlation between peritoneal T4 and TBG (r = 0.69; P less than 0.01) and between peritoneal total proteins and T4 (r = 0.80; P less than 0.001). Despite the finding that large amounts of protein are lost in peritoneal fluid, T4 and T3 losses were relatively modest and remained below their daily production rates, and none of the patients were overtly hypothyroid. Serum thyroid stimulating hormone (TSH) was mildly elevated in three of nine patients and was consistent with early thyroid failure. The patients' serum iodine levels were higher than normal but did not predict the patients' thyroid status. We conclude that major protein losses could predispose patients undergoing CAPD to thyroid failure and that long-term follow-up of thyroid function is warranted in these patients.

Iodine trapping and organification in patients with chronic renal failure

In vitro thyroid function tests were studied in 30 patients on regular hemodialysis. In addition, 8 patients (one not yet dialyzed) underwent perchlorate discharge testing and the salivary to plasma ratio of 131I was measured to identify defects in iodine uptake or organification. When compared with 35 healthy controls, uremic patients had lower total T4 (5.8 +/- 0.4 vs 8.6 +/- 0.3 micrograms/100 ml) and total T3 (81 +/- 6 vs 124 +/- 5 ng/100 ml) but T3RU was higher in dialysis patients (35.5 +/- 0.9 vs 28 +/- 0.5%, P less than 0.01). The TSH levels were not different from those of controls. The RAI uptake 1 h after perchlorate was significantly higher than the control valve (0.6 +/- 0.14 vs 0.06 +/- 0.04, P less than 0.05). Similarly, the salivary to plasma ratio of 131I was higher in patients with chronic renal failure (70 +/- 10:1 vs 40 +/- 4:1). It is concluded that there is altered iodine trapping in CRF patients but iodine organification appears to be normal.

Hemodialysis and thyroid functions in children

Evaluation of thyroid functions in 16 children receiving maintenance hemodialysis for a mean duration of 17 months showed that the serum T3, T4 and TSH were below normal concentrations for age. However, the measurements of these variables before and after the dialysis procedure did not show any significant changes. The possibility that the chronic uremia may give rise to abnormal TSH secretion and low T3, T4 concentrations is not tested in this study.