The effects of iopodate on the serum iodothyronine pattern in normal subjects

Abstract

The ingestion by normal subjects of 3 g of sodium iopodate, which is widely used in routine oral cholecystography, resulted in significant decreases of serum total and free T3 to a nadir on day 4 which averaged 43% and 40%, respectively, below initial mean values. Total and free rT3 increased markedly to a peak on day 3, 244% and 189%, respectively, above initial mean values. Total and free Ti and free T4 index rose to a maximum on day 4, but these changes were not statistically significant. A marked TSH increase was also seen, most evident on day 3. All these changes reverted to baseline values by day 14 at a time when serum total iodide was still markedly elevated. It is concluded that the changes observed after iopodate were not due to alterations in serum binding proteins nor to an effect on thyroid gland by the large iodine component of iopodate, but were consistent with an effect on the peripheral metabolism of T4. Difficulty in ipterpreting routine thyroid function tests may occur for up to 14 days after oral cholecystography with iopodate. Sodium iopodate is a contrast medium commonly used in oral cholecystography. Its chemical struc¬ ture bears some similarity to that of thyroxine (T4) and 61.4% of its weight consists of iodine. Wu et al. (1978) observed the effects of iopodate ingestion for a week and showed that serum 3, 5, 3' triiodothyronine (T3) decreased, 3, 3', 5' triiodothyronine (rT3) increased and T4 increased in euthyroid subjects. Similar changes were observed in thyrotoxic patients excepting that serum T4 decreased. They concluded that iopodate alters peripheral metabolism of T4 and, in thyrotoxicosis, it may decrease thyroid activity also.Because of its widespread use in clinical practice iopodate may commonly complicate the interpreta¬ tion of routine thyroid function tests. We therefore decided to document the magnitude and duration of such interference by observing the effects of oral iopodate on routine thyroid function tests. We also studied the effects on the serum concentrations of total and free rT3, free T3, free T4, thyroid stimu¬ lating hormone (TSH) and total iodide to help elucidate the mechanism of the interference. Subjects and MethodsSix healthy adult male volunteer subjects were studied whose average age was 25 years. Sodium iopodate was given in 3 separate 1 g oral doses between 09.00 and 12.00 h. Similar studies were conducted on two other healthy volunteers given an equivalent oral dose of iodine in the form of potassium iodide. Blood was sampled by venepuncture at 09.00 h on each day of observation namely 10 and 6 days before iopodate, on the day of taking iopodate and on days 1, 3, 4, 7, 14 and 21 after the doses. Samples were taken with minimal venostasis after the subjects had been sitting for 5 min. Serum was stored in aliquots and analyzed soon after the last sample was collected. All samples from each subject were analyzed in the same batch.Concentrations of total T4, total T3, rT» and the TSH were determined by specific radioimmunoassay (RIA) using second anti...

The Use of Oral Radiographic Contrast Agents in the Management of Hyperthyroidism

“…The most widely studied agents in humans are sodium ipodate and iopanoic acid. With the administration of either agent, serum triiodothyronine (T 3 ) and free T 3 levels fall while rT 3 levels increase (1,2,(14)(15)(16) (Fig. 1).…”

“…Thus, there is reduction in peripheral as well as intrapituitary conversion of thyroxine (T 4 ) to T 3 and diminished degradation of rT 3 . Basal and TRH-stimulated thyrotropin (TSH) values have been shown to increase or remain unchanged (2,(14)(15)(16)(22)(23)(24)(25)(26). In hyperthyroid subjects with Graves' disease, where there is increased expression of 59-MDI, T 4 levels decrease or remain unchanged (10,(27)(28)(29)(30)(31).…”

“…T 3 levels occasionally reach the hypothyroid range with nadir levels 3 to 11 days after one dose of either ipodate or iopanoic acid (2,14,15,24,25). T 3 (and rT 3 ) levels return to baseline 1 to 3 weeks after intake and T 3 may even reach the hyperthyroid range.…”

“…TSH levels remain elevated even when the T 3 levels have normalized, suggesting that the inhibitory effect on the pituitary (type II 59-MDI activity) lasts longer than the effect on the periphery (type I 59-MDI) (25). T 4 and free T 4 levels increase in 3 to 7 days returning to baseline in 3 to 6 weeks (2,14,15,24,25).…”

Sodium Ipodate in the Preparation of Graves’ Hyperthyroid Patients for Thyroidectomy

“…Thirdly, as 500 mg of SI contains 308 mg iodine, large amounts of iodine are released during its metabolism, resulting in the inhibition of the organifica tion of iodine (Wolff-Chaikoff effect) [5] and of thyroid hormone release [6], Furthermore, SI seems to inhibit the hormonal secretion of the thyroid also by a direct effect which is independent of the inhibition induced by iodine [3,7,8]. It has been shown that during treatment with SI plasma T4 values decrease by 20-31 % in thyro toxic Graves' patients [1,[9][10][11][12][13], but remain unchanged or even rise in healthy adults [1,14,15]; on the other hand, plasma T3 values decrease by 67-76% in thyro toxic Graves' patients and by 25-55% in healthy adults [1,[9][10][11][12][13][14][15].…”