Effect of treadmill exercise on circulating thyroid hormone measurements.

OBJECTIVES: Effects of exercise on circulating thyroid hormone (TH) values remain controversial. We sought to observe the effect of treadmill exercise on serum TH values in highly selected subjects. METHODS: Twenty-six healthy male military recruits aged 23-27 (mean, 25) years were studied. All had maintained identical diet and physical activity for a week before the test. Serum samples were drawn before (baseline) and immediately, 1, 4, 24, 24 and 48 h after maximal exercise (on a treadmill, Bruce protocol). All subjects completed the protocol with normal ECG results. Specimens were analyzed to measure 3,3',5-triiodothyronine (T(3)), thyroxine (T(4)), free T(4) (FT(4)), free T(3) and thyroid-stimulating hormone (TSH) in the same assays. To determine the possible effect of hemodynamic changes, hematocrit (Hct)-adjusted data were also compared. RESULTS: Hemoconcentration, as reflected by increased Hct, was found immediately after exercise. No significant changes of serum mean TH values before and after exercise were found except for TSH, which increased significantly immediately after exercise (1.72 vs. baseline 1.42 IU/l, p < 0.01). Values for T(3), T(4), and TSH increased significantly immediately after exercise, as compared to other postexercise values. However, the changes became insignificant after Hct adjustment. The FT(4) values showed a reciprocal increase after exercise that became significant after Hct correction. Significantly negative correlation was found between FT(4) and TSH values, but these values were still well within the normal range. CONCLUSIONS: Maximal treadmill exercise does not greatly affect the determination of concentrations of circulating THs.

Potential interference of agents on radioiodide thyroid uptake in the euthyroid rat.

UNLABELLED: The objective of this research was to investigate the merits of controlled studies with euthyroid rats as a means of determining the influence of dose and time after administration of agents that may interfere with radioiodide uptake in the thyroid. METHODS: Potassium iodide (KI), propylthiouracil (PTU), diatrizoate meglumine, and iohexol were selected to represent interfering agents. Two dose levels per agent were investigated. Doses used were 1 and 2 mg/kg of body weight for KI, 3.5 and 7 mg/kg of body weight for PTU, 1 mL/kg (282 mg I/kg) and 2 mL/kg (564 mg I/kg) of body weight for diatrizoate meglumine, and 1 mL/kg (300 mg I/kg) and 2 mL/kg (600 mg I/kg) of body weight for iohexol. The 24-h radioiodide thyroid uptake was determined after (131)I was given at 1, 8, 15, and 22 d after administration of interfering agents. RESULTS: The percentage radioiodide uptake value for the thyroid decreased significantly compared with controls for all agents and both doses on day 1 but returned to control levels by day 22 for all agents and both doses The time to return to normal varied between agents and doses. CONCLUSION: We conclude that the interfering agent, the dose given, and the length of time after administration influence the potential for an agent to affect radioiodide uptake in the thyroid. Further studies with the rat, preferably hyperthyroid, would be beneficial in generating data to reduce confusing contradictory information on the length and severity of interference of agents in radioiodide thyroid studies.

The effect of formulation on reduced radioiodide thyroid uptake.

UNLABELLED: This investigation compared in vitro dissolution profiles from sodium iodide capsules with radioiodide thyroid uptake in patients with thyroid abnormalities, using sodium iodide capsules prepared with a formulation exhibiting complete release of radioiodide in vitro and a formulation exhibiting incomplete release. METHODS: In vitro dissolution profiles for radioactive sodium iodide capsules with 2 different formulations were determined using the U.S. Pharmacopeia (USP) XXIV dissolution test. The 2 formulations studied in vitro were sodium phosphate dibasic powder with 1% magnesium stearate and calcium phosphate dibasic powder with 3% magnesium stearate. The thyroid uptake of radioiodide from capsules exhibiting complete release or incomplete release of radioiodide was determined in patients with thyroid disorders. RESULTS: In the dissolution studies, by 20 min after initiation of the test, >95% of the radioactive iodide was released from capsules of sodium phosphate dibasic powder. The capsules of calcium phosphate dibasic powder reached 75% at 65 min, with no further release occurring thereafter. In the in vivo studies, the mean thyroid uptake at 1 h for sodium phosphate dibasic powder with 1% magnesium stearate (complete-release formulation) was 12.7%, compared with 9.3% for calcium phosphate dibasic powder with 3% magnesium stearate (incomplete-release formulation) (P < 0.05). At 24 h, the value was 56.6% for the complete-release formulation, compared with 50.3% for the incomplete-release formulation (P < 0.01, Wilcoxon signed rank test). At 1 h, the abdominal activity for the complete-release formulation was 3.4%, compared with 8.8% for the incomplete-release formulation (P < 0.01). At 24 h, the value was 0.4% for the complete-release formulation, compared with 1.0% for the incomplete-release formulation (P < 0.01). CONCLUSION: The data suggest that the incomplete dissolution profile observed in vitro may correlate with reduced bioavailability of radioiodide in vivo. The USP dissolution test can be applied to radioiodide sodium iodide capsules as a quality assurance procedure.