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Circulating Iodide Concentrations during and after Pregnancy

Professor of Medicine 35, Vas. Sofias Avenue GR-10675 Athens, Greece

Liberman et al. (1)report in the October issue that "Pregnancy, at least in iodine-sufficient regions, does not have an important influence on circulating concentrations of iodide." This statement has far reaching implications on what is known and accepted about iodine metabolism, and so the authors would like perhaps to clarify some points.

1. Estimation of the plasma inorganic iodine (PII) as the difference between total serum iodine (TI) and protein-bound iodine (PBI) has been known for several decades, but not used extensively. It was judged not accurate enough, despite the paper by my late mentor Bob Vought and colleagues (2). Indeed, the level of the PII is usually so much lower than the TI and PBI, so that a small error in the estimation of either the TI or the PBI means a proportionally much higher one in the PII.
   
2. A low PII in pregnancy has been found not only in Scotland by Aboul-Khair et al. (3), but also in Greece by us (4), and by others, and is consistent with the fact that the renal iodide clearance increases considerably during pregnancy. So, it is difficult to accept a PII level similar to that before pregnancy if the iodine intake does not increase markedly. In the paper of Liberman et al. (1) there was not a marked increase in the iodine intake in pregnancy, as judged by the urinary iodine excretion, and so one wonders how the PII has not decreased.
   
3. The PII obtained by the isotope dilution technique and multiplied with the thyroidal iodide clearance rate leads to absolute iodine uptake rate estimates compatible with the amounts of iodide utilized by the thyroid, as discussed in the 1950s by Riggs (5) and in the 1960s by Wayne et al. (6), Rall et al. (7), and others. It should be noted that the thyroidal iodide clearance rate, as usually measured over a 1- or 2-h period, corresponds more to the net one and not the unidirectional. With the PII values found by Liberman et al. (1), about 10 times higher than the PII values by the isotope dilution technique in iodine-sufficient countries, the thyroid is expected to trap and retain 10 times more iodide than it utilizes.
   

Footnotes

Address correspondence to: Demetrios A. Koutras, Professor of Medicine, 35, Vas. Sofias Avenue, GR-106 75 Athens, Greece.

Received December 10, 1998.

References

1. Liberman CS, Pino SC, Fang SL, Braverman LE, Emerson CH. 1998 Circulating iodide concentrations during and after pregnancy. J Clin Endocrinol Metab. 83:3545–3549.[Abstract/Free Full Text]
2. Vought RL, London WT, Lutwak L, Dublin TD. 1963 Reliability of estimates of serum inorganic iodine and daily fecal and urinary iodine excretion from single casual specimens. J Clin Endocrinol Metab. 23:1218–1228.
3. Aboul-Khair SA, Crooks J, Turnbull AC, Hytten FE. 1964 The physiological changes in thyroid function during pregnancy. Clin Sci. 27:195.
4. Koutras DA, Pharmakiotis AD, Koliopoulos N, Tsoukalos J, Souvatzoglou A, Sfontouris J. 1978 The plasma inorganic iodine and the pituitary thyroid axis in pregnancy. J Endocr Invest. 1:227–231.[Medline]
5. Riggs DS. 1952 Quantitative aspects of iodine metabolism in man. Pharmacol Rev. 4:284–370.[Abstract/Free Full Text]
6. Wayne EJ, Koutras DA, Alexander WD. 1964 Clinical aspects of iodine metabolism. Oxford: Blackwell Scientific Publications.
7. Rall JE, Robbins J, Lewallen CG. 1964 The thyroid. In: Pincus G, Thimann KV, Astwood EB, eds. The hormones, vol V. New York and London: Academic Press; 159–439.

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