The plasma inorganic iodine and the pituitary-thyroid axis in pregnancy.

Abstract

The plasma inorganic iodine (PII), the serum T4, the resin T3 in vitro uptake (RT3U), the free — thyroxine index (FTI) and the serum TSH level before and 30 min after the iv injection of 200 μg TRH were measured serially for several months in 9 pregnant women, whereas in 12 others the TRH test was performed once. The PII was low (<0.08 μg/100 ml) in 6 of the 9 cases, normal in two and greatly increased (probably because of iodine contamination) in one. Excluding this latter case, the PII was significantly lower than the control value for nonpregnant women in Athens (mean ± SE: 0.08 ± 0.01 compared to 0.12 ± 0.01, p<0.02). In spite of this low PII, there was no increased TSH, either before or after the administration of TRH. On the contrary, the TRH response was blunted in 3 of 21 cases. It is concluded that in pregnancy there is a lower PII, because of the increased renal iodide clearance rate, and so iodine supplementation is recommended in countries with a marginal iodine intake, as in Europe. In spite of this low PII, indicating a state of relatively acute iodine deficiency, the thyroid may adapt without an increased serum TSH level. Hence the fact that the serum TSH is normal in endemic goitre with mild iodine deficiency is best explained not by the chronicity of the condition and the establishment of a new steady-state, but by other mechanisms including probably thyroid autoregulation.