THYROID FUNCTION TESTS DURING PREGNANCY, THE NORMAL MENSTRUAL CYCLE AND IN WOMEN USING ORAL CONTRACEPTIVES

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Abstract
SUMMARY: Endogenous and exogenous oestrogenic effects on thyroid function tests were studied. [125I]Tri-iodothyronine charcoal uptake (T3 uptake) and protein-bound iodine (PBI) were measured in serum obtained from women during the menstrual cycle and the different trimesters of pregnancy and from women taking oral contraceptives for varying periods of time. The free thyroxine index (FTI) was calculated for the pregnant subjects and the free thyroxine factor (FTF) evaluated for all samples studied. Significant deviations from control values in T3 uptake, PBI, FTI and FTF could not be correlated with relatively large changes in the levels of oestrogens. T3 uptake results appeared to reflect changing oestrogenic patterns more closely than the corresponding PBI measurements. Evidence presented suggests that the FTF is preferred to the FTI for clinical use in the diagnosis of thyroid status although further modification to the FTF formula involving PBI and [125I]T3 charcoal uptake measurements may be necessary.

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