Smoking during Pregnancy–Effects on the Fetal Thyroid Function

ABSTRACT. Infants delivered at term by mothers smoking at least 10 cigarettes daily during pregnancy ( n =46) were found to be growth retarded compared to infants of non-smoking mothers ( n =49), birthweights 3445pM385 (SD) g and 3667pM392 g respectively ( p <0.05) in the two groups. Cord serum thyrotropin (TSH) was significantly decreased (8.2±4.(1 U/l vs. 10.3±4.9 U/l) and free thyroxine index (FT₄I)/TSH ratio significantly increased (18.8±9.0 vs. 14.4±7.6) ( p <0.05) in the smoking group compared to infants of non-smokers. Cord serum thyroxine (T₄) and FT₄I were higher in the smoking group (149.0±22.4 nmol/1 and 125.5± 14.9 respectively) compared to infants of non-smoking mothers (140.6±21.6 nmol/1 and 120.0±16.5 respectively), with borderline statistical significance (0.05< p <0.10). The results indicate that infants of smoking mothers may have a hyperfunction of the thyroid gland at birth compared to infants of non-smokers, with a negative feed-back on TSH production from the pituitary gland. Increased metabolic rate and oxygen consumption caused by fetal thyroid hyperfunction may be pathogenetic factors for the fetal growth retardation caused by maternal smoking.