Association of maternal urinary fluoride concentrations during pregnancy with size at birth & potential mediation effect by maternal thyroid

[helvella]

Relatively unusually, I'm posting this partly because of the "it's not thyroid" aspect!

We have had many, many posts regarding fluoride and whether, how, it might affect the thyroid gland and thyroid hormone levels. The fact that the increased birth size and weight in this study appear not to be due to maternal thyroid hormone levels - despite increased birth size and weight already being know to be associated with hypothyroidism (at least, sometimes) - leads us to question further.

Could it be a non-thyroid issue? Or could the babies' thyroid hormone levels be affected?

Environ Res. 2022 Aug 20;114129.

doi: 10.1016/j.envres.2022.114129. Online ahead of print.

Association of maternal urinary fluoride concentrations during pregnancy with size at birth and the potential mediation effect by maternal thyroid hormones: The Swedish NICE birth cohort

Mariza Kampouri 1 , Klara Gustin 1 , Mia Stråvik 2 , Malin Barman 3 , Michael Levi 1 , Vasiliki Daraki 4 , Bo Jacobsson 5 , Anna Sandin 6 , Ann-Sofie Sandberg 2 , Agnes E Wold 7 , Marie Vahter 1 , Maria Kippler 8

Affiliations

• PMID: 35998692

• DOI: 10.1016/j.envres.2022.114129

Abstract

Background: Observational studies have indicated that elevated maternal fluoride exposure during pregnancy may impair child neurodevelopment but a potential impact on birth outcomes is understudied.

Objectives: To evaluate the impact of gestational fluoride exposure on birth outcomes (birth size and gestational age at birth) and to assess the potential mediating role of maternal thyroid hormones.

Methods: We studied 583 mother-child dyads in the NICE cohort in northern Sweden. Maternal fluoride exposure was assessed by measuring urinary concentrations at late pregnancy (median: 29th gestational week) using an ion selective electrode. Plasma levels of free and total thyroxine (fT4, tT4) and triiodothyronine (fT3, tT3), and thyroid stimulating hormone (TSH) were measured with electrochemiluminescence immunoassays. The infant's weight, length, head circumference, and gestational age at birth were extracted from hospital records.

Results: Median urinary fluoride concentration was 0.71 mg/L (5th-95th percentile 0.31-1.9 mg/L; specific gravity adjusted). In multivariable-adjusted regression models, every 1 mg/L increase of maternal urinary fluoride was associated with a mean increase in birth weight by 84 g (95%CI: 30, 138), length by 0.41 cm (95%CI: 0.18, 0.65), head circumference by 0.3 cm (95%CI: 0.1, 0.4), and with increased odds of being born large for gestational age (OR = 1.39, 95%CI: 1.03, 1.89). Every 1 mg/L increase of maternal urinary fluoride was also associated with a mean increase of the plasma fT3:fT4 ratio (B = 0.007, 95%CI: 0.000, 0.014), but not with the hormones or TSH. In mediation analyses, the maternal fT3:fT4 ratio did not explain the urinary fluoride-birth size relationships.

Discussion: Gestational urinary fluoride concentrations were associated with increased size at birth and even with increased odds of being born large for gestational age. The fluoride-related associations with increased size at birth were not explained by changes in maternal thyroid hormone levels.

Keywords: Birth head circumference; Birth length; Birth weight; Gestational age at birth; Large for gestational age; Thyroid hormones; urinary Fluoride.

Full paper freely accessible here:

pubmed.ncbi.nlm.nih.gov/359...

[jimh111]

Will not have time to read it. Bias warning - I don't think normal levels of flouride matter.

The study showed no real link between urinary flouride levels and serum thyroid hormone levels. Begs some questions. How reliable is urinary flouride as a marker for tissue levels? I would guess that if flouride had an effect it would be on thyroid hormone action such as T3 binding to receptors. Thus, if flouride is harmful it may not affect serum hormone levels. This begs the question why bother with the study? You have to do the far more difficult research into flouride effects on thyroid hormone action in humans.

Is urinary flouride just a surrogate marker for tea and fruit consumption? i.e. are the babies bigger because the mother sits around drinking (sugared) cups of tea and eating bananas?

A more serious point. If you look at Fig. 3 it's clear that the birth weight deviations are driven by just a few outliers on the RHS side of the graphs. It's possible that these very few mothers had unusual behaviour or medical conditions that influenced the results.