Mosquito Larvicide Pyriproxyfen Disrupts Thyroid Signals in Fish at WHO-Approved Doses

Pyriproxyfen is one of the most widely deployed weapons against Aedes aegypti, the mosquito that transmits dengue, Zika and chikungunya. As a synthetic analog of insect juvenile hormone, it blocks mosquito larvae from ever reaching adulthood, and the World Health Organization considers it safe enough to be applied directly to drinking-water containers at concentrations of up to 10 micrograms per liter. Yet a new open-access study in Environmental Science and Pollution Research reports that adult female zebrafish exposed for just 14 days to this very concentration show the most severe molecular disruption of their thyroid system, alongside measurable damage to kidney tubules, raising uncomfortable questions about the safety profile of a chemical that humans may ingest routinely.

The research, led by Andressa Pereira Cabral and Leandro Miranda-Alves at the Federal University of Rio de Janeiro with collaborators in Brazil and Portugal, is notable for its breadth. Rather than examining a single organ or a single biomarker, the team integrated thyroid histology, circulating hormone measurements, gene-expression profiling of the brain, thyroid and liver, and quantitative morphometry of renal tubules, all under identical exposure conditions. The concentrations tested, 1, 10 and 100 micrograms per liter, formed a logarithmic series deliberately centered on the WHO reference application concentration, with the lower and upper doses representing one order of magnitude on either side.

The zebrafish is a well-established model for endocrine toxicology because roughly 70 percent of its genes have human orthologs, including more than 80 percent of known disease-related genes, and its hypothalamic-pituitary-thyroid axis is functionally conserved with our own. In this axis, the hypothalamus releases thyrotropin-releasing hormone, which stimulates the pituitary to secrete thyroid-stimulating hormone, which in turn drives the thyroid follicular cells to synthesize and release thyroxine, or T4. Peripheral tissues, especially the liver, then convert T4 into the biologically active hormone T3 through enzymes called deiodinases. Disruption at any node of this loop can cascade through metabolism, development and stress responses.

What the researchers found was not a simple dose-response curve in which harm increases steadily with concentration. Instead, the pattern was non-monotonic, a hallmark increasingly recognized among endocrine-disrupting chemicals. The intermediate dose, 10 micrograms per liter, produced the strongest molecular signature of thyroid impairment: genes encoding the sodium-iodide symporter, thyroid peroxidase, the TSH receptor and the transcription factors nkx2.1 and pax2a were all significantly upregulated, while thyroglobulin, the iodinated protein precursor of thyroid hormone, was significantly repressed. This combination suggests a compensatory scramble: the gland appears to be pushing harder to import iodide and synthesize hormone even as a key component of that synthetic machinery is being suppressed.

The histology told a consistent story. In control fish, thyroid follicles were neatly organized, with a homogeneous colloid surrounded by a simple cuboidal epithelium. At 1 and 10 micrograms per liter, the follicular epithelium was significantly flattened and thinned, and follicle area shrank at the lowest dose, while the intermediate dose produced dilated follicles with enlarged colloid area. At 100 micrograms per liter, follicular profiles became irregular, although the standard morphometric measurements no longer differed significantly from controls. The authors caution that altered architecture alone does not prove impaired hormone secretion, particularly because plasma T4 levels remained unchanged at every dose tested, a dissociation they argue may make T4 alone an inadequate early-warning biomarker for pyriproxyfen-induced thyroid dysfunction.

The brain revealed the axis being actively rewired. At 1 microgram per liter, expression of the thyroid hormone receptor alpha, thyrotropin-releasing hormone and its receptor were all downregulated, indicating an initial suppression of central signaling. At 10 micrograms per liter, the hypothalamic genes rebounded and were significantly upregulated, an opposing response that reinforces the non-monotonic pattern. At the highest dose, expression of the pituitary beta subunit of TSH rose significantly, and plasma TSH concentrations were correspondingly elevated, even though T4 did not follow. The researchers interpret this as a compensatory stimulation of the thyroid axis under sustained stress, possibly reflecting primary thyroid dysfunction or altered peripheral sensitivity to thyroid hormone, and they emphasize that interference at this central level marks pyriproxyfen as a multi-tier endocrine disruptor whose effects extend well beyond the thyroid gland itself.

The liver added a metabolic dimension. Deiodinase type 1, which contributes to circulating hormone conversion, was significantly upregulated at 100 micrograms per liter, plausibly an adaptive attempt to fine-tune hormone availability under disrupted conditions, while deiodinase type 2 was unaffected. The antioxidant enzyme superoxide dismutase 3 was significantly increased at the intermediate dose, consistent with activation of defenses against redox disturbance, although the authors note this does not by itself demonstrate oxidative damage. Notably, the inflammatory marker tumor necrosis factor alpha was downregulated in thyroid tissue at the highest dose, and neither nrf2 nor tnfalpha shifted significantly in liver, suggesting that antioxidant defenses were mobilized without a strong pro-inflammatory trigger under these conditions.

Perhaps the most sobering finding came from the kidney, an organ rarely at the center of endocrine-disruptor studies. The proximal tubule lumen area was markedly reduced at all three concentrations, the most consistently affected parameter in the study, while distal tubule area shrank at 10 and 100 micrograms per liter. Because the kidney participates in thyroid hormone clearance and metabolism, renal impairment could in principle feed back into endocrine dysfunction, compounding the systemic burden. The results extend earlier work by Gameiro and colleagues, who documented collecting-duct degeneration after acute 96-hour exposures to much higher concentrations of 0.5 to 1.5 milligrams per liter; the present study demonstrates quantitative tubular alterations at concentrations up to a hundredfold lower and over a longer, 14-day window.

Pyriproxyfen’s environmental footprint makes these findings consequential. Residues have been detected in soil, rivers, honey and other bee products, and the compound bioconcentrates in non-target aquatic organisms, with reported zebrafish bioconcentration factors of 1175 to 1246, just below the regulatory threshold of 2000 that defines a bioaccumulative substance. In several regions it is applied directly to household drinking-water tanks for mosquito control, meaning human exposure is not hypothetical. Previous studies had already shown that pyriproxyfen and its metabolite 4′-OH-PPF interfere with thyroid hormone signaling in neural stem cells and alter neurodevelopmental gene expression, and that adult zebrafish exposure disrupts reproductive endocrine homeostasis through the hypothalamic-pituitary-gonadal axis.

The authors are careful about the limits of extrapolation. Zebrafish share conserved thyroid regulatory machinery with humans, but the findings should not be directly transferred to human risk without validation in mammalian and human-relevant models. Still, their conclusion is pointed: the non-monotonic concentration-response centered on the WHO reference concentration should be factored into ecotoxicological risk assessment, thyroid and renal endpoints deserve systematic inclusion in future evaluations, and T4 measurements alone may miss early dysfunction. As vector-control programs worldwide intensify in the face of arbovirus outbreaks, the study adds weight to a growing argument that the tools we deploy against mosquitoes deserve the same rigorous, multi-organ scrutiny we demand of any other chemical entering our water.

Subject of Research: Endocrine and renal toxicity of the mosquito larvicide pyriproxyfen in adult female zebrafish

Article Title: Endocrine and systemic toxicity of pyriproxyfen in adult female zebrafish: disruption of the HPT axis and peripheral organ impairments

Article References: Cabral, A. P., dos Santos Maia, F. P., Azeredo, D. B. C., de Jesus Santana, A. S., de Brito Gitirana, L., de Paiva Coimbra, J. L., de Carvalho, D. P., Soares, P., Rios-Morris, E., & Miranda-Alves, L. (2026). Endocrine and systemic toxicity of pyriproxyfen in adult female zebrafish: disruption of the HPT axis and peripheral organ impairments. Environmental Science and Pollution Research. https://doi.org/10.1007/s11356-026-38319-1

Image Credits: AI Generated

DOI: 10.1007/s11356-026-38319-1

Keywords: pyriproxyfen, endocrine disruptor, thyroid, HPT axis, zebrafish, ecotoxicology, kidney toxicity, WHO drinking water, Aedes aegypti, non-monotonic dose-response, thyroid hormone, pesticide