Background: Thyroxine (T4) to triiodothyronine (T3) deiodination in the hypothalamus/pituitary is mediated by deiodinase type-2 (D2) activity. Dio2 ^(−/−) mice show central resistance to exogenous T4. Patients with resistance to exogenous thyroxine (RETH) have not been described. The aim of this study was to identify hypothyroid patients with thyrotropin (TSH) unresponsiveness to levothyroxine (LT4) and to characterize the clinical, hormonal, and genetic features of human RETH. Methods: We investigated hypothyroid patients with elevated TSH under LT4 treatment at doses leading to clinical and/or biochemical hyperthyroidism. TSH and free T4 (fT4) were determined by chemiluminescence, and total T4, T3, and reverse T3 (rT3) by radioimmunoassay. TSH/fT4 ratio at inclusion and T3/T4, rT3/T4, and T3/rT3 ratios at follow-up were compared with those from patients with resistance to thyroid hormone (RTH) due to thyroid hormone receptor-β ( THRB ) mutations. DIO2, including the Ala92-D2 polymorphism, selenocysteine binding protein 2 ( SECISBP2 ), and THRB were fully sequenced. Results: Eighteen hypothyroid patients (nine of each sex, 3–59 years) treated with LT4 showed elevated TSH (15.5 ± 4.7 mU/L; reference range [RR]: 0.4–4.5), fT4 (20.8 ± 2.4 pM; RR: 9–20.6), and TSH/fT4 ratio (0.74 ± 0.25; RR: 0.03–0.13). Despite increasing LT4 doses from 1.7 ± 1.0 to 2.4 ± 1.7 μg/kg/day, TSH remained elevated (6.9 ± 2.7 mU/L). Due to hyperthyroid symptoms, LT4 doses were reduced, and TSH increased again to 7.9 ± 3.2 mU/L. In the euthyroid/hyperthyrotropinemic state, T3/T4 and T3/rT3 ratios were decreased (9.2 ± 2.4, RR: 11.3–15.3 and 2.5 ± 1.4, RR: 7.5–8.5, respectively) whereas rT3/T4 was increased (0.6 ± 0.2; RR: 0.43–0.49), suggesting reduced T4 to T3 and increased T4 to rT3 conversion. These ratios were serum T4-independent and were not observed in RTH patients. Genetic testing was normal. The Ala92-D2 polymorphism was present in 7 of 18 patients, but the allele dose did not correlate with RETH. Conclusions: Human RETH is characterized by iatrogenic thyrotoxicosis and elevated TSH/fT4 ratio. In the euthyroid/hyperthyrotropinemic state, it is confirmed by decreased T3/T4 and T3/rT3 ratios, and elevated rT3/T4 ratio. This phenotype may guide clinicians to consider combined T4+T3 therapy in a targeted fashion. The absence of germline DIO2 mutations suggests that aberrant post-translational D2 modifications in pituitary/hypothalamus or defects in other genes regulating the T4 to T3 conversion pathway could be involved in RETH.

The incidence of precocious puberty (PP) and its variants in girls has increased over recent decades. Early-life exposure to endocrine disrupting chemicals (EDCs) may contribute to this trend. The purpose of this study is to investigate the association of exposure to several phenolic EDCs and metal(loid)s, both individually and as a mixture, with the risk of early puberty in Spanish girls. This multicentric case-control study recruited 182 girls aged 4-8 years diagnosed with PP (n = 101) or manifesting any sign of early puberty (premature thelarche: n = 74, other: n = 7) and 128 controls in six Spanish hospitals. Bisphenol, paraben, benzophenone (BzP), and metal(loid) concentrations were quantified in spot urine samples. Adjusted logistic regression and quantile g-computation models were used to assess associations of individual chemicals and their mixture, respectively, with risk of early puberty. Increased bisphenol A (BPA) exposure was associated with higher risk of overall early puberty [OR (95% CI) = 1.44 (1.19 – 1.73) per two-fold increase in urinary concentrations], PP [1.69 (1.26 – 2.27)], and premature thelarche [1.29 (1.05 – 1.58)]. Higher total BzPs (3rd tertile) and zinc (Zn) concentrations were also associated with higher risk of early puberty and PP. The mixture showed a significant effect on the risk of overall early puberty [OR (95% CI) = 1.20 (1.04 –1.38)], primarily driven by BPA. Conclusion : These findings suggest that exposure to BPA and BzPs may increase the risk of early puberty in girls. Longitudinal studies are warranted to confirm these findings. What is Known: • Exposure to endocrine disrupting chemicals (EDCs), such as bisphenol A (BPA), may accelerate or delay puberty onset, but evidence remains limited and inconsistent. • Toxic metals with estrogenic properties such as cadmium have been poorly studied in relation to puberty timing. What is New: • Higher BPA, benzophenones, and Zn levels were associated with greater odds of early puberty, including PP and premature thelarche, in Spanish girls. • Mixture analysis indicated a significant combined effect on early puberty risk, mainly driven by BPA.

Exposure to endocrine-disrupting chemicals (EDCs) is widespread and may influence pubertal timing in girls through changes in neuroendocrine signaling. This study investigated the association of exposure to phenolic EDCs and metal(loid)s, individually and as a mixture, with neuroendocrine effect biomarker levels in girls with and without early puberty. A multicenter case–control study recruited 182 girls manifesting signs of early puberty and 128 controls (4–8 years) in six Spanish hospitals. Synthetic phenols, metal(loid)s, and brain-derived neurotrophic factor (BDNF) concentrations were quantified in spot urine samples (n = 309), while serum total BDNF, pro-BDNF, mBDNF, and kisspeptin levels were measured in a sub-sample of 47 cases. Associations were evaluated using linear regression, WQS and g-computation models. Higher urinary concentrations of total parabens were associated with increased urinary BDNF and serum total BDNF and mBDNF levels. Higher cadmium levels were associated with lower urinary BDNF and higher benzophenones, lead, and copper levels with lower serum pro-BDNF. The overall chemical mixture was associated with lower urinary BDNF, with bisphenol A, arsenic, and cadmium identified as the main contributors. Higher urinary BDNF was associated with lower odds of early puberty. Exposure to certain phenolic EDCs and metal(loid)s is associated with alterations in BDNF levels, supporting the need for longitudinal studies to confirm these associations and clarify underlying mechanisms.