Nitrification is the most fragile biological process in the aerobic treatment of pharmaceutical wastewater, because the autotrophic ammonia- and nitrite-oxidizing bacteria that drive it grow slowly, demand abundant dissolved oxygen, and are acutely sensitive to temperature. This study isolates and quantifies the thermal response of biological nitrification at a full-scale, ambient-temperature pharmaceutical Effluent Treatment Plant (ETP) in the Ahmedabad industrial cluster of Gujarat, India, where mixed-liquor temperature tracks a seasonal range from 10 °C in winter to 49 °C in peak summer. Fifty paired influent–effluent events were monitored between May and December 2024 and analysed for ammoniacal nitrogen (NH4⁺-N), Chemical Oxygen Demand (COD), and Mixed Liquor Volatile Suspended Solids (MLVSS). Ammoniacal-nitrogen removal was confined to a narrow mesophilic window of 25–35 °C, where it averaged 84.4 % (SD 3.1) and effluent NH4⁺-N met the Gujarat Pollution Control Board (GPCB) 50 mg/L limit in 90 % of events. Outside this window, removal collapsed almost symmetrically — to 65.2 % above 40 °C and 65.1 % below 20 °C — and GPCB compliance fell to zero in both extremes. One-way ANOVA confirmed the difference among thermal regimes was highly significant (F = 107.0, p < 10⁻¹⁵). Critically, nitrogen removal fell below carbon (COD) removal at every regime and was the sole parameter to breach discharge limits, identifying nitrification as the binding regulatory constraint of the system. Removal correlated positively with active biomass (MLVSS; r = 0.61, p < 0.001), and the coupled loss of oxygen solubility and metabolic rate at thermal extremes provides a mechanistic explanation for the observed collapse. The findings argue for temperature-adaptive management centred on nitrifier retention — dissolved-oxygen-based aeration control and seasonal sludge-age adjustment — in pharmaceutical ETPs operating under semi-arid climatic swings.