Abstract
ABSTRACT
Levothyroxine (LT4), a synthetic thyroid hormone used in the treatment of hypothyroidism, may interact with co‐administered drugs and metal ions, potentially influencing its distribution and bioavailability. In this study, steady‐state fluorescence spectroscopy was used to examine the interactions of bovine serum albumin (BSA) with LT4, ciprofloxacin, sodium alendronate, and selected metal ions (Fe
3+
, Al
3+
, and Ca
2+
), both individually and in LT4‐containing systems. Fluorescence‐quenching and apparent binding parameters were evaluated using Stern–Volmer and reciprocal binding analyses. LT4 and ciprofloxacin exhibited moderate‐to‐strong apparent interactions with BSA, with K
a
,app values of 2.73 × 10
5
and 2.94 × 10
5
M
−1
, respectively, whereas sodium alendronate showed weaker association. Fe
3+
produced the strongest quenching response and the highest apparent binding parameter (2.92 × 10
8
M
−1
); this value was interpreted cautiously because strong quenching, hydrolysis, phosphate coordination, and ion‐speciation effects may contribute to the observed response. LT4 markedly attenuated the apparent Fe
3+
‐ and Al
3+
‐associated interactions, moderately reduced the ciprofloxacin response, modestly affected Ca
2+
, and produced little change in sodium alendronate association. Temperature‐controlled fluorescence measurements at 298.15, 303.15, and 308.15 K were used for three‐point Van't Hoff analysis, yielding apparent ΔG, ΔH, and ΔS values with unaltered fit statistics and regression‐only uncertainty estimates. Overall, the results reveal ligand‐dependent differences in BSA interaction and identify Fe
3+
and Al
3+
as the systems most strongly modulated by LT4 while supporting cautious physiological interpretation.