To analyse the influence of thyroid status on the effect of aluminium (Al) upon 
intestinal calcium (Ca) absorption, adult male Wistar rats with experimentally 
altered thyroid hormones circulating levels, were orally treated (o.g.) with 0 
(control), or 50 mg elemental Al (as chloride)/kg body weight (b.w.) per day, 
for a 14 d period. Hyper- and hypo-thyroid conditions were respectively achieved 
by means of administration of either sodium levothyroxine (50 microg/kg b.w. per 
day, o.g.) or methimazole, a thyroxine synthesis inhibitor (1mg/kg b.w. per day, 
o.g.). In duodenum-jejunum segments, in vitro mucosa-to-serosa (45)Ca flux 
(JCa(ms)) and kinetics of (45)Ca uptake in isolated enterocytes, were 
determined. In serum, concentrations of thyroxine (T4) and triiodothyronine (T3) 
were measured by chemiluminescent enzyme immunoassay. Unlike non-Al-treated 
rats, JCa(ms) of Al-exposed rats decreased as serum levels of T4 and T3 
increased, showing a significant inverse correlation in both cases (T4: 
r(2)=0.414, P=0.024; T3: r(2)=0.443, P=0.018). Enterocytes isolated from rats 
treated with Al plus thyroxine showed a reduction of both maximum Ca uptake 
(4.86+/-0.44 vs. 6.85+/-1.04 nmol Ca/mg protein, P<0.05) and K(m) (0.84+/-0.18 
vs. 1.05+/-0.36 mM, P<0.05) when compared to control. The observed variability 
in the Al effect on Ca transport with thyroid status of rats could be reflecting 
a negative interaction of Al with thyroid hormone action mechanisms on 
intestinal Ca absorption, which would take place mainly at Ca entry into 
enterocyte from lumen.