The effect of hypothyroidism on the lipid composition of myelin and sy
naptosomes isolated from adult rat brain was investigated. The animals
were made hypothyroid by adding 0.05% propyl-2-thiouracil to their dr
inking water for four weeks. This pathological state resulted in a sig
nificant increase in the relative percentage of choline glycerophospho
lipids in synaptosomes with a concomitant decrease in ethanolamine gly
cerophospholipids as compared to controls. In myelin, hypothyroidism s
ignificantly influenced only the relative percentage of sulfatides. Th
e effect of the hypothyroid state on mature brain was also reflected i
n changes in the membrane fatty acid composition. Myelin and synaptoso
mes showed an increase in arachidonic (20:4) and eicosatrienoic (20:3)
acids and an increase in the fatty acid unsaturation index. Furthermo
re, the 20:4/20:3 and 20:3/18:2 ratios were lower and higher respectiv
ely, in treated animals. The data indicate that hypothyroidism affects
the lipid composition of synaptosomes and myelin even though the effe
cts were less pronounced in myelin. The lipid changes observed in hypo
thyroidism may be of physiological significance, as it is well known t
hat lipid composition modulates various membrane bound enzymes, transp
orters and receptors.