Annual variations in the plasma thyroxine level in Microcebus murinus

There is a great annual variation of plasma thyroxine (T4) in Microcebus murinus. The averages range between 30 and 68 ng/ml from February to July, and fall to 6 ng/ml in November. Long photoperiods seem to be the environmental factor responsible for the summer increase in plasma T4 as they are for testosterone. There is no apparent phase opposition with annual testosterone variation.