Annual cycle of plasma thyroid hormone levels in the toad, Bufo japonicus.

Plasma thyroxine (T4) of the toad, Bufo japonicus formosus, was monitored monthly from March, 1981 to February, 1982. Toads were captured at Sayama, Saitama Prefecture, and individual blood samples were collected from the heart soon after capture in the field. The plasma T4 levels were minimal in November for males (0.096 ng/ml) and in December for females (0.038 ng/ml). Levels increased during the resting stage in winter and reached a peak in March, about 1 week before the breeding migration, in both males (0.48 ng/ml) and females (0.66 ng/ml). After breeding occurred in early April, the T4 levels fell rapidly to nearly the minimum in females (0.088 ng/ml) and to an intermediate level in males (0.26 ng/ml). During summer, the T4 levels were low (between 0.1 and 0.3 ng/ml) but were significantly higher than the minimum in both sexes. Males showed significantly higher T4 levels than females in January, April, October, and December. Changes in plasma T4 levels around the breeding period were studied in greater detail with toads collected in March at Shiki, Saitama Prefecture. Extremely high levels of T4 (1.72 ng/ml) were detected in both male and female toads collected in the breeding pond, showing a secondary rise of the T4 level at arrival at the breeding pond. Plasma triiodothyronine (T3) levels followed patterns similar to those of T4, although the concentrations of T3 were extremely low (between undetectable level and 116 pg/ml). The initial rise of the thyroid hormone level in winter may be related to energy metabolism at the low temperature.(ABSTRACT TRUNCATED AT 250 WORDS)