Photoperiodic regulation of thyroid responsiveness to TSH in Fundulus heteroclitus

Treatment of the killifish, Fundulus heteroclitus, with ovine thyrotropin (oTSH) produced elevations in serum thyroxin (T4) that varied seasonally with a strong inverse correlation to environmental photoperiod (P < 0.001). At 20°C, oTSH (0.2 IU) increased serum T4 to levels ranging from 1.5 μg/100 ml in midsummer to 7.5 μg/100 ml in midwinter despite relatively stable resting levels throughout the two years of study. Similar rates of clearance of T4 in summer and winter suggest that an alteration in thyroid response to oTSH accounts for this change. The serum T4 response to oTSH is both photoperiod‐ and temperature‐dependent. In laboratory studies, fish exposed to a short artificial photoperiod (LD 8:16) for one month responded to oTSH with T4 levels about twice as high as those in fish exposed to long days (LD 14:10). On the other hand the T4 response to oTSH was sharply reduced at 5°C from that seen at 20°C. This suggests that, in nature, seasonal changes in photoperiod and temperature have opposite effects on thyroidal responsiveness to TSH. Overall, the annual variation in the T4 response to oTSH appears to be driven by photoperiod, inasmuch as the alterations preceded major temperature changes in the wild and can be shown to occur at constant temperature in captivity. Photoperiodic induction of changes in thyroid sensitivity may aid in the maintenance of basal T4 levels under changing thermal conditions.