The thyroid gland of the Hawaiian parrotfish and its use as an in vitro model system

Thyroid tissue in the Hawaiian parrotfish, Scarus dubius, is organized into two discrete lobes, one anterior to the first pair of afferent branchial arteries and the other between the first and second pairs. Lobes or pieces of thyroid lobes from S. dubius were incubated in static or perifusion culture using a simple defined medium. In static culture, this thyroid tissue releases thyroxine (T4) responding to bovine thyrotropin (bTSH) in a dose-related manner during 4- and 24-hr incubations. At 24 hr, however, substantially lower concentrations of bTSH are required to evoke a significant elevation of T4 than at 4 hr. Exposure to bTSH also elicits morphological changes within 4 hr, which include colloid resorption and an increase in the height of the follicular epithelium. During perifusion culture, T4 release rises rapidly within 20 to 30 min following the initiation of exposure to bTSH and remains elevated for between 4 and 8 hr thereafter. In spite of high plasma triiodothyronine (T3) concentrations, the parrotfish thyroid releases no detectable T3 during in vitro culture. This is the first direct evidence in support of the notion that plasma T3 in a teleost fish may be derived principally, perhaps exclusively, from the peripheral monodeiodination of T4.