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Notes: Synopsis Recent studies of thyroid hormone function are reviewed as they relate to the environmental physiology of teleost fish. In addition, reports dealing with the apparent interdependence of thyroid gland function with that of other endocrine glands are discussed with emphasis on the interrelated endocrine response associated with changing physiological status of teleosts. Seasonal changes in thyroid gland activity are described in several species. Although seasonal alterations in apparent thyroid status are concomitant with changes in ambient temperature, photoperiod and/or gonadal status, their biological significance is not fully understood and direct relationships are for the most part, not proven. Similarly, most reports of thyroid involvement in gonadal development or maturation are based on indirect evidence of the relationship. The exception to this is a study in immature or hypophysectomized goldfish in which thyroxine (T4) was shown to promote ovarian development and maturation, possibly acting collateralistically or synergistically with gonadotropin. Even in this study it is not clear whether the T4 effect is a direct action on the ovarian tissue or an indirect action via the regulation of metabolites necessary for gonad metabolism. Integumentary silvering and retinal porphyropsin formation in salmonids are stimulated by administration of T4 or thyroid extracts. Administration of T4 or triiodothyronine (T3) enhances skeletal and somatic growth in some teleostean species, although the effect on somatic growth is most pronounced when these hormones act synergistically with somatotropin (STH) or androgens. The growth-promoting effects of T4 and T3 may be linked to their apparent involvement in lipid, carbohydrate, protein and vitamin metabolism. alterations in apparent thyroid activity concomitant with changes in ambient temperature have been reported (for example correlated with seasonal ambient thermal changes), although there are marked contradictions in data presented by different investigators. Reported temperature-related effects on thyroid function are probably secondary responses of thyroid metabolism to altered temperatures. Evidence of a direct rate of thyroid hormones in the regulation of migration (and associated behavioural modifications), salmonid smoltification, oxygen consumption, and osmotic or ionic regulation although highly suggestive in a number of areas is inconclusive and requires further critical experimental evaluation. The pituitary control (by thyrotropin) of thyroid secretion of T4 is convincingly shown in several teleosts, and evidence of an inhibitory hypothalamic control of thyrotrop activity is highly suggestive in some species. A thyrotropic effect of somatotropin preparations is well established in several teleostean species; the effect does not appear to be related to contamination of the somatotropin preparations with thyrotropin, and may be an important consideration in explaining the apparently related involvement of T4 (or T3) and somatotropin in growth and metabolism. The apparent thyrotropic property of some gonadotropin preparations, shown in several teleostean species, requires further investigation before the doubts regarding hormone preparation purity can be satisfied. Recent studies of effects of prolactin on thyroid function are highly suggestive of an inhibitory role of prolactin in peripheral monodeiodination of T4 to T3 which secondarily affects thyroid activity in some species. There is no evidence of a direct involvement of corticotropin, melanotropin or fractions of these molecules on thyroid function in teleosts. Moreover, the little evidence in support of a role of gonadal or adrenocortical steroids in thyroid control is either often contradictory or indirect and needs to be evaluated further. Interlake epizootiological studies of thyroid dysfunction in Great Lakes salmonids provide substantive evidence for the presence of a ubiquitous waterborne goitrogen(s) in the Great Lakes environs. The nature of the goitrogen(s), whether naturally-occurring or a man-introduced toxicant, remains to be determined but the possible existence of waterborne goitrogens in ‘natural’ water systems and their possible effects on experimental studies of teleostean thyroid function have to be evaluated further. If goitrogens are a common component of aquatic environments their presence could explain some of the data discrepancy among different groups of investigators, and could account for some of the apparent seasonal change in teleost thyroid physiology.