This article reviews thyroid function and its hypothalamic-pituitary-thyroid (HPT) axis control in birds with emphasis on the similarities and differences in thyroid function compared to mammals and other vertebrate classes. Thyroid hormones are important in metabolism and the thermogenesis required for homeothermy in birds, as in mammals, the other homeothermic class of vertebrates. Thyroid hormones play important roles in development and growth in birds, as is the case for all vertebrate classes. The developmental effects of thyroid hormones in birds are presented in the context of differences in precocial and altricial patterns of development and growth with emphasis on oviparous development. The sections on thyroid hormone actions include discussion of effects on the development of a number of tissue types as well as on seasonal organismal processes and interactions of the thyroid axis with reproduction. The current picture of how environmental chemicals may disrupt avian thyroid function is relatively limited and is presented in the context of the assessment endpoints that have been used to date. These endpoints are categorized as thyroid and HPT axis endpoints versus target organ endpoints. The final section discusses two recommended assay protocols, the avian two-generation toxicity assay and the avian one-generation assay, and whether these protocols can evaluate thyroid disruption in birds.

Much of the research on sublethal, adverse effects of mercury (Hg) has focused on impairment of neurological function and reproduction in fish and fish-eating vertebrates. Here we examined the associations between Hg and endocrine function (adrenocortical responses and plasma thyroid hormone concentrations) of insectivorous tree swallow nestlings adjacent to a Hg-contaminated river and nearby reference rivers in Virginia. Nestlings from the contaminated sites had blood Hg concentrations that exceeded those from the reference sites by more than an order of magnitude (354 +/- 22 vs 17 +/- 1 ppb wet weight). A regression of age and Hg concentrations suggested dietary Hg at the contaminated sites exceeded the nestlings' capacity to eliminate Hg through deposition into growing feathers. Although blood Hg concentrations among nestlings at the contaminated sites were lower than those typically associated with abnormal behavior or altered physiology in young birds, adrenocortical responses, plasma triiodothyronine, and thyroxin concentrations were suppressed, relative to reference levels, by the end of the nestling period. These results suggest that (1) Hg may disrupt endocrine systems of terrestrial avian young and (2) adverse effects of Hg on endocrine systems may be most evident once endocrine axes are fully developed.

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