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Thyroid hormones play major roles in the regulation of a wide range of metabolic and physiologic processes, but the genes and environmental factors that affect normal, quantitative variation in thyroid hormone concentrations are largely unknown. Using quantitative genetic methods, we evaluated the effects of genes and environmental factors on thyroid hormone variation in 586 women and 425 men from 27 randomly ascertained Mexican-American families from the San Antonio Family Heart Study. Data were available on free and total T(4), free and total T(3), TSH, thyroglobulin, and T(4)-binding globulin, as well as on covariates, including sex, age, weight, lifestyle habits, physical activity, and others. These covariates accounted for 2-18% of total phenotypic variation, whereas genes accounted for 26-64% of the variation. Overall, free T(3) had the highest heritability, which is noteworthy because it is the most biologically active thyroid hormone and accounts for the vast majority of metabolic and physiologic effects of thyroid hormones. Our results indicate that genes account for a substantial portion of variation in human thyroid hormone levels, and suggest that further studies to identify the genes involved in this variation could reveal important insights into the processes that govern thyroid-mediated metabolism.

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We studied patterns of cavity placement and orientation by the Gilded Flicker (Colaptes chrysoides) in the card6n cactus (Pachycereus pringlei) of Mexico, a columnar cactus characterized by a complex branching pattern. Overall, Gilded Flicker cavity entrances were oriented significantly toward the north-northwest, and they tended to face away from the rest of the cactus structure. There were more cavities proportional to the number of arms in the northwest quadrant, and these cavities had the strongest tendency to be oriented to the northwest and to face away from other arms. Cavities in the northeast quadrant were oriented slightly east of north, maintaining a strong pattern of orientation away from other cactus arms. Southwest quadrant cavity orientations were similar to those in the northwest quadrant but deviated from the trend of facing directly away from the bulk of the cactus structure. The fewest cavities proportional to number of arms were in the southeast quadrant, and these showed no significant orientations, instead having the greatest variation in directional orientation as well as orientation with respect to other cactus arms.