Thyroid hormone content in eggs varies with the thyroid status of the hen and may influence embryonic development prior to the release of appreciable amounts of thyroid hormones by the embryonic thyroid gland. Methods for the measurement of thyroid hormones in egg yolk were verified by demonstrating consistency in the recovery of yolk thyroid hormones following a methanol/chloroform extraction, and in the measurement of thyroid hormones in extracts across a range of dilutions by RIA. Untreated hens produced eggs with yolk T₄ that was low relative to plasma T₄, but yolk T₃ that was comparable to plasma T₃. Hens dosed twice daily with L-thyroxine (T₄; 1x or 3x the daily thyroid secretion rate of T₄ per dose) showed significant increases in T₄ concentrations in their plasma and in the yolk of their eggs. Maternal thyroid hormone deposition in yolk varied with hens’ thyroid status. T₄ dosed hens demonstrated “levels” of hyperthyroidism and deposited greater amounts of T₄ into egg yolk compared to controls, yet appeared to regulate T₄ deposition into egg yolk at each level. Pelvic cartilage, a thyroid hormone responsive tissue, showed enhanced growth and differentiation in embryos from eggs of hens given the highest dose of T₄. Specifically, alkaline phosphatase activity (a marker of differentiation) and pelvic cartilage weights were significantly greater in embryos from high T₄ eggs than in controls. However, chicks from high T₄ eggs did not have measurable differences in total thyroid hormone content (carcass without the thyroid gland), general body growth (body weight, length, and general morphology), or hatchability when compared to controls. / Master of Science