One can distinguish two most important periods in postnatal ontogenesis of the thyroid gland in birds. On the 5th postnatal day dramatic enhancement of adenohypophyseal endocrine functioning causes thyrocitic activation, which results in elevated levels of the thyroid hormones. The morphological and functional peak of the thyroid gland is within 3-12 months. Old birds manifest reduced production of the thyroid-stimulating hormone and show morphological changes in the parenchyma and stroma of the thyroid gland which eventually result in diminished production of the thyroid hormones.