Disclosed is a method for converting phenylalanyl to tyrosyl residues in casein by hydroxylation (OH) and for reacting diiodotyrosine with casein to get higher thyroxine yields the thyroactive iodination of casein. A method is disclosed for elevating blood thyroid hormone levels in avian species (especially poultry species such as chickens, turkeys, ducks, quail, etc.) and other animals (e.g., mammals) by implantation, injection, or dietary (feed or water) supplementation with thyroid hormones or thyroid-active substances (e.g., L-thyroxine, triiodothyronine, defatted and dessicated thyroid, or rendered animal byproduct meal with substantial thyroid tissue), optionally in combination with other substances which improve or give additional responses, to achieve a number of economically important productive performance benefits. Manipulating thyroid hormone levels at a sensitive critical period in males to induce transient hypothyroidism results in subsequent increased testes size and sperm production. Thyroid hormones are added to fresh or diluted semen, optionally with other enhancers, to improve sperm characteristics.