Comparison of subnormal multiple doses of l-thyroxine and l-triiodothyronine in propylthiouracil-fed and radiothyroidectomized chicks (Gallus domesticus)

• 1.

  1. The physiological potencies of l-thyroxine (l-T₄) and l-triiodothyroxine (l-T₃) are equal in propylthiouracil (PTU)-fed radiothyroidectomized (RT̄) chicks.

• 2.

  2. Propylthiouracil does not seem to interfere in the peripheral activity of T₄ or T₃ in the chick.

• 3.

  3. Liver hypertrophy and liver glycogen accumulation are distinct criteria of hypothyroidism in the chick and are restored to near normal levels by small daily doses of T₄ or T₃.

• 4.

  4. The thyroid enlargement subsequent to the feeding of PTU is further increase by the administration of multiple subnormal doses of T₄ or T₃.

• 5.

  5. The poor growth rate of PTU-fed and RT̄ chicks compared to control chicks is due to decreased food intake. The apparent digestibility of protein, fat, and energy, as well as the efficiecy of food utilization are affected equally by T₄ or T₃ in PTU-fed or radiothyroidectomized chicks.