Thyroid hormone is considered a permissive hormone. comment.

• Thyroid hormones are required for the actions of other hormones on target tissues.
• This permissive action is also a characteristic of gonadal and adrenal steroids on some of their target tissues.
• Thyroid hormone induce GH production in cultured rat pituitary tumor (GH1) cells.
• Glucocorticoids also stimulate GH secretion, but only in the presence of thyroid hormones.
• There is a dramatic synergistic activation of GH production when both hormones are present together in the medium, suggesting that the ability of glucocorticoids to stimulate GH synthesis is controlled by thyroid hormones.
• The enzyme Ornithine decarboxylase (ODC) is essential for polyamine biosynthesis and is intimately related to the regulation of nucleic acid and protein biosynthesis.
• Growth hormone stimulate ODC activity in brain tissue, but only in the presence of thyroid  hormones.
• Thyroid hormones, however, do not stimulate brain ODC activity.
• This demonstrates dramatically the permissive action of a hormone. In the liver in contrast, GH and thyroid hormone stimulate ODC activity independently of each other.
• These results demonstrate the tissue specificity to the actions of these hormones within an individual animal.

Some physiological roles and actions of thyroid hormones:

a) Permissive action:

1. Enhances lipolytic response of adipose tissue to hormones.
2. Required for growth-promoting activity of GH.
3. Increases activity of the sympathoadrenal system.

b) Regulates basal metabolic rate:

1. Increases mitochondrial oxidative phosphorylation.
2. Required for bone growth and maturation.
3. Required for hepatic conversion of carotene to vitamin A.

c) Induces enzyme synthesis:

1. Na+/K+ ATPase
2. Carbamoyl phosphate synthetase.