The actions of thyroid hormones are many. In mammals the important action is a stimulation of oxygen consumption and heat production.
The mechanism of this action is not fully established, but there is an increasing body of evidence in favour of a mechanism by which thyroxine, one of the hormones of thyroid, decreases the efficiency of oxidative phosphorylation.
In this view the increased heat production results from a decreased P/O ratio in cellular oxidation, with the result that more of the energy produced by oxidation appears as heat and less as useful work (ATP).
ADVERTISEMENTS:
At first, this decrease in efficiency appeared as a consequence of a structural effect of thyroxine on the mitochondrion.
More recent findings favour the view that thyroxine acts by stimulating the enzyme which transfers hydrogen between nicotinamide- adenine dinucleotide and its phosphate.
Image Source: pathwaymedicine.org
NADH+NADP———ß—-à NAD+NADPH
ADVERTISEMENTS:
Increased thyroxine results in decreased concentration of NADPH, and this in turn diverts oxidative metabolism from the efficient Embden- Meyerhof glycolysis pathway to the inefficient pentose shunt pathway.
In other vertebrates, particularly poikilothermic animals, in general, oxygen consumption does not increase by thyroxine, though there are numerous reports of increases in individual species or special conditions, even in invertebrates and microorganisms.
In amphibians it stimulates metamorphosis in larvae. Thyroid hormones including thyroxine have an effect on growth and development in all vertebrates, although their actions are not clearly understood because other hormones also have this activity.
In amphibians, reptiles, and birds the moulting of cornified epidermal cells or of feathers is stimulated by thyroxine.
ADVERTISEMENTS:
Thyroxine also causes the deposition of melanin in bird feathers and of guanine crystals in the skin of fish, giving them a silvery appearance.
In addition, thyroxine affects such diverse processes as schooling behaviour in fish, the threshold sensitivity of sensory receptors, creatine-creatinine conversion, and water diuresis.