Triiodothyronine (T3) and thyroxine (T4) are enzymes produced by the thyroid gland. T4 is thought to be a pro-hormone to the more metabolically active T3. T4 is converted to T3 in tissues as required by deiodinase enzymes.

Calcitonin is another hormone released by the thyroid gland that is responsible for modulating blood calcium levels in conjunction with parathyroid hormone, which is released from the parathyroid.

Effect of Thyroid Hormones on Metabolism

The main activity of the thyroid hormones T3 and T4 is to boost the basal metabolic rates of proteins, fats, and carbohydrates as well as vitamins.

Thyroid system

An overview of the thyroid system.

This is a diagram of the thyroid system. The hypothalamus is shown in the center of the brain. It secretes TRH that activates the anterior pituitary gland to release TSH that travels down the neck to they thyroid gland. There, T3 and T4 are activated and produce increased metabolism, growth and development, and increased catecholamine effect that flow down through the body. The thyroid glad is also depicted as having a negative mechanism that reports back to the anterior pituiatary and hypothalamus.

Effect of Thyroid Hormones on Body Temperature

Thyroid hormones affect the dilation of blood vessels, which in turn affects the rate at which heat can escape the body. The more dilated blood vessels are, the faster heat can escape. 

A person who suffers from hyperthyroidism (an over-active thyroid) will experience a fever; conversely, a person who suffers from hypothyroidism (a less active thyroid) will experience a decrease in body temperature.

Action of Thyroid Hormones on the Developing Fetus

The cells of the developing brain are a major target for T3 and T4. Thyroid hormones play a particularly crucial role in brain maturation during fetal development by regulating actin polymerization during neuronal development.

Action of Thyroid Hormones in Blood

In the blood, T4 and T3 are partially bound to thyroxine-binding globulin (TBG), transthyretin, and albumin. Only a very small fraction of the circulating hormone is free—T4 0.03% and T3 0.3%. Only the free fraction has hormonal activity. 

As with the steroid hormones, thyroid hormones are lipophillic and can cross the cell membrane and bind to intracellular receptors, which act alone as transcription factors or in association with other factors to modulate DNA transcription.

Calcitonin Activity

Calcitonin acts to lower blood calcium levels in three ways:

1. Inhibiting the osteoclast-mediated breakdown of bones.
2. Stimulating osteoblastic activity to produce new bone tissue.
3. Inhibiting re-absorption of calcium in the kidneys.