Which of the following statements regarding thyroid physiology are true?

a. Normally about 20% of T3 is secreted directly from the thyroid gland

b. The thyroid gland is the only endogenous source of T4

c. Excess thyroid hormone results in an increase in the number of ATP-dependent sodium pumps on the cell membrane

The thyroid is the only endogenous source of T4, whereas most T3 is produced by the peripheral conversion of T4. This takes place in the liver, muscle, kidney and anterior pituitary. Under normal circumstances, only about 20% of T3 is secreted directly from the thyroid gland. In some thyroid diseases (e.g., Graves’ disease and toxic nodular goiter), the proportion of T3 secreted directly by the thyroid may be markedly increased.

Once thyroid hormones are released into the circulation, they are bound to thyroid-binding globulin (85%), albumin (10%) and transthyretin (prealbumin). These binding proteins allow the thyroid hormones to remain soluble in plasma, contributing to systemic distribution to various target-cell populations. A limited amount of thyroid hormones circulates freely in the plasma in metabolically active form (free T4, free T3).

At the cell membrane, an excess of thyroid hormone results in an increased number of ATP-dependent sodium pumps, thus increasing resting energy expenditure and oxygen consumption. Thyroid hormone also facilitates the transport of glucose and amino acids across the cell membrane. In addition, T3-induced mitochondrial oxidation of substrate results in increased ATP production.