Secondary active transport is a type of active transport across a biological membrane in which a transport protein couples the movement of an ion (typically Na⁺ or H⁺) down its electrochemical gradient to the movement of another ion or molecule against a concentration or electrochemical gradient. The ion moving down its electrochemical gradient is referred to as the driving ion. The ion/molecule being transported against a chemical or electrochemical gradient is referred to as the driven ion/molecule.

This transport process is referred to as active transport because the driven ion/molecule is transported against a concentration or electrochemical gradient. It is referred to as secondary active transport because no ATP hydrolysis is involved in this process (as opposed to primary active transport). The energy required to drive transport resides in the transmembrane electrochemical gradient of the driving ion.

Secondary active transport is also referred to as ion-coupled transport. Those utilizing Na⁺ as the driving ion are called Na⁺-coupled transporters. Those utilizing H⁺ as the driving ion are called H⁺-coupled transporters.

Two types of secondary active transport exist: cotransport (also known as symport) and exchange (also known as antiport). Na⁺/glucose cotransporter and H⁺/dipeptide cotransporter are examples of cotransporters. Na⁺/Ca²⁺ exchanger and Na⁺/H⁺ exchanger are examples of exchangers.

Cotransport
Symport
Exchange
Antiport

Lecture notes on Secondary Active Transport

Electrogenic pumps are primary active transporters that hydrolyze ATP and use the energy released from ATP hydrolysis to transport ions across biological membranes leading to the translocation of net charge across the membrane.

For example, the Na⁺/K⁺ ATPase (sodium pump) is an electrogenic pump because during every transport cycle, it transports 3 Na⁺ ions out of the cell and 2 K⁺ ions into the cell. This leads to the movement of one net positive charge out of the cell making this process electrogenic.

Electrogenic

Hypothyroidism refers to a pathophysiological condition in which the thyroid gland does not produce sufficient amounts of the thyroid hormones (underactive thyroid), leading to abnormally low levels of the thyroid hormones (T₃ and T₄).

The symptoms of hypothyroidism may include reduced metabolic rate, chronic fatigue, weight gain, myxedema, and depression.

Three main forms of hypothyroidism exist: (1) Primary hypothyroidism, (2) Secondary hypothyroidism, and (3) Tertiary hypothyroidism.

In primary hypothyroidism, the thyroid gland itself is incapable of producing normal levels of the thyroid hormones.

Secondary hypothyroidism is caused by low levels of thyroid stimulating hormone (TSH). TSH, produced by the anterior pituitary gland, is required to stimulate the thyroid gland to produce the thyroid hormones.

Tertiary hypothyroidism is caused by low levels of thyrotropin releasing hormone (TRH). TRH, produced by the hypothalamus, is required to stimulate the anterior pituitary gland to produce TSH which, in turn, is required to stimulate the thyroid gland to produce the thyroid hormones.

Primary hypothyroidism
Secondary hypothyroidism
Tertiary hypothyroidism

Hyperthyroidism
Euthyroidism