Lipid- Versus Water-Soluble Hormones

IP3 = inositol trisphosphate

DAG = diacylglycerol

Hormone Receptors

Hormone specificity

A hormone affects only cells that possess receptors specific to that particular hormone.

For example, both adrenocorticotropic hormone (ACTH) and luteinizing hormone (LH) increase the secretion of steroid hormones. However, ACTH does so only in the adrenal cortex and LH only in gonadal tissue.

Hormone activity

Under normal conditions, receptors are not saturated; that is, extra receptors exist. Therefore:

• • Normally, the number of hormone receptors is not rate-limiting for hormone action.

  • The plasma concentration of free hormone is usually indicative of activity.

Resistance to hormone action

• • Abnormalities in receptors or events distal to the ligand-receptor interaction, often due to chronic elevation of circulating hormone (e.g., type II diabetes) or drug therapy.

  • Under these conditions receptors are often saturated.

  • Reduction of hormone levels often produces some recovery in sensitivity.

  • The clinical presentation is often one of normal or elevated hormone levels but with reduced or absent peripheral manifestations of the hormone and a failure of replacement therapy to correct the problem.

Permissive action

A phenomenon in which one type of hormone must be present before another hormone can act; for example, cortisol must be present for glucagon to carry out gluconeogenesis and prevent hypoglycemia.

Measurement of Hormone Levels

Plasma analysis

• • Provides information at the time of sampling only and may not reflect the overall secretion rate

  • When hormone secretion is episodic, single sampling may reflect peaks (erroneous hyperfunction) or nadirs (erroneous hypofunction). Pulsatile secretion, diurnal and cyclic variation, age, sleep entrainment, and hormone antagonism must all be considered in evaluating circulating levels.

    • Growth hormone is secreted in pulses and mainly at night. This is not reflected in a fasting morning sample. However, growth hormone stimulates the secretion of IGF-I which circulates attached to protein and has a long half-life (20 hours). Plasma IGF-I measured at any time during the day is usually a good index of overall growth hormone secretion.

    • Thyroid is a fairly constant system and T4 has a half-life of about 6–7 days. Thus, a random measurement of total T4 is usually a good estimate of daily plasma levels.

Urine analysis

• • Restricted to the measurement of catecholamines, steroid hormones, and water-soluble hormones such as hCG and LH.

  • A distinct advantage of urine analysis is that it provides an integrated sample.

    • A “24-hour urine free cortisol” is often necessary to pick up a low-level Cushing’s syndrome and to eliminate the highs and lows of the normal circadian rhythm.