Hormone Functioning

Hormones are chemical messengers produced by endocrine glands that regulate a wide range of physiological activities, including growth, metabolism, reproduction, and homeostasis. They are released into the bloodstream and act on specific target tissues, even at very low concentrations. 

1.0What are Hormones?

• Definition: Hormones are organic substances secreted in minute quantities into the bloodstream to act on distant target organs.
• They are not used as energy sources but act as regulators of biochemical reactions.
• Example: Insulin controls blood glucose levels, and Thyroxine regulates metabolism.

2.0Types of Hormones and Their Chemical Nature

1. Steroid Hormones

• Derived from cholesterol, steroid hormones like estrogen, testosterone, and cortisol are lipid-soluble. 
• They diffuse across cell membranes and bind to intracellular (nuclear) receptors to regulate gene expression.

2. Peptide and Protein Hormones

• These hormones, such as insulin and growth hormone, are made of amino acid chains. 
• They are water-soluble and cannot cross cell membranes, so they bind to cell-surface receptors and initiate signalling via second messengers such as cAMP or Ca²⁺.

3. Amino Acid–Derived Hormones

• These include hormone types such as thyroxine and Adrenaline, derived from amino acids and capable of acting through both intracellular and cell-surface receptors.

3.0Mechanisms of Hormone Action

• There are two types of hormones: (i) water-soluble hormones and (ii) lipid-soluble hormones.
• Hormones exert their effects on target tissues by binding to specific proteins known as hormone receptors, which are found only in those tissues. Receptors located on the cell membrane of target cells are called membrane-bound receptors. In contrast, those located inside the target cell are known as intracellular receptors, often found in the nucleus.
• When a hormone binds to its receptor, a hormone-receptor complex forms. Each receptor is specific to a single hormone, ensuring precise interactions. The formation of this complex triggers biochemical changes in the target tissue, thereby regulating its metabolism and overall physiological function.

4.0Regulation of Hormone Levels

• Feedback Mechanisms: Hormones are regulated via negative and positive feedback loops. For instance, thyroid hormones inhibit TSH production (negative feedback), while oxytocin release during childbirth is enhanced by positive feedback.
• Endocrine Axes (e.g., HPG Axis): The hypothalamic–pituitary–gonadal (HPG) axis is a key regulatory system. GnRH from the hypothalamus stimulates LH and FSH release from the pituitary, which in turn regulates sex hormone production. Feedback loops fine-tune hormone levels.

5.0Hormones and Their Functions

• Insulin and Glucagon: Regulate blood glucose—insulin lowers it, glucagon raises it.
• Thyroid Hormones (T3/T4): Regulate metabolism, growth, and neurological development.
• Growth Hormone (GH): Promotes growth by stimulating IGF‑1 production; acts via cell surface receptors and signalling cascades like JAK‑STAT.
• Adrenal Hormones (Cortisol, Aldosterone, Epinephrine): Control stress response, metabolism, and fluid balance.
• Sex Hormones (Estrogen, Testosterone): Regulate reproductive development and secondary sexual characteristics; estrogen acts via nuclear receptors to modulate gene expression.