Hormones and Glands: Hypothalamus, Thyroid, and More

Posted on Mar 27, 2025 in Biology

Hypothalamus and Pituitary Gland

The hypothalamus controls the body’s hormonal activity and acts as a liaison between the nervous and endocrine systems. Neurons in the hypothalamus receive information from other brain areas and hormones in the blood. They respond by secreting hormones that reach the pituitary gland, regulating its hormone production. The pituitary gland, an endocrine gland, controls the activity of many other endocrine glands and is itself controlled by the hypothalamus.

The pituitary gland has two distinct zones:

  • Anterior lobe (adenohypophysis): Produces several hormones, including:
    • TSH: Acts on the thyroid.
    • FSH: Acts on the gonads.
    • ACTH: Acts on the adrenal cortex.
    • PRL: Acts on the mammary glands after childbirth.
  • Posterior lobe (neurohypophysis): Stores and releases two peptide hormones synthesized in the hypothalamus:
    • Oxytocin: Promotes uterine contractions.
    • Vasopressin (ADH): Serves the nephrons to reabsorb water.

Thyroid Gland

The thyroid gland is located beneath the front of the trachea and larynx. It synthesizes two types of hormones:

  • Thyroxine (T4) and Triiodothyronine (T3): T4 has four iodine atoms, and T3 has three. Their production is controlled by TSH from the pituitary gland. These hormones promote metabolism and maturation of the central nervous system (CNS). Iodine deficiency can lead to thyroid hypofunction, causing cretinism (mental retardation, chubby appearance) in children. In adults, it can cause myxedema (thick skin, hair loss, overweight). Hyperthyroidism (overactive thyroid) causes accelerated metabolism and gland enlargement.
  • Calcitonin: A protein hormone that regulates calcium levels in the blood, working with parathyroid hormone.

Gonads

These endocrine glands produce sex hormones (steroids) and are regulated by the adenohypophysis. There are several types:

Parathyroid Glands

Four small glands attached to the back of the thyroid. They synthesize parathyroid hormone (PTH), which regulates calcium and phosphorus balance.

Adrenal Glands (Suprarenal Capsules)

Attached to the top of the kidneys, they have two functional areas: the medulla and the cortex.

  • Medulla: Secretes epinephrine (adrenaline) and norepinephrine in response to emergency situations. These hormones increase metabolism, blood glucose levels, heart rate, and respiratory rate.
  • Cortex: Produces three types of steroid hormones derived from cholesterol: mineralocorticoids, glucocorticoids, and sex hormones (in lower proportions).
  • Mineralocorticoids: Regulate blood sodium (Na) and potassium (K) levels.
  • Glucocorticoids: Cortisol is the most important, defending the body against aggression.

Pancreas

A mixed gland with two parts:

  • Exocrine: Releases pancreatic juices.
  • Endocrine: Builds and releases hormones.

Cells are grouped, forming islets of Langerhans:

  • Beta cells (β): Produce insulin.
  • Alpha cells (α): Manufacture glucagon, which increases blood glucose levels.

Insulin is released when blood glucose levels rise and has several functions: facilitating glucose entry into cells, stimulating glycogen formation and storage in the liver, and stimulating protein synthesis and fat storage.

Glucagon’s action is opposite to insulin, increasing blood glucose concentration from stored glycogen.

Ovaries: Manufacture two types of hormones:

  • Estrogens: Produced by maturing follicles, stimulated by FSH, and stimulate thickening of the uterine walls.
  • Progestogens: Synthesized in cells of the corpus luteum, stimulated by LH, and linked to the ovulatory cycle.

Testes: Formed by seminiferous tubules, which contain cells that produce testosterone, stimulated by LH. Testosterone is involved in the development of sexual organs and characteristics.