The Pineal Gland

The pineal gland is a small endocrine gland present in the brain. It plays an important role in regulating biological rhythms.

Location

The pineal gland is located on the dorsal side of the forebrain, between the two hemispheres of the brain.

Hormone Secreted

It secretes a crucial hormone called Melatonin.

Functions of Melatonin

Melatonin is mainly responsible for regulating the biological clock of the body.

Major functions include:

  • Regulation of 24-hour (diurnal) rhythm of the body
  • Regulation of the sleep–wake cycle
  • Regulation of body temperature rhythms
  • Influence on metabolism
  • Influence on menstrual cycle
  • Influence on defense capability of the body
  • Influence on pigmentation in some organisms

The Thyroid Gland

The thyroid gland is one of the largest endocrine glands of the body and plays a crucial role in regulating metabolism.

Structure

It is a butterfly-shaped gland composed of two lobes located on either side of the trachea (windpipe) in the neck. The two lobes are connected by a thin flap of connective tissue called the isthmus.

Composition

Microscopically, it is made of follicles and stromal tissues. The follicular cells synthesize two main hormones:

  1. Tetraiodothyronine or Thyroxine (T4T_4) (contains 4 iodine atoms).
  2. Triiodothyronine (T3T_3) (contains 3 iodine atoms).

The Essential Element

Iodine is absolutely essential for the normal rate of hormone synthesis in the thyroid.

Functions of Thyroid Hormones

  • The Engine Regulator: Their main job is to regulate the Basal Metabolic Rate (BMR) (how fast your cells burn energy).
  • Support the process of Red Blood Cell (RBC) formation.
  • Control the metabolism of carbohydrates, proteins, and fats.
  • Maintain water and electrolyte balance.
  • Thyrocalcitonin (TCT): The thyroid also secretes a protein hormone called TCT, which regulates blood calcium levels by lowering them.

Disorders of the Thyroid

  • Hypothyroidism (Underactive / Low secretion):
    • Simple Goitre: Enlargement of the thyroid gland, usually due to a deficiency of iodine in the diet.
    • Cretinism: If a mother suffers from hypothyroidism during pregnancy, it leads to defective development and stunted growth (cretinism), mental retardation, low IQ, abnormal skin, and deaf-mutism in the growing baby.
    • Irregular Menstruation: Very common in adult women with hypothyroidism.
  • Hyperthyroidism (Overactive / Excess secretion):
    • Exophthalmic Goitre (Graves' Disease): A form of hyperthyroidism characterized by enlargement of the thyroid gland, outward protrusion of the eyeballs (Exophthalmos), dangerously increased BMR, and severe weight loss.

The Parathyroid Gland

The parathyroid glands are small endocrine glands associated with the thyroid gland.

Location

In humans, there are four parathyroid glands. They are located on the back side (dorsal surface) of the thyroid gland (one pair embedded in each lobe).

Hormone Secreted

They secrete a peptide hormone called Parathyroid Hormone (PTH). PTH is a peptide hormone.

Regulation

PTH is not controlled by the brain; its secretion is strictly regulated by the circulating levels of Calcium (Ca2+Ca^{2+}) ions in the blood.

Function (The Calcium Raiser)

  • PTH is a Hypercalcemic hormone—it acts to increase Ca2+Ca^{2+} levels in the blood.
  • It does this by stimulating bone resorption (dissolving/demineralising the bone to steal its calcium).
  • It also tells the renal tubules (kidneys) to reabsorb Ca2+Ca^{2+} instead of peeing it out, and increases Ca2+Ca^{2+} absorption from digested food in the gut.

The Balancing Act

PTH (which raises calcium) and TCT from the thyroid (which lowers calcium) work together to maintain perfect Calcium homeostasis in the body.

Pineal, Thyroid, and Parathyroid Glands

1. Melatonin vs. Melanin:

  • Melatonin = Pineal gland (Sleep cycle).
  • Melanin = Pigment in skin (Controlled by MSH from the pituitary).

2. The Calcium Seesaw:

  • PTH Pulls calcium into the blood (Hypercalcemic).
  • TCT Throws calcium out of the blood into the bones (Hypocalcemic).

3. Thyroid Disorders Snapshot:

  • Hypothyroidism + Iodine deficiency = Simple Goitre (Swollen neck).
  • Hypothyroidism + Pregnancy = Cretinism (Stunted, mentally retarded baby).
  • Hyperthyroidism = Graves' Disease / Exophthalmic Goitre (Bulging eyes, high metabolism, weight loss).

💡 Questions and Answers

Q1. Which hormone regulates the sleep–wake cycle?

A1: The hormone melatonin regulates the sleep–wake cycle. It is secreted by the pineal gland and controls the body's biological clock.

Key points:

  • Melatonin regulates circadian rhythm
  • Secreted by pineal gland

Q2. What is the cause of simple goitre?

A2: Simple goitre occurs due to deficiency of iodine in the diet. Because iodine is required to produce thyroid hormones, the thyroid gland enlarges when iodine is lacking.

Key points:

  • Cause → iodine deficiency
  • Leads to thyroid enlargement

Q3. Which element is essential for the synthesis of thyroid hormones?

A3: The element iodine is essential for the synthesis of thyroid hormones T3 and T4.

Key points:

  • Iodine required for thyroid hormone synthesis

Q4. Is PTH a hypercalcemic or hypocalcemic hormone?

A4: PTH is a hypercalcemic hormone, meaning it increases calcium levels in the blood.

Key points:

  • PTH increases blood Ca²⁺

Q5. What are the symptoms of Graves' disease?

A5: Graves' disease is a form of hyperthyroidism. It causes enlargement of the thyroid gland, protrusion of the eyeballs (exophthalmos), increased metabolic rate, and weight loss.

Key points:

  • Hyperthyroidism
  • Exophthalmos
  • Weight loss