Thyroid hormones play a key role in regulating the body’s metabolism, influencing various systems, including cardiovascular and central nervous function. Produced by the thyroid gland and regulated by the pituitary gland’s secretion of Thyroid Stimulating Hormone (TSH), these hormones are essential for maintaining bodily functions.

Normal Thyroid Hormone Levels

• Total T3: 80–220 ng/dl
• Free T4: 0.8–1.8 ng/dl

Any alteration in thyroid hormone levels can lead to diseases like Hypothyroidism, Hyperthyroidism, or Thyroiditis, which affect the body’s overall function. Symptoms vary based on whether thyroid hormone levels are elevated or reduced.

Thyroid Gland Structure and Function

The thyroid gland comprises several critical components:

• Thyrocytes (Follicular Cells): These cells are responsible for the production of thyroid hormones.
• C-cells (Parafollicular Cells): These produce calcitonin, a hormone involved in calcium regulation.
• Colloid: A gel-like substance that stores thyroid hormones for up to 2-3 months when the gland is inactive.
• Thyroglobulin: A protein within the colloid that binds to thyroid hormones, facilitating their storage.

The thyroid gland’s structure supports its role in hormone synthesis and storage, ensuring a constant supply of thyroid hormones to regulate metabolic functions.

Thyroid Hormone Synthesis: A Step-by-Step Process

The creation of thyroid hormones involves multiple steps, which occur inside the thyroid follicles:

1. Thyroglobulin Secretion: Follicular cells secrete thyroglobulin into the colloid.
2. Iodine Trapping: Iodine enters follicular cells through the Na+/I- symporter (NIS), which is stimulated by TSH. The Pendrin transporter moves iodide to the colloid.
3. Iodine Oxidation: The enzyme Thyroid Peroxidase (TPO) oxidizes iodide into iodine (I2).
4. Organification of Iodine: Iodine attaches to tyrosine residues on thyroglobulin, forming Monoiodotyrosine (MIT) and Diiodotyrosine (DIT).
5. Coupling Reaction: The MIT and DIT molecules undergo a coupling process, forming:
   • T3 (MIT + DIT)
   • T4 (DIT + DIT)
   • rT3 (DIT + MIT)
6. Endocytosis and Digestion: The colloid, containing thyroid hormones, enters the follicular cells. In the cells, enzymes break down thyroglobulin, releasing T3, T4, and rT3 into the bloodstream.
7. Thyroid Hormone Release: The thyroid hormones are transported into the bloodstream, usually bound to carrier proteins.
8. Peripheral Conversion: In tissues outside the thyroid, T4 is converted to the more active form, T3, by the action of deiodinase enzymes.

This intricate process ensures the production of thyroid hormones that regulate various bodily functions.

Functions of Thyroid Hormones

Thyroid hormones are involved in numerous physiological processes. Here’s how they influence key systems:

• Cardiovascular System: Thyroid hormones increase heart rate and contractility, leading to a rise in systolic blood pressure and a decrease in diastolic blood pressure due to lower total peripheral resistance.
• Central Nervous System: Crucial for brain development, thyroid hormones help in the maturation of the CNS. In pregnancy, a deficiency in thyroid hormone can result in cretinism in the baby.
• Muscle Function: They stimulate protein catabolism, which can lead to muscle weakness (thyrotoxic myopathy).
• Bone Formation: Thyroid hormones promote bone growth and enhance the action of growth hormone (GH).
• Metabolism: They increase glucose absorption in the gut and help remove cholesterol by increasing LDL receptors in the liver.
• Protein Synthesis and Degradation: Thyroid hormones regulate both the creation and breakdown of proteins, maintaining metabolic balance.

Clinical Significance of Thyroid Hormones

Changes in thyroid hormone levels are associated with several clinical conditions:

Hyperthyroidism

Hyperthyroidism occurs when thyroid hormone levels are too high, as seen in conditions like Graves’ disease, Toxic Multinodular Goiter, and Toxic Adenoma. Common symptoms include:

• Weight loss
• Heat intolerance
• Excessive sweating
• Fatigue
• Palpitations

In severe cases, signs like lid retraction and exophthalmos (bulging eyes) may also be present.

Hypothyroidism

When thyroid hormone levels are low, hypothyroidism can lead to symptoms such as:

• Weight gain
• Dry skin and hair
• Heavy menstrual bleeding (Menorrhagia)
• Periorbital edema (swelling around the eyes)
• Cold intolerance
• Bradycardia (slow heart rate)

Pregnancy and Hormonal Changes

In pregnancy or conditions with hyperestrogenism, Thyroid Binding Globulin (TBG) levels increase, which may reduce the levels of free thyroid hormone. This means that individuals with hypothyroidism may need an increased dose of thyroid hormone to compensate for the higher amount of bound hormone in the bloodstream.

Thyroid hormones are vital for maintaining the body’s metabolic functions and regulating numerous physiological processes, from heart rate to bone health. Understanding how these hormones are synthesized, how they function, and how imbalances can lead to various conditions is essential for diagnosing and managing thyroid-related disorders effectively.

Frequently Asked Questions: 

1. What is the function of the thyroid hormone synthesis?

Thyroid hormone affects virtually every organ system in the body, including the heart, CNS, autonomic nervous system, bone, GI, and metabolism. In general, when the thyroid hormone binds to its intranuclear receptor, it activates the genes for increasing metabolic rate and thermogenesis.

2. What are the thyroid hormones and their functions?

The thyroid gland releases triiodothyronine (T3) and thyroxine (T4). These hormones play an important role in regulation of your weight, energy levels, internal temperature, skin, hair, nail growth, metabolism and is an important part of the endocrine system.

3. What hormones are synthesized by the thyroid gland?

Thyroid is a small, bow-shaped gland that sits in the front of your neck. Your thyroid gland makes 3 thyroid hormones: thyroxine (T4), triiodothyronine (T3) and calcitonin. Thyroid problems, such as an overactive or underactive thyroid, are common.

4. What is the clinical importance of thyroid?

It’s a part of your endocrine system and controls many of your body’s important functions by producing and releasing (secreting) certain hormones. Your thyroid’s main job is to control the speed of your metabolism (metabolic rate), which is the process of how your body transforms the food you consume into energy.