Thyroid Hormones - Histology

What are Thyroid Hormones?

Thyroid hormones are chemical substances produced by the thyroid gland that regulate various physiological processes in the body. The primary thyroid hormones are triiodothyronine (T3) and thyroxine (T4). These hormones play critical roles in metabolism, growth, and development.

How are Thyroid Hormones Synthesized?

The synthesis of thyroid hormones occurs in the thyroid follicles, which are spherical structures lined by follicular cells. The process involves several steps:

Uptake of iodide from the bloodstream into the follicular cells.
Oxidation of iodide to iodine by the enzyme thyroid peroxidase (TPO).
Iodination of tyrosine residues in the protein thyroglobulin to form monoiodotyrosine (MIT) and diiodotyrosine (DIT).
Coupling of MIT and DIT to form T3 and T4.
Release of T3 and T4 into the bloodstream upon stimulation by thyroid-stimulating hormone (TSH).

What is the Role of Thyroid Hormones in Metabolism?

Thyroid hormones are essential regulators of metabolism. They increase the basal metabolic rate (BMR) by stimulating the synthesis of enzymes involved in metabolic pathways. This leads to an increase in oxygen consumption and heat production, which is why hyperthyroidism is often associated with symptoms such as weight loss and heat intolerance.

How do Thyroid Hormones Affect Growth and Development?

Thyroid hormones are crucial for normal growth and development, especially in children. They stimulate the growth of long bones and the maturation of the brain. A deficiency in thyroid hormones during development can lead to conditions such as cretinism, characterized by stunted physical and mental growth.

What is the Histological Structure of the Thyroid Gland?

The thyroid gland is composed of numerous thyroid follicles, each surrounded by a single layer of follicular cells. These cells synthesize and secrete thyroid hormones. The follicles contain a central lumen filled with colloid, a protein-rich substance containing thyroglobulin. Between the follicles are parafollicular cells (C cells), which secrete the hormone calcitonin.

What are Common Thyroid Disorders?

Common thyroid disorders include:

Hypothyroidism: A condition characterized by insufficient production of thyroid hormones. Symptoms include fatigue, weight gain, and cold intolerance.
Hyperthyroidism: Overproduction of thyroid hormones, leading to symptoms such as weight loss, heat intolerance, and increased heart rate.
Goiter: An enlargement of the thyroid gland, which can occur in both hypo- and hyperthyroidism.
Thyroid cancer: A malignant growth in the thyroid gland that requires medical intervention.

How are Thyroid Hormone Levels Regulated?

Thyroid hormone levels are regulated by the hypothalamus-pituitary-thyroid axis. The hypothalamus releases thyrotropin-releasing hormone (TRH), which stimulates the pituitary gland to secrete TSH. TSH, in turn, stimulates the thyroid gland to produce T3 and T4. Negative feedback mechanisms involving T3 and T4 ensure that hormone levels remain within a normal range.