Thyroid Hormones: Types, Function, Diagnosis, Treatment, and More

The thyroid gland secretes hormones that regulate various physiological functions. The primary hormones it produces, Triiodothyronine (T3) and Thyroxine (T4), play a crucial role in regulating metabolism, maintaining energy levels, and supporting growth and development.

This article delves into the mechanisms and functions of these hormones, providing insights into their types, roles, production processes, and broader implications for health.

Types of Hormones Secreted by the Thyroid Gland

The two hormones secreted by the thyroid gland are as follows:

• Triiodothyronine (T3)

The thyroid gland produces this hormone in smaller amounts (about 20%). T3 is responsible for most of the physiological effects attributed to thyroid hormones, including regulating metabolism, heart rate, and body temperature.

• Thyroxine (T4)

The thyroid gland produces this hormone in larger quantities compared to T3. It serves as a precursor to the more active form, T3. Approximately 80% of the hormones released by the thyroid are T4.

A lesser-known hormone called reverse T3 (rT3) is also produced, although its exact function remains unclear.

Functions of Thyroid Hormones

Various bodily functions are dependent on thyroid hormones. Some of them are:

• Metabolism Regulation

Thyroid hormones influence the rate at which an individual’s body processes nutrients and uses energy. They help regulate an individual's metabolism. Inadequate hormone production can slow down the calorie-burning process, affecting an individual's metabolism and appetite.

• Growth and Development

Hormone secretion from the thyroid gland affects bone health and brain development in children, and it is essential for their overall growth and development.

• Temperature Regulation

These hormones regulate the heat production in the body’s tissues to maintain the required body temperature.

• Heart Function

Thyroid hormones increase heart rate and cardiac output, contributing to overall cardiovascular health.

Mechanism of Hormone Production in the Thyroid Gland

The production of thyroid hormones relies mostly on iodine and Thyroid-stimulating Hormone (TSH). Here is the mechanism explained:

Role of Iodine

Iodine is essential for the synthesis of thyroid hormones. After absorption from the diet, it is transported to the thyroid gland and is absorbed by follicular cells through the sodium-iodide symporter. Inside the cells, iodine undergoes oxidation and binds to tyrosine residues, forming iodinated tyrosines, which serve as precursors for synthesising T3 and T4.

Role of Thyroid-Stimulating Hormone (TSH)

The anterior pituitary gland secretes thyroid-stimulating hormone (TSH), which stimulates the thyroid gland to produce and release T3 and T4. TSH binds to receptors on follicular cells, triggering the endocytosis of thyroglobulin and its hydrolysis to release active hormones into circulation. This process maintains hormonal balance and homeostasis.

Understanding the Thyroid Hormone Imbalances

Imbalances in thyroid hormone levels can cause the following health issues:

• Hypothyroidism

Also referred to as underactive thyroid, this condition occurs due to insufficient thyroid hormone production. In most cases, autoimmune disorders like iodine deficiency, Hashimoto’s disorder, thyroiditis, etc., cause this hormonal imbalance. Some common symptoms of hypothyroidism are depression, fatigue, cold intolerance, weight gain, and fatigue.

• Hyperthyroidism

Also called overactive thyroid, hyperthyroidism occurs due to excessive production of thyroid hormone. Conditions like Grave’s disease mainly cause this disorder. Some common symptoms of hyperthyroidism are anxiety, weight loss, heat intolerance, and increased heart rate.

Diagnosis of Thyroid Hormone Disorders

Here are the common ways to diagnose thyroid hormone disorders:

• Blood Tests

Measurement of serum levels of TSH, T3, and T4 helps determine whether an individual has hypothyroidism or hyperthyroidism. Elevated TSH levels with low T3/T4 indicate hypothyroidism, while low TSH with high T3/T4 suggests hyperthyroidism.

• Imaging Studies

Ultrasound or radioactive iodine uptake scans help assess structural abnormalities or functional status of the thyroid gland.

• Biopsy

In some cases, doctors suggest a fine needle aspiration biopsy to detect if nodules or suspicious masses in the thyroid gland are cancerous or not.

Treatment Options for Thyroid Hormone Disorders

There are multiple treatment methods for thyroid hormone disorders based on the type and cause of the disorder.

Treatment for hyperthyroidism aims to reduce excess thyroid hormones. Beta-blockers help control heart rate and blood pressure by blocking hormone effects. Antithyroid drugs, such as methimazole and propylthiouracil, limit hormone production. Radioiodine therapy targets and damages thyroid cells to reduce hormone levels.

For hypothyroidism, the primary treatment is thyroid hormone replacement, typically using levothyroxine to supplement hormone levels. Surgery, such as thyroidectomy, is another option for thyroid disorders. This permanent procedure removes the thyroid, but patients must continue lifelong medication to replace the hormones the body no longer produces.

Lifestyle Changes and Thyroid Health

The following lifestyle adjustments can improve the thyroid health in an individual:

• Incorporating iodine-rich foods (like fish and dairy) can help maintain adequate iodine levels for hormone production. However, excessive iodine intake can worsen certain thyroid conditions.
• Working out on a daily basis can help manage weight and improve overall metabolic health.
• Chronic stress may affect hormone levels. Practising yoga or meditation can be beneficial to relieve stress and regulate hormone production.

Current Research on Thyroid Hormones

Recent studies highlight the complexity of the relationship between thyroid-stimulating hormone (TSH) and thyroid hormones T3 and T4. These studies challenge the assumption that TSH levels directly reflect thyroid function. This is because the interaction between these hormones can vary among individuals, leading to different physiological responses.

This variability suggests the need for a more personalised approach to diagnosing and treating thyroid conditions. Consequently, TSH may not always be a reliable indicator of thyroid health, particularly in cases of non-thyroidal illness or during treatment with levothyroxine (LT4).

Emerging Treatments for Thyroid Disorders

Recent approaches for the treatment of thyroid disorders are moving towards individualised care instead of relying solely on standard TSH targets. Studies suggest that patients, particularly those who do not experience relief with LT4 alone, may benefit from a combination of T3 and T4 treatments.

This advanced approach aims to offer personalised care based on an individual's hormonal profile and symptoms.

Conclusion

The thyroid hormones T3 and T4 are crucial regulators of numerous physiological processes within the body. Understanding their functions, diagnostic approaches, and treatment options is essential for maintaining proper hormone levels and supporting overall bodily function.