Thyroid Hormones and its Physiological Significance

A recent trend has been observed among various researchers

Introduction

Thyroid hormones secreted by thyroid glands are significant for the proper development of brain in infants, regulates the metabolic activity and has effects upon every organ of the body in adults. The major hormones synthesized by thyroid gland are thyroxine (T4), 3,5,3'-triiodothyronine (T3), and reverse 3,5,3'-triiodothyronine (rT3), which are in turn controlled by thyroid stimulating hormone (TSH) secreted from the anterior pituitary gland. Iodine can be considered to have important role for the functioning of thyroid hormone. Thyroid hormone binds to intracellular receptors of mitochondria and results in increased breakdown of nutrients and generation of ATP. Thyroid hormone specifically acts upon beta receptors on the heart, which causes elevated heart rate. In gastrointestinal tract, thyroid hormone causes increase in GI motility. The hormone is necessary for neural development in the brain; it helps in neurogenesis, neuronal migration, neuronal and glial cell differentiation, myelination, and synaptogenesis (Shahid et al, 2019).

Hypotheroidism and its root causes:

Hypothyroidism, a common thyroid hormone deficiency disorder affects mostly women. The literature survey revealed that the incidence rate of clinical hypothyroidism is 0.5 – 1.9% among women and < 1% among men. Hypothyroidism can be classified based on onset time – congenital and acquired, level of endocrine dysfunction – primary, secondary and central and based on severity of the disease – clinical and subclinical. The incidence rate of sub clinical hypothyroidism is 3 – 13.6% among women and 0.7-5.7% in men (Athanassiou et al, 2010). The disease prognosis can be easily done by measuring the thyroid hormone levels in blood. The primary causes of hypothyroidism are chronic autoimmune thyroiditis, inadequate intake of iodine, thyroidectomy, treatment with radioactive iodine, radiotherapy, drugs and thyroid gland agenesis or dysgenesis. The secondary or central cause of the disease includes conditions such as pituitary gland adenomas, history of surgery or radiotherapy of pituitary gland, head trauma history, pituitary apoplexy, Hypothalamic tumors, History of surgery or radiotherapy of hypothalamus (Athanassiou et al, 2010). Here disorder of thyroid gland causes decreased production and synthesis of thyroid hormones. 50% of the hypothyroidism cases occur due to autoimmune dysfunction resulting in chronic autoimmune thyroiditis. The residual cases are due to drug abnormalities, radiation therapy treatment. Both post partum and silent thyroiditis resulting in hypothyroidism are considered to be part of chronic autoimmune thyroiditis. The frequency is found to be higher among women both in middle aged (30 to 50 years) and children (Athanassiou et al, 2010).

Hashimoto's thyroiditis (HT):

The rationale of choosing this model:

Chronic autoimmune thyroiditis also known as Hashimoto thyroiditis occurs due to destruction of thyroid tissues by both cell mediated and antibody mediated immune system. Hashimoto's thyroiditis (HT), one of the most common autoimmune disorder is found to be associated with gastric disorders among 10% to 40% of patients (Mincer et al, 2019). The production of anti thyroid antibodies which in turn attacks the thyroid cells results in formation of progressive fibrosis. It is considered to be one of the most common cause of hyperthyroidism after the age of six years in United States and also in those parts of the world where iodine intake can be considered to be adequate. Literature revealed the rate of incidence to be 3.5 per 1000 people per year among women and 0.8 per 1000 people per year among men. The incidence rate of thyroid disease generally increases with age (Mincer et al, 2019).

Clinical Manifestations:

The pathophysiology of the disease shows lymphocyte infiltration and fibrosis as typical features of the disease. Diagnosis depends upon clinical manifestations correlated with laboratory data such as elevated TSH and normal to low thyroxine levels. The case report presented similar facts such as serum TSH level was 15 mU/L (Ref.range:0.4 – 4.5 mU/L), free thyroxine (FT4): 5 pmol/L (10 - 20 pmol/L) and total triiodothyronine (T3): 0.9 nmol/L (0.9 - 2.5 nmol/L) which supported the diagnosis. Physical examinations of the patient revealed that both the thyroid lobes are diffusely enlarged with firm consistency which matches with the pathophysiology of the disease. Significantly higher levels of anti-thyroid peroxidase (TPO) antibodies can be observed among patients suffering from Hashimoto thyroiditis. The report data about the anti-thyroglobulin (anti-Tg) antibody and anti-thyroid-peroxidase (anti-TPO) levels were significantly elevated justified the above statement. The patient belongs to the middle aged group which also matches with the prevalence data. myxedema can be considered as the classic skin characteristic in relation with hypothyroidism, caused primarily due to increased glycosaminoglycan deposition. Though uncommon but found among the severe cases, skin can be scaly and dry. Reduced hair growth, dull and brittle hair and diffused or partial alopecia are also very common. The patient here also reported that hair felt drier, cold dry rough skin. Bradycardia, fatigueness, exercise intolerance, decreased muscle strength are among the other symptomatic features observed among patients suffering from Hashimoto thyroiditis. The female patient also reported overall muscle aches and stiffness around her neck. Among the vital signs: blood pressure 140/95 mmHg, pulse rate 98 per minute, respiration rate 16 per minute, and body temperature 36.5℃, BMI: 26.1 matches with the symptomatic features of the disease. Anemia can be observed among 30% to 40% of the patients. Creatine kinase, prolactin hormone, total cholesterol, LDL, and triglyceride levels can also become elevated among patients (Liu et al, 2016) (Yoo et al. 2016.) Other tests report data such as total cholesterol 7mmol/L (3- 5.5 mmol/L), complete blood count showed decreased RBC count and low Hb level matches with the literature reports about the disease.

Available Treatment:

The preferred mode of treatment for hyperthyroidism is replacement of thyroid hormone. The most prescribed drug is titrated levothyroxine sodium to be administered orally. The drug has half life of 7 days and is administered on a daily basis but it should never be given along with iron, aluminum hydroxide, calcium. Patients suffering from cardiovascular disease and elderly patients should be given lower doses of drugs. On the contrary during pregnancy the dose of thyroxine can be increased by 30%. The disease is a life time disorder and the key management is to follow up the levels of thyroid hormone. In addition the drug dose may cause lymphoma therefore follow up examination of the neck area is highly recommended (Mincer et al, 2019).

References:

• Shahid MA, Sharma S (2019). Physiology, Thyroid Hormone. In: StatPearls Treasure Island (FL): StatPearls Publishing, pp.1-12.
• Athanassiou IK. Hypothyroidism new aspects of an old disease, Hippokratia, 14(2), p82-87.
• Mincer DL, Jialal I (2019). Hashimoto Thyroiditis In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing, pp. 1- 17.
• Liu M, Murphy E, Amerson EH (2016). Rethinking screening for thyroid autoimmunity in vitiligo. J. Am. Acad. Dermatol. 75(6):1278-1280.
• Yoo WS, Chung HK (2016). Recent Advances in Autoimmune Thyroid Diseases. Endocrinol Metab (Seoul). 31(3):379-385.