Thyroid disorders comprise the great burden of endocrine disorders. The thyroid gland is the largest endocrine gland in the human body. It is butterfly-shaped gland and located on both sides of the windpipe in the middle of the lower neck. It stores and secretes two important hormones- Thyroid (T3 and T4) and Calcitonin.
Its role is to decrease the amount of calcium in the blood. It has the opposite action to parathyroid hormone, Calcitonin hormone helps to regulate the amount of calcium that circulates in the bloodstream, which increases the amount of calcium in the blood when levels fall.
Thyroid hormone influences every cell, tissue and organ in the body and regulates the body’s metabolism—the rate at which the body produces energy from nutrients and oxygen.
The thyroid gland uses iodine from the foods you eat to make two main hormones:
- Triiodothyronine (T3)
- Thyroxine (T4)
Regulation of Thyroid Harmone
Now, let us go into some basic stuff to actually know about the secretion and regulation of Thyroid hormone.
Hypothalamus is a master switchboard controlling all other endocrine glands via the pituitary gland – the master gland of the body. It is important that T3 and T4 levels are neither too high nor too low. Two glands in the brain—the hypothalamus and the pituitary communicate to maintain T3 and T4 balance.
The hypothalamus produces TSH Releasing Hormone (TRH) that signals the pituitary to tell the thyroid gland to produce more or less of T3 and T4 by either increasing or decreasing the release of a hormone called thyroid stimulating hormone (TSH).
TRH act on the pituitary to secrete, the Thyroid secreting a hormone, abbreviated as TSH, which in turn act on Thyroid gland to regulate the release of thyroid hormone. Suppose, for any reason, your body is not able to make thyroid hormone, this would send a positive feedback to pituitary gland via Hypothalamus, TSH is increased in the body in the application of increasing the hormone in the body. An opposite cascade of events takes place when the hormone level in the body is increased than to the normal level. In nutshell,
- When T3 and T4 levels are low in the blood, the pituitary gland releases more TSH to tell the thyroid gland to produce more thyroid hormones.
- If T3 and T4 levels are high, the pituitary gland releases less TSH to the thyroid gland to slow production of this hormone
T3 (5mg/day) is produced in smaller amounts than T4 (80 mg/day) but is more active than T4. Once inside its target cell, T4 is converted to the more active T3(25mg/ day). T3 and T4 can act on nearly any cell of the body, where they have a number of roles.
How Does Thyroid Act?
Their main role is to control the body’s metabolic rate and calorigenic, which is essentially how much energy the body is using. T3 and T4 affect how your body breaks down the main sources of fuel in the body – carbohydrates, fats and protein.The blood level of cholesterol is inversely proportional to the Thyroid hormones.T3 and T4 increases Oxygen consumption increase metabolic rate increases heat production and basal metabolic rate of the body.Severe excess of Thyroid hormones can raise BMR by 60-100%. Lack of Thyroid hormone can decrease BMR by 40-50%.
T4 helps in Maintenance of lactation.
- Growth and DevelopmentThyroid hormones are especially important in children, as they are necessary for normal growth and development. T3 and T4 work with growth hormone to promote the growth of bones in children and adolescents, and also make the cells of the body more responsive to the hormone adrenaline.Kids deficient in the thyroid hormone have stunted growth and are dwarf with disproportionate limbs and body ratio. (Cretinism). Normal levels of thyroid hormone are essential to the development of the fetal and neonatal brain.
- Cardiovascular SystemThyroid hormones increase heart rate, cardiac contractility and cardiac output. Enhancement of blood flow to many organs.
- Nervous systemBoth decreased and increased concentrations of thyroid hormones lead to alterations in mental state. Too little thyroid hormone and the individual tends to feel mentally sluggish, while too much induces anxiety and nervousness.
- Reproductive systemNormal levels of thyroid hormone are essential for gonadal development, secondary sexual development. Hypothyroidism, in particular, is commonly associated with infertility and disturbances in menstruation.
Types of Thyroid Disorders
Hypothyroidism occurs when the thyroid gland does not make enough of the thyroid hormones to maintain your normal body metabolism.
Types of Hypothyroidism
1. According to the time of onset
2. According to the level of endocrine dysfunction
– Secondary or central
3. According to the severity
– Severe or clinical
– Mild or subclinical hypothyroidism.
What causes Hypothyroidism?
Hypothyroidism is a commonly seen condition. It may be categorized into two categories:
- Primary hypothyroidism
- Secondary hypothyroidism
Primary hypothyroidism results from under secretion of thyroid hormone. Reasons for low thyroid hormone:
- Autoimmune thyroiditis (Hashimoto’s thyroiditis)
The most common cause of hypothyroidism is a disorder known as thyroiditis—an inflammation of the thyroid gland. The most common cause of thyroiditis is autoimmune (Hashimoto’s thyroiditis) when the body natural defence system starts working against one’s own body – mistakes cells in the thyroid gland for harmful invaders and destroy the thyroid gland. The pituitary gland then releases TSH to tell the thyroid gland to make more thyroid hormone. This demand on the thyroid gland can cause it to enlarge (Goitre). The prevalence of antibodies is higher in women and increases with age.
• Pregnancy-induced and Postpartum thyroiditis
• Certain Drugs may destroy thyroid tissues (e.g. amiodarone, lithium)
• Iatrogenic causes (e.g. radioactive iodine, thyroidectomy)
- Congenital hypothyroidism (Cretinism)
The most severe and devastating form of hypothyroidism is seen in young children with congenital thyroid deficiency. If that condition is not corrected by supplemental therapy soon after birth, the child will suffer from cretinism, a form of irreversible growth and mental retardation.
- Iodine deficiency in the diet
Hypothyroidism also can result from a diet that does not have enough iodine, although this type of hypothyroidism is common in Himalayan and sub-Himalayan regions (Endemic Goitre).
Myxedema or myxoedema is a term used synonymously with severe hypothyroidism.
Secondary hypothyroidism is caused by lack of TSH production from the pituitary gland.
What are the Symptoms of Hypothyroidism?
The symptoms of hypothyroidism are slow to develop. Common symptoms of hypothyroidism include the following:
- Tiredness, Fatigue or weakness and sleepiness
- Weight gain in spite of decreased appetite
- Joint pains and muscle cramps
- Change in menstrual periods
- Loss of libido
- Feeling cold when others do not
- Puffiness around the eyes
- Brittle nails, Hair loss and cold dry skin
- Depression, Emotional liability, mental impairment, Forgetfulness, impaired memory, inability to concentrate
- Menstrual disturbances, impaired fertility
- Blurred vision
- Decreased hearing
- Fullness in the throat, hoarseness
2. Subclinical Hypothyroidism
Subclinical hypothyroidism refers to mildly increased serum TSH levels (usually less than 10 mIU/L) in the setting of normal free T4. This condition is usually asymptomatic but regularly follow up is necessary.
What are the various Treatments available for Hypothyroidism?
In most cases, hypothyroidism is treated with medication that contains thyroid hormone. The dosage of the medication is increased slowly until a normal level of thyroid hormone has been reached. The American Thyroid Association recommends screening at age 35 years and every 5 years thereafter, with closer attention to patients who are at high risks, such as the following :
- Pregnant women
- Women older than 60 years
- Patients with type 1 diabetes or another autoimmune disease
- Patients with a history of neck irradiation