What is hypothyroidism?
Hypothyroidism is a medical condition where the thyroid hormone (T3 and T4) levels in the blood are low and TSH is high due to an under-active thyroid. The thyroid gland secretes thyroid hormones to regulate the function of various organs in our body. It is regulated by an anterior pituitary hormone TSH (Thyroid Stimulating Hormone), which in turn is monitored by the hypothalamus via TRH (Thyrotropin Releasing Hormone). Together TRH, TSH, T3 & T4 make the hypothalamus-pituitary-thyroid axis. Any abnormalities in this feedback mechanism can be detected by measuring the hormone levels in the blood.
Duration and incidence of hypothyroidism
Hypothyroidism is a chronic or life long illness. It is very common, detected in almost ten million people in India every year. Although it can affect either of the genders and any age group, it is more commonly seen in women and the elderly.
The signs and symptoms of hypothyroidism are different for infants, children & teens, adults and the elderly. This is because thyroid hormones have an effect on the growth and functioning of almost all tissues in our body.
Symptoms of hypothyroidism common to all age groups are listed below:
- Cold intolerance
- Easily fatigued
- Weight gain
- Puffed-up face
- Dry skin
- Thinning of hair, with global hair loss
- Reduced bowel movements – chronic constipation
- Muscle pains, weakness and stiffness
- Voice change – deep hoarse voice
- Irregular menstrual flow
- Bradycardia – slow heart rate
- Poor memory
- Mental fog
- Thyroid swelling – goiter
- Dyslipidemia on blood tests
The thyroid affects growth, therefore, infants, children and teens will exhibit some symptoms that are different from adults. These are mentioned below:
Hypothyroidism in infants
During pregnancy, if the thyroid gland is deficient in the foetus, the mother’s hormones will make up for it. At birth, there are little or no signs of hypothyroidism. However, it will soon manifest with the following symptoms:
- Jaundice: Thin yellow skin and sclera (whites of the eyes)
- Macroglossia: Large, protruding tongue
- Umbilical hernia: soft swelling below the belly button
- Breathing difficulty
- Hoarse cry
- Poor muscle tone and hyporeflexia
- Delayed milestones – global developmental delay. If left untreated leads to retarded physical and mental growth
Symptoms and signs in children
A mix of the symptoms seen in adults, and growth & developmental delay is seen in children. Mental retardation, delayed eruption of permanent teeth, short stature and delayed puberty are symptoms specific to this age group.
Causes of hypothyroidism
Any condition that results in reduced circulating T3 & T4 hormones can cause hypothyroidism. These conditions can be broadly classified as the following:
- Pituitary / Hypothalamus
This condition is seen when the body’s demand for thyroid hormone increases and the thyroid is unable to cope with it, for example, pregnancy, growth spurts, recovery from major surgery or infection. It is diagnosed by blood tests which are TSH, T3 & T4. Treatment is supplementation with T4 for 6 months or till this metabolic state resolves. Lifelong or long term treatment is given if the thyroid hormone production cannot cope with increased demands and TSH increases on stopping thyroxine.
Low levels of iodine in food and drinking water is a cause of hypothyroidism. Iodine is necessary for thyroid hormone production, in its absence, T3 & T4 cannot be produced in adequate amounts. Iodine demand increases during growth spurts and pregnancy. Therefore, iodine deficiency thyroid disorders are most commonly seen in women of childbearing age. Iodination of table salt was adopted as a national strategy in India to fight this disease in 1986. Since then, India has seen a significant drop in iodine deficiency disorders of thyroid. However, it is still prevalent in certain regions of the country due to regional dietary practices.
Another environmental cause for thyroid disorders including hypothyroidism is radiation poisoning due to nuclear power plant accidents and nuclear atom bombings and testing.
Congenital hypothyroidism is due to absent or underdeveloped thyroid gland (dysgenesis of the thyroid) or faulty hormone production (dyshormonogenesis) in a normally developed gland due to enzymatic defect. The most commonly affected genes are PAX8 & TSHR. It is an inherited disorder that should be diagnosed at birth to prevent mental and physical growth retardation known as cretinism.
The most common reason for hypothyroidism in adults is an autoimmune disorder of the thyroid known as Hashimoto’s thyroiditis. An autoimmune disorder is due to immune surveillance which has gone wrong. The body’s immune system identifies antigens on the cells of the thyroid as foreign and attacks these cells. This results in immune-mediated cell death, leading to hypo functioning thyroid gland. No cause has been identified for autoimmunity, it is most likely a multi-factorial interaction of genetics and the environment.
Viral thyroiditis is a common reason for inflammation and destruction of thyrocytes. Initially, it may present with a hyper-functional gland, but, in the long term, thyroid function reduces and causes hypothyroidism.
Iatrogenic causes mean medically induced reasons. This includes surgical removal of the thyroid gland for treatment of benign or malignant neoplasms, radio-iodine ablation of thyroid for treatment of Grave’s disease, medical treatment of hyperthyroidism with anti-thyroid drugs such as neomercazole. Thyrotoxicosis is a state where there is an unregulated stimulation of thyroid hormone receptors on various tissues. This is brought about by high levels of thyroid hormones due to constant stimulation of the thyroid gland. Ablation of thyroid followed thyroxine supplementation is an effective treatment.
Pituitary / Hypothalamus
Thyroid-stimulating hormone is secreted by the anterior pituitary gland. This may get affected due to the direct effect of the tumour or due to treatment of the tumour by surgery or radiation. Hypothalamic inflammation or lesion can affect the hypthalamus-pituitary-thyroid axis.
Diagnosis of Hypothyroidism
No matter the cause, hypothyroidism is always diagnosed by blood tests. The role of imaging such as ultrasound of the neck or MRI of the brain is in differentiating the various differential diagnoses. These blood tests (TSH, T3 & T4) will also be frequently advised by the doctor to monitor the progress of the patient on treatment.
Treatment of hypothyroidism
Thyroid hormone supplementation is the treatment of hypothyroidism. The dose needs to be titred as per the requirements of the patient. Frequent follow-ups are necessary to maintain and adjust the hormone levels in the healthy range.