Hypothyroidism
Hypothyroidism is the most common disorder of thyroid function. It is reported to be more
common in women than in men. In hypothyroidism, there is decreased production
and secretion of the thyroid hormones by the thyroid gland.
The principle
role of the thyroid gland is to regulate tissue metabolism through the production
of the thyroid hormones thyroxine (T4) and in smaller amounts tri-iodothyronine
(T3). In infants and children thyroid hormones are also needed for normal growth
and development. Hypothyroidism can be classified on the basis of etiology, age
of onset and on the basis of its severity. The coming pages of this section provide
detailed discussion about classification of hypothyroidism.
Etiology
Hypothyroidism can be classified on the basis of etiology as primary, secondary
or transient.
• Primary Hypothyroidism
• Central / Secondary Hypothyroidism
• Transient Hypothyroidism
Primary Hypothyroidism
This is
hypothyroidism caused by disorders of the thyroid gland itself.
Causes:
• Destruction of Thyroid Tissue
• Chronic Autoimmune Thyroiditis
• Goitrous
• Atrophic
• Post Thyroidectomy
• Post Radioactive
Iodine Therapy for Thyrotoxicosis
• Neck Radiation
• Iodine Deficiency
• Disorders of Hormone
Synthesis-Enzyme Defects
• Antithyroid agents, Lithium, Iodine, Radiocon-Trast
Dyes containing Iodine, Amiodarone
The commonest cause of primary hypothyroidism
in iodine-sufficient areas is chronic autoimmune thyroiditis and in iodine-deficient
areas - iodine deficiency itself.
Central / Secondary Hypothyroidism
Decreased
thyroid hormone production and secretion by the thyroid gland due to inadequate
stimulation by thyroid stimulating hormone (TSH) because of pituitary or hypothalamic
disorders.
Causes:
- Pituitary Disorders - decreased TSH
- Hypothalamic Disorders-Decreased Thyrotropin Releasing Hormone (TRH)
Transient
Hypothyroidism
In this type, there is decreased thyroid hormone production
and secretion for a transient period of time.
Causes:
- Silent
Thyroiditis
- Postpartum Thyroiditis
- Subacute Thyroiditis
- After withdrawal of Thyroid Hormone Therapy in Euthyroid Patients
Age
of onset
Hypothyroidism is also classified on the basis of age of onset
as
Congenital Hypothyroidism
Cretinism
Juvenile Hypothyroidism
Adolescent Hypothyroidism
Adult Hypothyroidism.
Congenital Hypothyroidism
Causes:
- Congenital Hypothyroidism
- Transient Permanent
- Iodine Deficiency Thyroid Dysgenesis
- Iatrogenic
Maternal Exposure to131 I
- Maternal/Neonatal Dyshormonogenesis
- Iodine Deficiency
- Maternal RAI Therapy
- Congenital Toxoplasmosis
- TSH Receptor
- Blocking
Antibodies
- Hypothalamic Pituitary Disorders
- Idiopathic
This type of
hypothyroidism has been in existence since antiquity. This was portrayed in the
ancient sculptures of goitrous dwarfs in 400 BC in South America. It was also
described in writings about goitre in ancient Roman empire in the first century.
Congenital
hypothyroidism presents at birth. It may be transient or permanent.
Cretinism
Severe
iodine deficiency causing hypothyroidism in infancy and presenting as mental retardation,
neurological maldevelopment and impaired growth is called cretinism. The infant
having this form of hypothyroidism is called a cretin. It is rightly stated about
a cretin that "What was supposed to be made into the image of God, has turned
into an Imp".
- Epidemiology It is associated with endemic goitre
and severe iodine deficiency.
- Clinical Manifestations These consist
of mental deficiency, together with either of the following :
- Predominant
Neurological Syndrome - which consists of disorders of stance and gait and disorders
of hearing and speech.
- Predominant Hypothyroidism and Stunted Growth
- Prevention
- Prevention of Endemic Cretinism with Adequate correction of Iodine
Deficiency.
Juvenile Hypothyroidism
Occurs in childhood
and manifests mainly as growth retardation along with other generalised features
of hypothyroidism
Adolescent Hypothyroidism
Hypothyroidism
during adolescence presents with delayed puberty with/without short stature and
menstrual irregularities in females.
Severity: Hypothyroidism is also classified
on the basis of severity as
Subclinical Hypothyroidism
Overt or
frank hypothyroidism
• Subclinical Hypothyroidism
In
this, patients are asymptomatic and are identified in screening especially in
patients at risk. Spontaneous subclinical hypothyroidism is more common in women
and the incidence increases with age and is associated with the presence of antithyroid
antibodies.
• Overt or Frank Hypothyroidism
All causes of
hypothyroidism can manifest with moderate to severe symptoms and signs. However,
the clinical manifestations are variable and sometimes nonspecific too.
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