Underactivity of the thyroid is called hypothyroidism. When the cause lies in the thyroid it is called primary hypothyroidism; when it is due to a lack of thyroid stimulating hormone (TSH) subsequent to pituitary failure, it is called secondary hypothyroidism. Primary hypothyroidism can be clinical or subclinical. Clinical hypothyroidism is when there are definite symptoms with a raised TSH outside the reference range and low serum thyroxine level. Some patients have few if any symptoms but have a serum TSH outside the reference range but have serum thyroxine levels within the reference range. These patients are said to be subclinically hypothyroid.
Diagram illustrating classification of hypothyroidism - click to enlarge
Primary hypothyroidism results from low levels of thyroid hormones in the circulation as a direct result of an underproduction by the thyroid gland itself. The main causes of primary hypothyroidism are given below.
1. Hashimoto's thyroiditisis the most common cause of hypothyroidism. It is six times more common in women than men and presents with all the symptoms of hypothyroidism.It may be associated with other endocrine organ deficiencies such as diabetes mellitus or Addison's disease (Schmidt's syndrome). The patients have a rubbery feeling thyroid and the thyroid function tests confirm hypothyroidism. The thyroid scan shows an irregular patchy uptake. Thyroid antibodies in the blood confirm the diagnosis. Treatment is thyroid replacement. The condition is potentially premalignant and any suspicious nodules should be biopsied to exclude lymphoma. The thyroid swelling usually regresses on thyroid medication and surgery is very rarely indicated. Non Hashimoto's thyroiditis such as postpartum thyroiditis, De Quervains thyroiditis and silent thyroiditis can all cause hypothyroidism.
2. Congenital - Poor development of the thyroid, aplasia, hypoplasia or defective synthesis of thyroid hormone - one in 4000 live births.
3. Drug causes:
4. Surgical removal of the thyroid.
5. Radioactive iodine treatment.
Secondary hypothyroidism results from an underproduction of thyroid hormones from the thyroid caused by deficient TSH stimulation by the pituitary. The main causes are listed below:
1. Destruction of the anterior pituitary gland due to tumour or surgery.
2. Very rarely isolated deficiency in TSH production by the anterior pituitary gland.
3. Peripheral resistance to thyroid hormones.