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Understand Thyroid – A Functional Myriad, and Clear Skepticism on Hypothyroidism

Dr. K. V. Narasimha Raju & Dr. B. Swapna

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Lack of iodine prevents production of both thyroxine and triiodothyronine. As a result, no hormone is available to inhibit production of TSH by the anterior pituitary; this causes the pituitary to secrete excessively large quantities of TSH. The TSH then stimulates the thyroid cells to secrete tremendous amounts of thyroglobulin colloid into the follicles, and the gland grows larger and larger. But because of lack of iodine, thyroxine and triiodothyronine production does not occur in the thyroglobulin molecule and therefore does not cause the normal suppression of TSH production by the anterior pituitary. The follicles become tremendous in size, and the thyroid gland may increase to 10 to 20 times normal size.

Idiopathic Nontoxic Colloid Goiter. Enlarged thyroid glands similar to those of endemic colloid goiter can also occur in people who do not have iodine deficiency. These goitrous glands may secrete normal quantities of thyroid hormones, but more frequently, the secretion of hormone is depressed, as in endemic colloid goiter.

Hypothyroidism, like hyperthyroidism, probably is initiated by autoimmunity against the thyroid gland, but immunity that destroys the gland rather than stimulates it. The thyroid glands of most of these patients first have autoimmune “thyroiditis,” which means thyroid inflammation. This causes progressive deterioration and finally fibrosis of the gland, with resultant diminished or absent secretion of thyroid hormone.

The most common and clinically significant types of thyroiditis are 1. Hashimoto Thyroiditis (Chronic Lymphocytic Thyroiditis), 2. Subacute Granulomatous Thyroiditis and 3. Subacute Lymphocytic Thyroiditis.

Hashimoto’s Thyroiditis. Hashimoto’s is an autoimmune disease in which the body’s immune system inappropriately attacks the thyroid tissue. In this condition, the thyroid gland is usually enlarged (goiter) and has a decreased ability to make thyroid hormones. In part, this condition is believed to have a genetic basis showing the tendency of running in families. Hashimoto’s is 5 to 10 times more common in women than in men. Blood samples drawn from patients with this disease reveal an increased number of antibodies to the enzyme, Thyroid Peroxidase (anti-TPO antibodies).

Lymphocytic Thyroiditis. When the thyroid gland inflammation is caused by a particular type of white blood cell known as a lymphocyte, the condition is referred to as lymphocytic thyroiditis. This condition is particularly common after pregnancy and can actually affect up to 8% of women after they deliver. In these cases, there is usually a hyperthyroid phase (in which excessive amounts of thyroid hormone leak out of the inflamed gland), which is followed by a hypothyroid phase that can last for up to six months. The majority of affected women eventually return to a state of normal thyroid function, although there is a possibility of remaining hypothyroid.

Post Therapy: Radioactive Iodine or Surgery
Patients who have been treated for a hyperthyroid condition (such as Graves’ disease) and received radioactive iodine may be left with little or no functioning thyroid tissue after treatment. The likelihood of this depends on a number of factors including the dose of iodine given, along with the size and the activity of the thyroid gland. If there is no significant activity of the thyroid gland six months after the radioactive iodine treatment, it is usually assumed that the thyroid will no longer function adequately. The result is hypothyroidism. Similarly, removal of the thyroid gland during surgery will be followed by hypothyroidism.

Pituitary or Hypothalamic Disease
If for some reason the pituitary gland or the hypothalamus are unable to signal the thyroid and instruct it to produce thyroid hormones, a decreased level of circulating T4 and T3 may result, even if the thyroid gland itself is normal. If this defect is caused by pituitary disease, the condition is called “secondary hypothyroidism.” If the defect is due to hypothalamic disease, it is called “tertiary hypothyroidism.”

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