The thyroid gland secretes two hormones that regulate protein synthesis, enzyme activity, and stimulate mitochondrial oxidation. These are thyroxine (T4) and triiodothyronine (T3). The thyroid gland secretes 20% of the circulating T3. The remaining 80% comes from degradation of T4 hormone. Approximately 40% of T4 is degraded and becomes T3.
T3 and T4 are carried in the blood by thyroxine-binding globulin (TBG) and albumin, which protects the hormones from being degraded. T3 is more potent than T4. Only unbound free T3 and T4 are active and produce a hormonal response.
A decreased amount of T3 and T4 is produced in a condition called hypothyroidism. This is caused by a disorder of the thyroid gland or a secondary lack of TSH secretion. Hyperthyroidism is an increase in circulatory T4 and T3 caused by an overactive thyroid gland or an excessive output of thyroid hormones.
Primary hypothyroidism is characterized by a decrease in T4 and an increase in TSH levels. Primary hypothyroidism is caused by acute or chronic inflammation of the thyroid gland, radioiodine therapy, excess intake of antithyroid drugs, and surgery.
Myexedema is severe hypothyroidism characteristic by lethargy, apathy, memory impairment, emotional changes, slow speech, deep coarse voice, edema of the eyelids and face, thick dry skin, cold intolerance, slow pulse, constipation, weight gain, and abnormal menses.
In children, hypothyroidism can have a congenital (cretinism) or prepubertal (juvenile hypothyroidism) onset.
Hypothyroidism is treated by administering levothyroxine sodium (Levothroid, Synthroid), which increases levels of T3 and T4. Levothyroxine sodium (Levothroid, Synthroid) is also used to treat simple goiter and chronic lymphocytic (Hashimoto's) thyroiditis.
Lyothyronine (Cytomel) is a synthetic T3 that is used for short-term treatment of hypothyroidism. It isn't used for maintenance therapy because lyothyronine has a short half-life and duration.
Liotric (Euthroid, Thyrolar) is a mixture of levothyroxine sodium and liothy-ronine sodium with no significant advantage over levothyroxine sodium. Thyroid and thyroglubin (Proloid) are seldom used.
A list of drugs utilized in the treatment of hypothyroidism is provided in the Appendix. Detailed tables show doses, recommendations, expectations, side effects, contraindications, and more; available on the book's Web site (see URL in Appendix).
Hyperthyroidism is an increase in circulating T4 and T3 levels resulting from an overactive thyroid gland or excessive output of thyroid hormones. Hyperthyroidism may be mild with few symptoms or severe leading to vascular collapse and death.
Graves' disease or thyrotoxicosis is the most common type of hyperthyroidism and is caused by a hyperfunctioning thyroid gland. Graves' disease is characterized by a rapid pulse (tachycardia), palpitations, excessive perspiration, heat intolerance, nervousness, irritability, exopthalmos (bulging eyes), and weight loss. Treatment involves surgical removal of a portion of the thyroid gland (subtotal thyroidectomy), radioactive iodine therapy, or antithyroid drugs that inhibit either the synthesis or the release of thyroid hormones.
Antithyroid drugs reduce the excessive secretion of T4 and T3 by inhibiting thyroid secretion. Thiourea derivatives (thioamides) are the drugs of choice used to decrease thyroid production.
Propylthiouracid (PTU) and methylthiouracil (Tapazole) are effective thioamide antithyroid drugs used for treating thyrotoxic crisis and in preparation for subtotal thyroidectomy. Methimazole does not inhibit peripheral conversion of T4 to T3 as does PTU, but it is 10 times more potent and has a longer halflife than PTU. Prolonged use of thioamides may cause a goiter because of the increased TSH secretion that inhibits T4 and T3 synthesis. Minimal doses should be given when indicated to avoid goiter formation.
Strong iodide preparations such as Lugol's solution are used to suppress thyroid function in patients having a subtotal thyroidectomy for Graves's disease. Sodium iodide administered intravenously is useful for the management of thyrotoxic crisis.
A list of drugs utilized in the treatment of hyperthyroidism is provided in the Appendix. Detailed tables show doses, recommendations, expectations, side effects, contraindications, and more; available on the book's Web site (see URL in Appendix).
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