These drugs are really quite simple if you can recognize the names and if you remember how the thyroid gland is controlled and how it synthesizes thyroid hormone (Figure 38–1).
In each thyroid cell there is active uptake of iodide. This iodide is then incorporated onto tyrosine residues in the protein thyroglobulin. The iodinated thyroglobulin then undergoes proteolysis to release thyroid hormone in the form of T3 and T4.
The thyroid gland helps maintain an adequate level of metabolism in tissues. Hypothyroidism (low levels of hormone) results in slow heart rate (bradycardia), cold intolerance, and physical slowing. In children, hypothyroidism can result in mental retardation and short stature. Hyperthyroidism (too much hormone) results in fast heart rate, nervousness, tremor, and excess heat production.
The thyroid gland stores thyroid hormone as thyroglobulin.
THYROID REPLACEMENT THERAPY
There are two major thyroid hormones, called T3 and T4. T3 is the most active form.
T4 is the major secretory product of the thyroid gland. T3 is secreted by the thyroid, but is also synthesized by extrathyroid metabolism of T4. Both T4 and T3 are bound to thyroxine-binding globulin and several other proteins in the plasma. T4 is often referred to as thyroxine and T3 as triiodothyronine.
LEVOTHYROXINE (a sodium salt of T4) is the most commonly used drug for thyroid replacement therapy. LEVOTHYROXINE is the drug of choice for the treatment of hypothyroidism.
Liothyronine (a sodium salt of T3) and liotrix (a mixture of T3 and T4) have also been used to treat hypothyroidism.
DRUGS THAT ARE THYROID DOWNERS
Treatment of hyperthyroidism is achieved by removing part, or all, of the thyroid gland, inhibiting synthesis of thyroid hormone, or by blocking release of hormone from the gland.
Surgery or radioactive iodine can be used to destroy the thyroid gland.
Remember that iodine is taken up selectively by the thyroid gland. Therefore, administration of radioactive iodine will result in the accumulation of radioactivity in the thyroid gland. This is very selective radiation therapy.
PROPYLTHIOURACIL and METHIMAZOLE inhibit thyroid synthesis.
Propylthiouracil and methimazole inhibit iodination of tyrosine groups and coupling of these groups to form thyroid hormone. They have no effect on the stored thyroglobulin or on the release of thyroid hormone. Therefore, there will be a delay between the onset of therapy and the clinical effect as the previously stored thyroglobulin is released. Propylthiouracil also inhibits the peripheral conversion of T4 to T3. Propylthiouracil causes severe hepatic toxicity and is ...