Antithyroid agents effects

The steps in thyroid hormone synthesis and the antithyroid agents effects upon them are summarized in Table VII-2-2. The clinical uses and their potential complications are presented in greater detail for the thioamides (propylthiouracil and methimazole) and iodine. 

Fukayama H, Nasu M, Murakami S, et al. 1992. Examination of antithyroid effects of smoking products in cultured thyroid follicles only thiocyanate is a potent antithyroid agent. Acta Endocrinol (Copenh) 127(6) 520-525. 

Methimazole (Tapazole) [Antithyroid Agent] Uses Hyperthy-roidism, thyrotoxicosis, pr for thyroid surgery or radiation Action Blocks T3 T4 formation Dose Adults. Initial 15-60 mg/d PO tid Maint 5-15 mg PO daily Peds. Initial 0.4-0.7 mg/kg/24 h PO tid Maint V h- U h of initial dose PO daily w/ food Caution [D, +/-] Contra Breast-feeding Disp Tabs SE GI upset, dizziness, blood dyscrasias Interactions t Effects OF digitalis glycosides, metoprolol, propranolol X effects OF anticoagulants, theophylline X effects W/ amiodarone EMS None OD May cause N/V, HA, abd pain, fever, and pale skin symptomatic and supportive... 

Aggravation of the extrapyramidal effects of antipsychotic agents have been described and it has been reported that the use of lithium in combination with haloperidol may result in irreversible neurological toxicity. Lithium can increase the hypothyroid effects of antithyroid agents or iodides. 

Antithyroid agents are used for the management of hyperthyroidism. The different agents are equally effective and have the same toxic potential. Their commonest adverse effects are skin rashes, while the most serious reaction is the occurrence in about 0.5% of the patients of a potentially fatal agranulocytosis. 

In addition, the metabohsm of OCAs results in the release of large amounts of E into the circulation. As described for KI, I released from OCAs may have effects at the thyroid gland and if used alone to treat hyperthyroidism, OCAs carry the same potential to induce increased secretion of thyroid hormone and exacerbation of thyrotoxicosis. When an OCA is used in the treatment of hyperthyroidism, large doses of antithyroid agents are usually administered concomitantly. However, the combination of OCAs and antithyroid drugs may cause resistance to the antithyroid drugs with time, presumably because of the elevation in intrathyroidal 1 content. Thus, it is recommended that the use of OCAs be reserved for short-term treatment of patients with severe thyrotoxicosis and significant comorbidity (e.g., myocardial infarction, sepsis, stroke) for rapid control of plasma Tj concentrations. 

A potential biomarker of exposure to PBDEs relates to their effect on the thyroid gland. As discussed in Sections 3.2.2.2, Endocrine Effects, thyroid changes in rats and mice include reduced serum thyroxine (T4) levels w itli no changes in scrum TSH (Damerud and Sinjari 1996 Fowles et al. 1994 Hallgren and Damemd 1998 WIL Research Laboratories 1984 Zhou et al. 2001, 2002). Additional studies are needed to characterize thyroid effects of PBDEs in humans and develop specific correlations between levels and duration of exposure and alterations in senun levels of T4. This potential biomarker is not specific to PBDEs because PBBs and other antithyroid agents can have similar effects. 

Polybrominated Diphenyl Ethers. The thyroid is a critical target for PBDEs in animals and, based on these data (see Section 3.2.2.2 Endocrine Effects), serum T4 level is a potential biomarker of effect in humans. Although this biomarker is not specific to PBDEs because other antithyroid agents can have similar effects, changes in T4 can be considered to indicate potential impairment of health. 

Table VII-2-2. Sites of Action and Effects of Antithyroid Agents...

Lithium carbonate, administered for affective and bipolar disorders, may enhance the effects of antithyroid drugs. Potassium iodide, used as an expectorant, is a major ingredient in many cough medications. Iodide derived from this source may enhance the effects of antithyroid drugs and lead to iodine-induced hypothyroidism. Iodine in topical antiseptics and radiological contrast agents may act in a similar manner. 

Although iodide is effective in treating hyperthyroidism for short periods, the effects of this drug begin to diminish after about 2 weeks of administration.35 Consequently, iodide is used in limited situations, such as temporary control of hyperthyroidism prior to thyroidectomy. In addition, iodide may cause a severe hypersensitive reaction in susceptible individuals. Therefore, the use of iodide has been replaced somewhat by other agents such as antithyroid drugs and beta blockers. 

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