NSC 830: Drugs Affecting the Thyroid BROOKE BENTLEY, PHD, APRN Hypothalamus-Pituitary-Thyroid Hormone Axis TSH: Normal = 0.5-5 mu/l Free T4: 1.3-3.8 ng/dl 1
Hypothyroidism: Thyroid Agents Natural Health food stores: dessicated thyroid preparations Cheaper; more natural Primarily derived from beef & pork thyroid glands Do not have consistent amts of thyroid hormones Content & bioavailability vary from dose to dose Serum fluctuations more likely to cause cardiac symptoms Should NOT be substituted for prescription drug Hypothyroidism: Thyroid Agents Natural 2
Hypothyroidism: Thyroid Agents Synthetic 1. levothyroxine (T4) Synthroid, Levoxyl 35% of T4 converts to T3 Longer ½ life (6-7 days) 2. liothyronine(t3) - Cytomel 3-4x more active Greater risk of cardiotoxicity 3. liotrix -Thyrolar A 4:1 mixture of T4 & T3 Prescribing levothyroxine for tx of hypothyroidism Adults: Initiate at 50 mcg/day Increase in increments of 25 mcg/day at 6 wkintervals Usually up to 100-150 mcg/day Target dose based on TSH levels & patient symptoms 3
Prescribing levothyroxine for tx of hypothyroidism Elderly: Initiate at 25 mcg/day Increase in increments of 12.5-25 mcg/day at 6-8 intervals Target dose based on TSH levels & patient response *** also, lower doses & longer intervals for changing doses in patients with CVD Prescribing levothyroxine for tx of hypothyroidism Pregnancy: Increasing the dose by 25-30% usually results in adequate coverage during pregnancy Check TSH in 4 weeks to determine any dosage adjustment 4
Prescribing levothyroxine for tx of hypothyroidism Congenital Hypothyroidism: Tablets may be crushed and added to infant formula Soy-based formula may impair absorption Need referral to pediatric endocrinologist Inappropriate Uses of Thyroid Hormones: Obesity Weight loss Depression Increase energy or mood 5
Take at same time QD Usually morning (before breakfast) Prevent insomnia Take on an empty stomach (1/2 hrbefore meals or 2 hrs after meal) Variably absorbed Fasting increases absorption High-fat & high-fiber decrease absorption Separate from other meds by >2 hrs Rx, OTC, vit/minerals (may decrease absorption of levothyroxine) Bile-acid sequestrants (colesevelam - WelChol) Iron Antacids (calcium, aluminum) Be aware of multiple drug interactions when prescribing levothyroxine 6
Do NOT switch brands Not bioidentical therapeutically If switch brands, retitrationof dose may be needed Color coded: not standardized across manufacturers Missed dose Single dose, then take that day as soon as remembered >3 missed doses, then inform NP Therapy is life-long Instruct patient to never stop taking it without first consulting the health care provider 7
Monitoring: Must achieve steady state to have reliable values Long ½ life (take 6 wksto achieve steady state) Initiate levothyroxine & check TSH level in 6 weeks Monitoring: Stable TSH may take 6-12 months to achieve Euthyroidgoal: TSH goal: 0.5-5 (preferably 1-2) mu/l Consider clinical symptoms Check TSH 6 wksafter any dosage adjustment ***infants/children checked more frequently Especially during growth spurts 8
Adverse Drug Reactions (ADR): 1. S/S hyperthyroidism Increased HR (check pulse if >100 hold dose & contact NP) Increased BP Nervousness, anxiety, tremors, insomnia Intolerance of heat, excessive perspiration Weight loss despite increased appetite Diarrhea Menstrual irregularities Assess TSH & adjust dose accordingly 2. Children: may experience temporary partial hair loss Precautions/Contraindications 1. Cardiovascular disease CAD, recent MI 2. Osteoporosis Long-term therapy assocwith decreased bone density in hip/spine 9
Hyperthyroidism: Antithyroid Agents Hyperthyroidism (thyrotoxicosis) Breakdown in feedback loop Excessive levels of thyroid hormones Most common cause = Graves disease (autoimmune) Severe = thyroid storm = life-threatening Goal of pharmacologic therapy is to either: 1. inhibit the synthesis of thyroid hormones 2. destroy thyroid gland tissue (radioactive iodine I 131) Hyperthyroidism Hyperthyroidism S/S= tachycardia, nervousness, anxiety, tremors, insomnia, weight loss, heat intolerance, diarrhea Treatment monitor: Clinically: S/S Lab: TSH 10
Antithyroid Agents 3 major categories: 1. thioamide derivatives propylthiouracil(ptu) methimazole(tapazole) 2. iodides (nonradioactive) Lugol s solution 3. radioactive iodine I 131 Antithyroid Agents propylthiouracil(ptu) MOA: blockage of thyroid hormone synthesis (inhibits incorporation of iodine) Does NOT destroy existing stores of thyroid hormones Once therapy is started, therapeutic effects will not occur until existing stores are depleted it may take several weeks Used only in adults 11
Antithyroid Agents propylthiouracil(ptu) Short ½ life (2 hours) TID Highly protein bound Low concentrations cross placenta or into breast milk Metabolized in liver with significant first-pass effect Antithyroid Agents propylthiouracil(ptu) AE: Drowsiness, HA, vertigo, diarrhea, nausea, arthralgia, paresthesias Agranulocytosis(decreased WBCs) Rare Occurs first 1-2 mo of therapy CBC; observe for S/S infection; reversible when PTU is dc d Teach patients to report: fever, chills, sore throat Drug-induced hepatitis BLACK BOX WARNING LFT(liver function tests) Teach patients to report: unusual HA, malaise, weakness, yellowing of eyes/skin Therapy required 6-18 months 12
Antithyroid Agents methimazole (Tapazole) Preferentially used in almost all cases (may be used in pediatrics) Fewer adverse effects MOA: blockade of thyroid hormone synthesis (inhibits incorporation of iodine) Does NOT destroy existing stores of thyroid hormones AE: Several weeks to see therapeutic effects Drowsiness, HA, vertigo, diarrhea, nausea, arthralgia, paresthesias Agranulocytosis Drug-induced hepatitis Longer ½ life: QD dosing Antithyroid Agents methimazole (Tapazole) Critical thinking Lipid soluble Not protein bound Freely cross placenta and into breast milk??? PTU preferred in pregnant women PTU = use in pregnancy methimazole = most others 13
Hyperthyroidism Beta-Blockers Treat SYMPTOMS Tachycardia Ex: 1. propranolol(inderal) 2. atenolol(tenormin) 14