Thyroid Hormone Replacement Therapy

For patients who have recently initiated levothyroxine therapy and whose serum TSH has normalized or in patients who have had their dosage or brand of levothyroxine changed, the serum TSH concentration should be measured after 8-12 weeks. When the optimum replacement dose has been attained, clinical (physical examination) and biochemical monitoring may be performed every 6-12 months, depending on the clinical situation, and whenever there is a change in the patient’s status. It is recommended that a physical examination and a serum TSH measurement be performed at least annually in patients receiving SYNTHROID (see WARNINGS, PRECAUTIONS, and DOSAGE AND ADMINISTRATION). Because of the increased prevalence of cardiovascular disease among the elderly, initiate levothyroxine sodium tablets at less than the full replacement dose see Dosage and Administration (2.3)and Warnings and Precautions (5.2) . Atrial fibrillation is the most common of the arrhythmias observed with levothyroxine overtreatment in the elderly.

Use In Specific Populations

Since postpartum TSH levels are similar to preconception values, the SYNTHROID dosage should return to the pre-pregnancy dose immediately after delivery see Dosage and Administration (2.3). Consumption of certain foods may affect SYNTHROID absorption thereby necessitating adjustments in dosing see Dosage and Administration (2.1). Soybean flour, cottonseed meal, walnuts, and dietary fiber may bind and decrease the absorption of SYNTHROID from the gastrointestinal tract. Start at a lower starting dosage and increase the dosage every 4 to 6 weeks as needed based on clinical and laboratory response. Personally, I have had experiences where I have written for brand-name SYNTHROID and the patients have been dispensed generic levothyroxine or even another branded levothyroxine product. It’s very important to make sure that you write “Dispense as written,” or whatever state-specific language, on the prescription when you prescribe the medication.

ADVERSE REACTIONS

The recommended daily dosage of SYNTHROID in pregnant patients is described in Table 3. To minimize the risk of hyperactivity, start at one-fourth the recommended full replacement dosage, and increase on a weekly basis by one-fourth the full recommended replacement dosage until the full recommended replacement dosage is reached. Evaluate the need for dosage adjustments when regularly administering within one hour of certain foods that may affect SYNTHROID absorption see Dosage and Administration (2.2 and 2.3), Drug Interactions (7.9), and Clinical Pharmacology (12.3). One is that the peak therapeutic effect of levothyroxine at a given dose may not be obtained for 4-6 weeks after medication is ingested. Dosing here should be individualized based on whether the patients were diagnosed with hypothyroidism prior to pregnancy or during the pregnancy.

Titrate the dose of levothyroxine sodium tablets carefully and monitor response to titration to avoid these effects see Dosage and Administration (2.4) . Consider the potential for food or drug interactions and adjust the administration or dosage of levothyroxine sodium tablets as needed see Dosage and Administration (2.1), Drug Interactions (7.1), and Clinical Pharmacology (12.3) . For secondary or tertiary hypothyroidism, serum TSH is not a reliable measure of SYNTHROID dosage adequacy and should not be used to monitor therapy.

Soybean flour, cottonseed meal, walnuts, and dietary fiber may bind and decrease the absorption of levothyroxine sodium tablets from the gastrointestinal tract. Grapefruit juice may delay the absorption of levothyroxine and reduce its bioavailability. Bile acid sequestrants and ion exchange resins are known to decrease levothyroxine absorption. Administer levothyroxine sodium tablets at least 4 hours prior to these drugs or monitor TSH levels. The recommended daily dosage of levothyroxine sodium tablets in pregnant patients is described in Table 3.

Animal studies have not been performed to evaluate the carcinogenic potential, mutagenic potential or effects on fertility of levothyroxine. The synthetic T4 in SYNTHROID is identical to that produced naturally by the human thyroid gland. Although there has been a reported association between prolonged thyroid hormone therapy and breast cancer, this has not been confirmed. Patients receiving SYNTHROID for appropriate clinical indications should be titrated to the lowest effective replacement dose. Therefore, a decrease in the dose of anticoagulant may be warranted with correction of the hypothyroid state or when the SYNTHROID dose is increased.

Reduce the SYNTHROID dosage or discontinue cat synthroid temporarily if signs or symptoms of overdosage occur. Concurrent use of tyrosine-kinase inhibitors such as imatinib may cause hypothyroidism. Monitor serum free-T4 levels and maintain in the upper half of the normal range in these patients.

The goal of treatment in pediatric patients with hypothyroidism is to achieve and maintain normal intellectual and physical growth and development.
Monitor for cardiac arrhythmias during surgical procedures in patients with coronary artery disease receiving suppressive SYNTHROID therapy.
Administer SYNTHROID to pediatric patients who cannot swallow intact tablets by crushing the tablet, suspending the freshly crushed tablet in a small amount (5 to 10 mL) of water and immediately administering the suspension by spoon or dropper.
The developmental and health benefits of breastfeeding should be considered along with the mother’s clinical need for SYNTHROID and any potential adverse effects on the breastfed infant from SYNTHROID or from the underlying maternal condition.

Addition of levothyroxine sodium tablets therapy in patients with diabetes mellitus may worsen glycemic control and result in increased antidiabetic agent or insulin requirements. Carefully monitor glycemic control, especially when thyroid therapy is started, changed, or discontinued see Warnings and Precautions (5.5). Addition of levothyroxine therapy in patients with diabetes mellitus may worsen glycemic control and result in increased antidiabetic agent or insulin requirements. Carefully monitor glycemic control after starting, changing, or discontinuing levothyroxine sodium tablets see Drug Interactions (7.2) .

1 Serious Risks Related to Overtreatment or Undertreatment with SYNTHROID

Toxic effects may include increased risk of cardiac arrhythmias and central nervous system stimulation. Levothyroxine sodium tablets may accelerate the onset of action of tricyclics. Administration of sertraline in patients stabilized on levothyroxine sodium tablets may result in increased levothyroxine sodium tablets requirements.

Thyroid hormones regulate multiple metabolic processes and play an essential role in normal growth and development, and normal maturation of the central nervous system and bone. The metabolic actions of thyroid hormones include augmentation of cellular respiration and thermogenesis, as well as metabolism of proteins, carbohydrates and lipids. The protein anabolic effects of thyroid hormones are essential to normal growth and development. Concurrent use of tricyclic (e.g., amitriptyline) or tetracyclic (e.g., maprotiline) antidepressants and SYNTHROID may increase the therapeutic and toxic effects of both drugs, possibly due to increased receptor sensitivity to catecholamines. Administration of sertraline in patients stabilized on SYNTHROID may result in increased SYNTHROID requirements.