Which parameter would indicate the optimal intended effect of therapy with levothyroxine?

PRIMARY HYPOTHYROIDISM DOSING GUIDELINES ADULT* DOSING CONSIDERATIONS2

*In adolescents in whom growth and puberty are complete.

• Dosing must be individualized and adjustments made based on periodic assessment of the patient’s clinical response and laboratory parameters2
  
• Exercise caution with administering to patients with underlying cardiovascular disease, the elderly, and those with concomitant adrenal insufficiency2

DISCUSS THE FOLLOWING WITH PATIENTS WHEN PRESCRIBING SYNTHROID

• Since substitutions can be made at the pharmacy, it’s important that you discuss with patients what their Synthroid pill looks like 
• Remind them that only Synthroid has the name “SYNTHROID” embossed on every pill and that each dose has a specific color2
• Synthroid is taken as a single daily dose, taken on an empty stomach, 30 minutes to 1 hour before breakfast2 

Select pill below to enlarge and see dosage

Section Text

The FDA has determined that certain levothyroxine products are therapeutically equivalent, and can be substituted at the pharmacy with the full expectation that the substituted product will produce the same clinical effect and safety profile as the reference product.9

Tablets shown not actual size and may not represent actual color.

DOSE ADJUSTMENTS AND TSH MONITORING

Careful dose titration is necessary to avoid the consequences of over- or undertreatment.2

• Dosing must be individualized and adjusted based on periodic assessment of the patient’s clinical response and laboratory parameters2
  
• Exercise caution with administering to patients with underlying cardiovascular disease, the elderly, and those with concomitant adrenal insufficiency2
  
• Evaluate need for Synthroid dose adjustments when regularly administering Synthroid within 1 hours of food that may affect absorption  (soybean flour, cottonseed meal, walnuts, dietary fibers, grapefruit juice)2
  
• Peak therapeutic effect at a given dose may not be attained for 4 to 6 weeks2
• During dose titration, TSH monitoring is generally recommended at 6- to 8-week intervals until normalization2
  
• Once optimum replacement dose is attained, monitoring may be performed every 6 to 12 months and whenever there is a change in the patient’s clinical status2
  
• Patients should be given the minimum dose necessary to achieve the desired response2

For secondary or tertiary hypothyroidism, TSH is not a reliable measure of Synthroid dose adequacy and should NOT be used to monitor therapy. Use free-T4 to monitor and titrate Synthroid dosing until clinically euthyroid and free-T4 is restored to upper half of normal range.2

FOOD AND SUPPLEMENT INTERACTIONS WITH SYNTHROID

Certain foods may decrease absorption and require dosing adjustments, including2:

• Soybean flour (infant formula)
• Cottonseed meal
• Walnuts
• Dietary Fiber

Grapefruit juice delays the absorption of Synthroid and reduces its bioavailability.2 

Iron, calcium supplements, and antacids can also decrease absorption.2

• Synthroid should not be taken within 4 hours of these agents2

DRUG INTERACTIONS WITH SYNTHROID

Many drugs can exert effects on thyroid hormone pharmacokinetics and metabolism (eg, absorption, synthesis, secretion, catabolism, protein binding, and target tissue response) and may alter the therapeutic response to Synthroid.2

Drugs That May Decrease T4 Absorption (Hypothyroidism)

Potential impact: Concurrent use may reduce the efficacy of SYNTHROID by binding and delaying or preventing absorption, potentially resulting in hypothyroidism.

Drugs That May Alter T4 and Triiodothyronine (T3) Serum Transport Without Affecting Free Thyroxine (FT4) Concentration (Euthyroidism)

Potential impact (below): Administration of these agents with Synthroid results in an initial transient increase in FT4. Continued administration results in a decrease in serum T4 and normal FT4 and TSH concentrations.

Drugs That May Alter Hepatic Metabolism of T4 (Hypothyroidism)

Potential impact: Stimulation of hepatic microsomal drug-metabolizing enzyme activity may cause increased hepatic degradation of levothyroxine, resulting in increased Synthroid requirements.

Drugs That May Decrease Conversion of T4 to T3

Potential impact: Administration of these enzyme inhibitors decreases the peripheral conversion of T4 to T3, leading to decreased T3 levels. However, serum T4 levels are usually normal but may occasionally be slightly increased.

Which medication teaching instructions should the nurse provide to a client who has been prescribed levothyroxine?

What is the impact of concurrently taking warfarin and thyroid hormone replacement therapy?

Which symptoms indicate thyroid storm would the nurse monitor a client for?

Which laboratory results would confirm overt hyperthyroidism?