NAPLEX Question of the Week: Perfect Timing

Levothyroxine is a common medication dispensed each day and can be subject to drug interactions. Are you up for the challenge?
NAPLEX Question of the Week: Perfect Timing

MW is a 38-year-old female who presents to your community pharmacy with a new prescription for Synthroid. She explains that she had been experiencing fatigue, hair loss, and constipation so she scheduled an appointment with her doctor. Laboratory findings and symptoms confirmed the presence of hypothyroidism. The patient’s PMH is significant for hyperlipidemia, diabetes, gastric ulcers, bipolar disorder, and obesity. Current medications include Zocor 40mg daily, Carafate 1g twice daily, Glucophage 500mg twice daily, and Lithobid 600mg twice daily. Additionally, she takes some over-the-counter products including Alli and Caltrate. Which of the following medications may interact with her new prescription for Synthroid? Select all that apply.

A. Carafate

B. Alli

C. Lithobid

D. Glucophage

E. Caltrate

F. Zocor



Answers: A, B, E

Brand/Generics covered: Synthroid (levothyroxine), Zocor (simvastatin), Carafate (sucralfate), Glucophage (metformin), Lithobid (lithium), and Alli (orlistat)

Rationale with Explanation:

Synthroid (levothyroxine) is the most prescribed drug for the treatment of hypothyroidism. While Synthroid is generally considered to be the drug of choice for patients with hypothyroidism, it has specific instructions for administration to ensure proper absorption. Synthroid should be administered on an empty stomach at the same time each day, at least 30 to 60 minutes prior to a meal. Absorption of the drug may be decreased if administered with foods such as soybean flour, cottonseed meal, walnuts, dietary fibers, and grapefruit juice. In addition, there are several drugs that can interfere with drug absorption including phosphate binders, orlistat, proton pump inhibitors, sucralfate, and antacids (See a complete list here – [7]).

Gastric acidity is required for adequate absorption of Synthroid. The patient is currently taking Carafate (sucralfate) which may decrease the serum concentration of Synthroid as this drug may affect gastric pH and cause hypochlorhydria. Other medications that may affect gastric acidity include PPIs and antacids. It is recommended that Synthroid be administered at least 4 hours prior to these drugs and to monitor for changes in thyroid function. Alli (orlistat), used in the treatment of obesity, can decrease levels of Synthroid and it is also recommended to separate administration of Synthroid and Alli by at least 4 hours and to monitor for changes in thyroid function. The patient is also taking Caltrate which contains calcium carbonate. Phosphate binders (e.g., calcium carbonate, ferrous sulfate, sevelamer) may bind to Synthroid and decrease the serum concentration. As a result, Synthroid and Caltrate should be administered at least 4 hours apart. While Lithobid (lithium) has the potential to interact with many drugs including SSRIs, NSAIDs, ACE inhibitors, just to name a few, it does not appear to interact with Synthroid. However it is important to keep in mind that lithium can affect thyroid function with report of both hypo and hyperthyroidism reported so monitoring long-term is important as this would be a drug-disease interaction even though not a drug-drug interaction.  Zocor and Glucophage do not appear to interact with Synthroid. 

It is also important to note that Synthroid has a narrow therapeutic index (available in 12 doses) and both subtherapeutic and supratherapeutic drug concentrations of Synthroid can result in adverse effects. Consequently, TSH levels should be monitored approximately 6 to 8 weeks after initiating treatment and dose changes. 

What does Synthroid look like? Each dose has a specific color. Don’t forget the pneumonic “Only White Violets or Yellow Roses Bring Teachers Back Looking Pretty Glad: 

Have a great week!

Dr. B

NAPLEX Exam Competencies Covered: Area 2 (Identify Drug Characteristics), 2.2 Commercial availability; prescription or non-prescription status; brand, generic, or biosimilar names; physical descriptions; or how supplied; Area 3 (Develop or Manage Treatment Plans), 3.8 Drug interactions 


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