NAPLEX Question of the Week: Thyroid Disorders

One of the more common endocrine disorders encounter in clinical practice involves thyroid disorders. This week's question involves laboratory interpretation with a patient receiving thyroid replacement.
NAPLEX Question of the Week: Thyroid Disorders

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GF is a 56 year old female who come to clinic for management of recently diagnosed hypothyroidism. Her other relevant past medical history includes hypertension, hyperlipidemia, and GERD for which she takes lisinopril/hctz 40/12.5mg daily, atorvastatin 40mg daily, and esomeprazole 20mg daily.  She was placed on levothyroxine 50mcg orally daily for initial therapy two months ago and had labs drawn yesterday which reveal a T4 level of 3.0 mcg/dL (normal range: 5-11 mcg/dL) and TSH of 12.5 mIU/mL (normal range 0.5-5 mIU/mL). She endorses adherence to current regimen and has had some improvement in her symptoms. Which of the following would most likely represent the best dosage adjustment for GF?

A. Her labs reflect underdosing of levothyroxine. Would recommend increasing levothyroxine to 75 mcg orally daily.

B. Her labs reflect underdosing of levothyroxine. Would recommend increasing levothyroxine to 100 mcg orally daily.

C. Her labs reflect overdosing of levothyroxine. Would recommend decreasing levothyroxine to 25mcg orally daily.

D. Her labs reflect overdosing of levothyroxine. Would recommend stopping levothyroxine therapy.

Answer with rationale:

The correct answer is A. 

Hypothyroidism is one of the most common diseases states encountered, with an estimated occurrence in 10 million Americans. As much as 10% of women may have some degree of hypothyroidism. Thyroid dosage adjustment is very precise with small dosage changes required with follow up in most circumstances. Have you thought about how many dosages of levothyroxine there are? An easy way to remember the different dosages is to correlate them with the color of the tablet such as 100mcg (yellow) or 88mcg (green). Patients often remember their medications by the color of the tablets/capsules.

Lab assessment is NAPLEX competency statement 1.1.4. For GF, her T4 is low and her TSH is high indicating that her replacement of 50mcg daily is too low. Therefore answers C and D are incorrect as this would be the case if she had low TSH and high T4 levels. Answer A is correct over Answer B as you would not want to double to dosage of the levothyroxine like often done with hypertension or hyperlipidemia medications. Small adjustments may have profound impacts on TSH and T4 levels. Additionally, large adjustments may increase the risk of atrial fibrillation or tachycardia which could increase the risk of coronary event in those patients who are at higher risk. An important pearl with regards to monitoring with levothyroxine is that it takes at least 4-6 weeks before full effects of a dose change to occur so labs should not be taken until this time period. 

Additionally competency statements can be found here for the NAPLEX:

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