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Lithium Subclinical Hypothyroidism — MRCPsych Paper B MCQ

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ModerateMood DisordersLithium Subclinical HypothyroidismMRCPsych Paper B

A 45-year-old woman with bipolar I disorder has remained well on lithium 800 mg daily. Her plasma lithium concentration is within the therapeutic range. Routine blood tests show a TSH of 8.5 mIU/L with a normal free T4. This is the first abnormal thyroid result. She has no symptoms of hypothyroidism and no thyroid enlargement. According to current NICE guidance, what is the most appropriate management?

Educational content. Not a substitute for clinical judgement or local policy.

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Correct answer: E — Continue lithium, consider thyroid peroxidase antibodies, and repeat TSH and free T4 in 3 months

Explanation lettering: E = shown as A · C = shown as B · A = shown as C · B = shown as E

B is correct. The raised TSH with normal free T4 indicates subclinical hypothyroidism. Because she is under 65, asymptomatic, has a TSH below 10 mIU/L and has only one abnormal result, NICE recommends confirmation with a second measurement 3 months later before treatment is considered; thyroid peroxidase antibodies may also help identify underlying thyroid disease. Levothyroxine should be considered if TSH is at least 10 mIU/L on two occasions 3 months apart, or as a 6-month trial if a persistent lower elevation is accompanied by hypothyroid symptoms. Effective lithium should not automatically be stopped, switched or reduced below the therapeutic range for this treatable abnormality. Routine 6-month monitoring alone is insufficient once impaired thyroid function has been detected.

Reference: NICE. Thyroid disease: assessment and management (NG145), recommendations 1.5.1–1.5.4. Published 2019; updated 2023. https://www.nice.org.uk/guidance/NG145/chapter/recommendations