Lithium Hypothyroidism — MRCPsych Paper B MCQ

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ModerateMood DisordersLithium HypothyroidismMRCPsych Paper B

A 28-year-old woman with bipolar I disorder has remained well on lithium maintenance treatment. Her serum lithium concentration is within the therapeutic range and renal function is normal. Routine monitoring shows a TSH of 12 mU/L and a free T4 of 8 pmol/L, which is below the laboratory reference range. What is the most appropriate management?

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Correct answer: D — Continue lithium and start levothyroxine replacement

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

The correct answer is B. A raised TSH with a free T4 below the reference range indicates overt primary hypothyroidism, a well-recognised and treatable adverse effect of lithium. The established approach, supported by the endocrine literature and UK prescribing practice, is that levothyroxine treatment is effective and lithium should not be stopped when it is providing good mood stabilisation. Stopping lithium immediately (A) risks affective relapse and is unnecessary for a correctable adverse effect. Switching to valproate (C) sacrifices effective prophylaxis and is particularly inappropriate in a woman of childbearing potential given valproate's teratogenicity and UK regulatory restrictions. Reducing the lithium dose (D) would not reliably reverse established hypothyroidism and could undermine mood stability. Watchful waiting (E) may be considered for mild, transient subclinical TSH elevation, but overt biochemical hypothyroidism warrants replacement irrespective of symptoms. Thyroid function should continue to be monitored at least six-monthly during lithium treatment.

Reference: Lazarus JH. Lithium and thyroid. Best Practice & Research Clinical Endocrinology & Metabolism. 2009;23(6):723-733. https://pubmed.ncbi.nlm.nih.gov/19942149/