In patients with hypothyroidism who are acutely unwell, current UK guidelines do not recommend routine automatic increases in thyroxine (levothyroxine) dosage during the illness. Instead, levothyroxine dosing should be maintained, with monitoring and adjustment based on thyroid function tests after recovery and clinical assessment. This is because acute illness can transiently alter thyroid function tests (non-thyroidal illness syndrome) without reflecting the true thyroid hormone requirements, and adjusting doses during acute illness may lead to inappropriate over- or under-treatment NICE CKS.
Thyroid function tests during acute illness can be unreliable due to physiological changes such as low T3 and fluctuating TSH, so testing should generally be deferred unless thyroid dysfunction is strongly suspected to be the cause or aggravating factor of the illness NICE NG145. Consequently, increases in levothyroxine dose are usually considered after clinical stabilization and reassessment of thyroid function, rather than during the unwell period.
However, exceptions exist. In special circumstances such as myxoedema coma or severe hypothyroid decompensation, urgent dose adjustment and specialist management are required. Additionally, pregnant women with hypothyroidism who become unwell or during pregnancy typically need specialist advice for dose adjustments NICE CKS.
Overall, the large reservoir of circulating thyroxine and the complexity of acute illness effects on thyroid tests warrant caution, favoring maintaining the patient's baseline levothyroxine dose during intercurrent illness and adjusting later based on clinical and biochemical assessment NICE CKS,NICE NG145.