Thyroid disorders and gastrointestinal and liver dysfunction: A state of the art review

• The function of the thyroid and the gut are closely intertwined in both health and disease.
• Thyroid dysfunction commonly causes various gastrointestinal symptoms.
• Conversely, some gastrointestinal diseases can mimic thyroid disorders (non-thyroidal illness).
• Thyroid function should be monitored in patients with gastrointestinal diseases and those on interferon therapies.
• Medullary thyroid carcinoma should be considered in the differential diagnosis of unexplained, chronic diarrhoea.

Thyroid disorders commonly impact on the gastrointestinal system and may even present with gastrointestinal symptoms in isolation; for example, metastatic medullary thyroid carcinoma typically presents with diarrhoea. Delays in identifying and treating the underlying thyroid dysfunction may lead to unnecessary investigations and treatment, with ongoing morbidity, and can potentially be life-threatening. Similarly, gastrointestinal diseases can impact on thyroid function tests, and an awareness of the concept and management of non-thyroidal illness is necessary to avoid giving unnecessary thyroid therapies that could potentially exacerbate the underlying gastrointestinal disease. Dual thyroid and gastrointestinal pathologies are also common, with presentations occurring concurrently or sequentially, the latter after a variable time lag that can even extend over decades. Such an association aetiologically relates to the autoimmune background of many thyroid disorders (e.g. Graves' disease and Hashimoto's thyroiditis) and gastrointestinal disorders (e.g. coeliac disease and inflammatory bowel disease); such autoimmune conditions can sometimes occur in the context of autoimmune polyglandular syndrome. Emphasis should also be given to the gastrointestinal side effects of some of the medications used for thyroid disease (e.g. anti-thyroid drugs causing hepatotoxicity) and vice versa (e.g. interferon therapy causing autoimmune thyroid dysfunction). In this review, we discuss disorders of the thyroid-gut axis and identify the evidence base behind the management of such disorders.

Fig. 1: The thyroid and the gut are closely intertwined in both health and disease. The hypothalamus (a) and anterior pituitary (b) centrally regulate the synthesis and secretion of thyroid hormones in the thyroid gland (c). The majority of the thyroid hormones are secreted as tetraiodothyronine or T4, which undergoes activation to the metabolically active triiodothyronine or T3 in peripheral tissues including the liver (d) and gastrointestinal tract (e).Figure optionsDownload high-quality image (61 K)Download as PowerPoint slide