A Case of Funny Thyroid Function Tests

H Bashiti, Royal United Hospital; A Iqbal, George Eliot Hospital Nuneaton; N Haya, Royal United Hospital

Abstract: The preferred treatment for hypothyroidism is oral levothyroxine (LT4) ingestion, in doses that ensure a sustained state of hormonal balance. Yet, despite physician’s best effort at dose titration, up to 20-50% of patients fail to achieve optimal thyroid function test. There are various factors, which influence the absorption of LT4, including: eating habits, medications and different functional as well as organic pathologies of the gastro-intestinal tract. Impaired absorption of levothyroxine through the gastrointestinal tract is one of the major causes of not achieving adequate thyroid function control. Approximately 62–82% of levothyroxine is absorbed after oral administration. This absorption occurs within the first 3 h of ingestion and is localised mainly in the jejunum and ileum. T4 is absorbed mainly from the small intestine, which explains the higher dose requirements in malabsorption.T4 and T3 conjugates are excreted in bile and partially deconjugated in the intestine, with the release of small amounts of T4 and T3 for reabsorption. Interference with enterohepatic circulation of thyroid hormone leads to deranged thyroid hormone absorption. Gastrointestinal conditions such as bile acid malabsorption syndrome can play an integral role in reducing levothyroxine absorption. Hypothyroidism that persists despite escalation of levothyroxine dose should prompt investigation into underlying gastrointestinal malabsorption or medication interference.

We present the case of labile thyroid function tests secondary to bile acid malabsorption syndrome.

A 49 year old Caucasian woman was referred to the Endocrinology Team with 24 years history of primary hypothyroidism. Other co-morbidities included premature ovarian failure, depression, anxiety and recent changes in bowel habit. To optimise her TFTs, primary care physician have altered her Levothyroxine dose several times over the past few years. However patient was left with ongoing symptoms of hypothyroidism, mild-moderate diarrhoea, and multiple vitamin deficiencies. She reported full compliance, no dietary changes or overt interacting medications. Malabsorption syndrome was suspected and further investigations were arranged by the gastroenterology team, including faecal elastase, Hydrogen Breath Test for lactose intolerance, and Radionuclide SeHCAT bile study. Patient’s SeHCAT bile study at 7-days showed an uptake of 2.7% which is significantly lower than the normal range of above 15%. Bile acid malabsorption syndrome was confirmed and this was thought to be the cause of her inadequate thyroid function control. Since starting a bile acid sequesterant (cholestyramine), her gastrointestinal signs and symptoms have settled and this was associated with a significant clinical and biochemical improvement in her thyroid function tests. Her recent TFTs have completely normalised.

Conclusion: These findings suggest that pre-existing malabsorption can reduce the bioavailability of levothyroxine. The need to use high LT4 doses in the substitutional treatment of hypothyroidism is often the very first sign of one of the pathologies that are connected with malabsorption syndrome, which might have been asymptomatic and undiagnosed previously.