Validity of the Rapid Thyroxine Absorption Test for the Differentiation Between Levothyroxine Non-compliance and Malabsorption in Thyroid-Stimulating Hormone Refractory Hypothyroidism

Introduction Thyroid-stimulating hormone refractory hypothyroidism is a common problem. This is due to either non-compliance or malabsorption with levothyroxine (LT4). The study aimed to assess the validity of the rapid LT4 absorption test in the differentiation between LT4 malabsorption and non-compliance. Methods A cross-sectional study was done from January to October 2022 at Faiha Specialized Diabetes, Endocrine, and Metabolism Center, in Basrah, Southern Iraq. Twenty-two patients with thyroid-stimulating hormone (TSH) refractory hypothyroidism were evaluated by rapid LT4 absorption test with measurements of TSH before 1000 μg LT4 intake, and free thyroxine (pmol/l) and total thyroxine before (nmol/l) (baseline TT4 and baseline FT4) and two hours after (2-HR TT4 and 2-HR FT4). The findings were compared with the following four-week-long supervised LT4 absorption test results. Results In the rapid LT4 absorption test, patients with (2-HR FT4 minus baseline FT4 ≤1.28 pmol/l (0.1 ng/dl) or 2-HR FT4 minus baseline FT4 1.28-6.43 pmol/l (0.1-0.5 ng/dl) plus 2-HR TT4 minus baseline TT4<72.08 nmol/l (5.6 µg/dl)), eight out of 10 patients were correctly diagnosed with malabsorption. And in those with (2-HR FT4 minus baseline FT4 ≥6.43 (0.5 ng/dl) or 2-HR FT4 minus baseline FT4 1.28-6.43 (0.1-0.5 ng/dl) plus 2-HR TT4 minus baseline TT4≥72.08 (5.6 µg/dl)), 11 out of 12 patients were correctly diagnosed as non-compliant. This criterion showed 88.8% sensitivity, 15.4% specificity, 80% positive predictive value, and 91.6% negative predictive value for diagnosing LT4 malabsorption. Conclusion The rapid LT4 absorption test showed good diagnostic accuracy in differentiating non-compliance from malabsorption when (2-HR FT4 minus baseline FT4) and (2-HR TT4 minus baseline TT4) were used as criteria.

Daily requirement of softgel thyroxine is independent from gastric juice pH

BACKGROUND

Softgel levothyroxine (LT4) preparation showed a better in vitro dissolution profile at increasing pH as compared to tablet LT4 preparation. Clinical studies suggested a better performance of softgel LT4 preparation in patients with gastric disorders but whether this finding is related to gastric juice pH variation in vivo is not known.

METHODS

Twenty-eight hypothyroid patients (24F/4M; median age=50 treated with tablet LT4 (median dose= 1.65 µg/kg/day) and with stable thyroid stimulating hormone (TSH) values on target (<0.8-2.5> mU/l) have been shifted to softgel LT4 preparation. The dose of softgel LT4 has been titrated to obtain a similar individual serum TSH value. All subjects followed a specific treatment schedule, taking LT4 in fasting condition and then abstaining from eating or drinking for at least 1 hour. Owing to the presence of long-lasting dyspepsia or of already known gastric disorders, all patients underwent endoscopy, upon informed consent. Gastric juice has been collected during endoscopy to measure gastric pH. Then we plotted the dose of LT4 with the gastric pH obtained in vivo, before and after the switch tablet/softgel preparation in all patients.

RESULTS

Upon the switch tablet/softgel preparation, the therapeutic LT4 dose was very slightly reduced (-6%) in the whole sample. However, the individual variations revealed the existence of two populations, one without any dose reduction (A) and the other showing a dose reduction >20% (B). Upon matching with the actual gastric pH, patients with normal pH (A: n=17; 14F/3M, median 1.52) no showed a lower softgel LT4 requirement. Instead, among patients with reduced gastric acid production (B: n=11; 10F/1M, median pH 5.02) the vast majority (10/11; 91%, p<0.0001) benefited from a lower dose of softgel LT4 (median = -23%, p<0.0001). Interestingly, the dose of LT4 in tablet correlated with pH value (Spearman's ρ =0.6409; p = 0.0002) while softgel dose was independent from gastric juice pH (Spearman's ρ =1.952; p = 0.3194).

CONCLUSIONS

These findings provide evidence that softgel LT4 preparation is independent from the actual gastric pH in humans and may represent a significant therapeutic option in patients with increased LT4 requirement, owed to disorders impairing the gastric acidic output.

Liquid levothyroxine formulations in patients taking drugs interfering with L-T4 absorption

PURPOSE

To describe the current knowledge on thyroid hormonal profile in patients on liquid L-T4 therapy and drugs known to interfere with L-T4 absorption.

METHODS

A PubMed/MEDLINE, Web of Science, and Scopus research was performed. Case reports, case series, original studies and reviews written in English and published online up to 31 August 2022 were selected and reviewed. The final reference list was defined based on the relevance of each paper to the scope of this review.

RESULTS

The available data showed that novel levothyroxine formulations circumvent gastric pH impairment due to multiple interfering drugs such as proton pump inhibitors, calcium or iron supplements, sevelamer, aluminum/magnesium hydroxide and sodium alginate.

CONCLUSION

New formulations can be taken simultaneously with drugs interfering with L-T4 absorption, in particular liquid formulations. Softgel capsules need more studies to support these data.

Levothyroxine Sodium Oral Solution Normalizes Thyroid Function in a Patient with Hashimoto's Disease, Gastritis, Diabetic Gastroparesis, and Small Intestinal Bacterial Overgrowth (SIBO)

Background

Levothyroxine (L-T4) is used as a standard-of-care treatment in patients with hypothyroidism. L-T4 is absorbed throughout the small intestine, and consistent drug absorption is required for successful treatment. Patients with Hashimoto’s disease (autoimmune thyroiditis) often have food and medication sensitivities, as well as comorbid gastrointestinal (GI) disorders detrimental to L-T4 absorption.

Case Presentation

This case report describes a 51-year-old female patient with long-standing Hashimoto’s disease and multiple sensitivities to food chemicals and medications. The patient suffered from GI symptoms and poor thyroid-stimulating hormone (TSH) control. She was switched to compounded thyroxine/triiodothyronine medication due to her multiple allergies, with only mild improvement in thyroid function and no symptom resolution. She was subsequently diagnosed with gastroparesis and small intestinal bacterial overgrowth (SIBO). Because of further worsening of her GI symptoms and weight loss, she was switched to levothyroxine sodium oral solution (Tirosint^®-SOL), which contains only three ingredients—levothyroxine, water, and glycerol—which was well tolerated and led to normalization of TSH levels.

Conclusion

Malabsorption of L-T4 is often seen in patients with Hashimoto’s disease-related hypothyroidism and comorbid GI conditions, such as gastroparesis and SIBO. L-T4 tablets and a compounded oral suspension were inefficiently absorbed, leading to suboptimal TSH control. Switching to levothyroxine sodium oral solution resulted in sustained TSH control with subsequent resolution of symptoms. No side effects or reactions to the medication were observed, despite the patient’s multiple allergies and sensitivities to food chemicals and medications.