Levothyroxine Absorption Test – Results and Follow Up of Patients from a Single Center : Indian Journal of Endocrinology and Metabolism

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Original Article

Levothyroxine Absorption Test – Results and Follow Up of Patients from a Single Center

Subramaniam, Karthik; Babu, Tom; Sreevalsan, K. C.1; Mathew, Shemy1

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Indian Journal of Endocrinology and Metabolism 27(1):p 32-36, Jan–Feb 2023. | DOI: 10.4103/ijem.ijem_188_21
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Hypothyroidism is one of the most common endocrine disorders in adults. Levothyroxine (LT4) supplementation is the current standard of care for its management. Under optimum conditions, 60%–80% of orally administered LT4 is absorbed from the duodenum, jejunum, and ileum.[1] This requires a fasting state, adequate pH in the stomach, and no interference in absorption from the lumen of the gut in a healthy individual.[2] However, LT4 absorption varies drastically and unpredictably when consumed with food and beverages, other medications (calcium salts, iron salts, aluminum salts, bile acid sequestrants), and in the presence of other gastrointestinal disorders (Helicobacter pylori infection, gastritis, celiac disease, lactose intolerance, biliary/pancreatic disorders, bowel shortening procedures). Hence, in patients with hypothyroidism, while ascertaining the reasons behind normal/high TSH with supraphysiological doses of LT4, after confirming adequate compliance, gastrointestinal causes have to be ruled out.

Noncompliance is the most common cause for not achieving euthyroidism in patients consuming an appropriate dose of LT4. It has been estimated that as much as 50% of patients might be nonadherent to medications by 1 year.[3] Levothyroxine absorption test might be one of the ways to convince and objectively demonstrate to the patients that, it is their non-adherence that is causing persistent biochemical and psychological problems.[4] Some of the excipients used in certain brands of levothyroxine include cellulose, talc, gelatin, colloidal silicon dioxide, sodium stearate, and lactose – to name a few.[5] Rarely, the reason behind malabsorption might be an allergy to the excipients used, and this may be resolved by changing the formulation. A liquid formulation of LT4 has been Food and Drug Administration (FDA) approved but is not yet available for commercial use in India.

In this paper, we analyze our institution’s data of patients who had undergone levothyroxine absorption test (LT4AT) and had regular follow-up at our institution.


We retrieved the data of all patients who had undergone LT4AT between 2018 and 2020. In all, there were 16 patients of which complete investigations for malabsorption were not performed in 2 patients, and 3 of them had no follow-up data. The data presented here is of the other 11 patients. All patients had long-standing hypothyroidism requiring thyroxine replacement with high titers of anti thyroperoxidase antibody and/or anti-thyroglobulin antibody along with sonographic evidence (heterogeneous echotexture with or without pseudonodular appearance) of chronic thyroiditis.

In brief, whenever any individual with LT4 dose >2 mcg/kg had normal or high TSH, he/she was subjected to detailed evaluation. Though there are no specific cut-offs for identifying malabsorption, the dose of 2 mcg/kg was based on a study from 2006.[6] History pertaining to interaction with food/beverages/naturopathic medicines/nutritional supplements; previous bowel surgeries; obvious gastrointestinal/malabsorption syndromes; allergies; severe organ dysfunctions; nutritional deficiencies; LT4 storage were elaborated. Compliance was assessed by cross-checking the pharmacy bills, visual inspection by the bystander at home, or by asking the individual to record the video of the act of consumption. In the majority of them, modifiable factors were corrected and rechecked after 3–6 months. If there were no obvious causes attributable to high dose requirement, then the individuals underwent the LT4 absorption test (LT4AT).

Individuals were called in fasting state and baseline TSH and total T4 samples were taken. Later, a dose of 15 mcg/kg (equivalent to a week’s dose) was given (except one patient - #3 in Table 1 who was given 12 mcg/kg) and was ensured under our supervision that it was taken. Hourly samples for a total T4 were taken for 5 h. Maximum increment in T4 was taken for the calculation. The absorption was calculated using the formula[7]

% absorption = [[maximum increment in T4 (mcg/dL) × 10]/total dose of thyroxine administered (mcg)] × Vd × 100

Vd being the volume of distribution which is calculated as BMI (in kg/m2) × 0.442.

Absorption of > 60% was taken as normal.[8]

Based on the results of absorption tests, the next described protocol was followed. If the calculated value was ≤60%, fasting serum gastrin, Helicobacter pylori – immunoglobulin G, tissue transglutaminase – immunoglobulin A, total Immunoglobulin A, stool examination for parasites were undertaken. When the above results were all negative, a lactose breath test was done and if negative, gastroduodenal biopsies were taken and subjected to histopathology. In case all of the above investigations turn negative, we labeled the patient to have no detectable cause of malabsorption. Celiac disease (CD) was diagnosed with positive antibody titer and duodenal biopsy suggesting villous atrophy; Helicobacter pylori infection (HpI) was diagnosed based on antibody positivity and after 2 weeks antibiotic course, eradication was confirmed with stool antigen testing; autoimmune gastritis (AG) was diagnosed when gastrin levels were high and anti-parietal cell antibody was positive. On the other hand, if the calculated value of the test was >60%, we labeled the patient as having no malabsorption.

The samples of total T4 and TSH were analyzed with electrochemiluminescence assay (Elecsys, Roche Diagnostics GmbH, Mannheim, Germany). Serum gastrin was measured by EIA (ELISA genie, Dublin, Ireland), immunoglobulin A was measured by immunoturbidimetry (Szybio, Wuhan Life origin biotech Co., Wuhan, China), Helicobacterpylori stool antigen by ELISA (Epitope diagnostics Inc, San Diego, USA), anti-parietal cell antibody by ELISA (Demeditec Diagnostics GmbH, Kiel, Germany), tissue transglutaminase was measured by ELISA (Demeditec Diagnostics GmbH, Kiel, Germany) and Helicobacter pylori IgG was measured by EIA (Sigma Aldrich Co, St. Louis, USA). During follow-up, celiac disease control was ascertained by the titer of tissue transglutaminase antibodies, and Helicobacter pylori eradication was ascertained by stool antigen test.


Characteristics of the whole cohort

The median age of this cohort was 24 years, the duration of hypothyroidism was 8 years, and 55% (6/11) were women. All the patients had autoimmune hypothyroidism. The median BMI was 25.73 kg/m2, and the median dose of LT4 before administering the absorption test was 3 mcg/kg/day. Everyone except two patients (patient # 4 and 8), had TSH above the normal range. Details are elaborated in Table 1.

Table 1:
Characteristics and details of patients who had undergone levothyroxine absorption test

Results of LT4 absorption test

None of the patients had baseline T4 below the 2.5th centile of the assay. In the LT4AT, 63% (7/11) had low absorption. In the group having malabsorption, median LT4 absorption was 36.5%, whereas it was 77.4% in those who had normal absorption. Two each (28%) had CD and HpI, whereas one (14%) had AG. One of the 2 who had CD, had North Indian ethnicity (Patient #1), where there is a higher prevalence of CD in the general population. In spite of doing the tests as elaborated above, we were not able to find the cause of malabsorption in two of the patients.

Follow-up of the patients

Among the patients with malabsorption, two of them with HpI were cured with antibiotics, onewith CD was controlled well with dietary restriction (patient #5), and another individual with CD was not compliant with the diet and hence had high titers of antibody (patient # 1). While recruitment, 6/7 had high TSH values but after evaluation and treatment, 5/7 (2 with HpI, 1 with CD, 1 with no cause being found, and 1 with AG) had TSH in euthyroid range. We were able to reduce the dose of LT4 in people with CD and HpI, but the dose was unchanged with the patient with AG. In patients where no cause was discernible, the dose needed to be increased to maintain euthyroidism.

After the absorption test, the TSH of all 4 patients without malabsorption was in euthyroid range on follow-up. Interestingly, in 3 of the patients, we were able to reduce the dose of LT4 and in the other patient, the dose was stable.


In our cohort of patients who were either requiring high doses of LT4 with/without biochemical euthyroidism, about 63% had reduced absorption of LT4 on testing in a standardized manner. Attributable cause for the reduced absorption was found in 70% of them, and complete resolution of the disorder took place in 30% of them. In total, euthyroidism with a replacement dose of LT4 ≤2 mcg/kg/day was achieved in 5/11 (45%), euthyroidism with >2 mcg/kg/day replacement of LT4 was achieved in 4/11 (35%), and in the remaining 2/11 (20%), TSH was not controlled.

Different protocols exist for testing LT4 – with bolus dosing, with daily dosing, and certain with weekly dosing.[9–12] In studies that have followed up a cohort of patients who had underwent LT4AT, the majority (90%–95% of patients tested) had normal absorption test results – meaning that they had pseudo-malabsorption.[8,10,11] In a study from Turkey, 2/5 patients who underwent LT4AT had lactose intolerance which was mitigated by diet modification.[13] As there are no well-designed studies to know the exact prevalence of pseudo-malabsorption in people with hypothyroidism, the reason for differences in the results of LT4AT with adequate follow-up, cannot be clearly elucidated. There might also be a role of differences in patient selection; criteria used to define “normal” absorption; ethnic differences in the prevalence of other malabsorptive syndromes – to mention a few.

Around 5%–7% of patients with autoimmune thyroid disease have antibody positivity for celiac disease.[14] The prevalence of CD in the southern part of India was almost 80 times lower than in the northern part (0.11 vs 8.53 per thousand, respectively).[15] In spite of relative rarity, CD was one of the common causes of malabsorption in our cohort. While one patient (#4) was compliant with gluten restriction and hence had dose reduction of LT4, another patient (#1) was not completely compliant (as her antibody titers did not normalize).

HpI has been associated with extra-intestinal disorders like type 1 diabetes, ischemic heart disease, and autoimmune thyroiditis. The presence of significant association of certain HLA phenotype and HpI in patients with autoimmune thyroiditis and Turner’s syndrome has been well established.[16] In patients requiring high doses of LT4 and yet uncontrolled TSH, a study had demonstrated that, after ruling out CD and subacute intestinal obstruction, eradicating HpI resulted in improvement of TSH values.[17] Hence, it is not surprising that both of the cases who completed the combination treatment for eradicating HpI had a reduction in dosage of LT4.

AG has been found in 10%–40% of people with Hashimoto’s thyroiditis.[18] This is one of the disorders warranting the usage of the liquid formulation of LT4 in order to increase its absorption across the mucosa of the gut. Apart from the diagnosis being made in the patient requiring high doses of LT4 (#4), management had not changed with LT4AT because of the nonavailability of liquid formulations in India. However, knowing the diagnosis paved way for screening iron deficiency anemia, vitamin B12 deficiency, and also for screening gastric malignancy in the future.

It has been emphasized, in the introduction that, noncompliance is the major cause for the inability to achieve euthyroidism. As the duration of the next follow-up testing is usually in months due to the long half-life of LT4 and as the effects of skipping the medications are not evident immediately, a subset of patients become complacent and noncompliant. Although the majority of this subset of patients acknowledge the fact, a minority require an objective way “to prove” to them that they were noncompliant. LT4AT serves such a purpose of being an objective test that might help the treating physician to explain to the patient and their family that there is no malabsorption in the gut resulting in uncontrolled TSH values. In our cohort, 3 (Patient # 9, 10, 11) of them who had uncontrolled TSH had euthyroid values on follow-up after LT4AT, and 3 (patient # 8, 10, 11) had dose reduction of LT4 (although not to physiological levels).

The mechanism of reduced LT4 absorption in CD has been attributed mainly to a reduction in absorbing surface of villi due to atrophy, loss of enzymes and proteins in brush border resulting in reduced transportation, alteration in luminal pH, and bacterial overgrowth.[19] Gastric pH plays an important role in LT4 absorption. As the pH increases above 1, the aqueous solubility of LT4 reduces.[5] Hence, in disorders where parietal cells are destroyed or their action is inhibited, the LT4 dose needs to be increased to maintain normal TSH. In HpI, the production of ammonia by the bacteria neutralizes the pH further apart from antral gastritis and pan gastritis.[20] When compliance, nutrient, and drug interferences have been excluded, few clinical clues may point towards specific etiology of malabsorption – abdominal distress, bloating with milk intake (lactose intolerance); B12 deficiency, microcytic or macrocytic anemia (AG, HpI); steatorrhea, anemia, osteopenia, neuropsychiatric disorders, atrophic glossitis (CD); prior abdominal surgeries, nutritional deficiencies, chronic diarrhea (small intestine bacterial overgrowth).

There were several limitations in the study. Firstly, the retrospective nature of the study resulted in inherent bias with the method. Secondly, in nonisotopic LT4AT the value of ≥60% being stated as normal is empirical. Thirdly, HpI was diagnosed with serology rather than biopsy and stool antigen test and in the diagnosis of AG, biopsy or gastric pH was not done. Lastly, in both patients in whom the cause of malabsorption was not known, other detailed tests were not performed.

In conclusion, this is one of the first cohorts reporting a high proportion of malabsorption in patients undergoing LT4AT. Irrespective of the outcome of LT4AT, 81% of the patients achieved euthyroidism on follow-up. Further studies in a larger cohort of patients with further detailed evaluation of malabsorption are warranted.

Ethics approval

Institutional review board ethical clearance was obtained prior to initiation of the study (IEC no 454/2020).

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.


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Celiac disease; gastritis; Helicobacter pylori; hypothyroidism; pseudo-malabsorption

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