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Is Thyroid Dysfunction Associated With Type 2 Diabetes?

New study finds incidence of thyroid disorders in people with diabetes no higher than than those without.

A new study published in PLoS One concluded that the incidence of thyroid dysfunction was not significantly associated with diabetes. The study’s objective was to assess the prevalence, incidence and predictive factors of thyroid disorders in patients with impaired glucose metabolism. Thyroid dysfunction was defined as clinical hypothyroidism, clinical hyperthyroidism, subclinical hypothyroidism and subclinical hyperthyroidism. Diabetes was defined as type 2 diabetes mellitus1.

The study included 5,786 people consisting of 2,376 men and 3,410 women, ages greater than 20 years in a cross-sectional phase implemented from March 1999 to December 2001 as a part of a larger study called the Tehran Thyroid Study (TTS). The participants were than followed for three years in different phases. To determine prevalence of thyroid disorder, they reviewed data from both patients with diabetes and with prediabetes who were over 30 years old, and participants of the phase 1 of TTS. Follow-up assessments were done every three years, after which incidence of thyroid disorder was then calculated and correlations with age, sex, smoking, blood pressure, body mass index, thyroid peroxidase antibody (TPOAb), thyroid stimulating hormone (TSH), insulin resistance index, triglycerides and cholesterol were assessed.

The study excluded patients with a history of thyroidectomy, radioactive iodine intake for non-specific cause, patients taking glucocorticoids or lithium, and pregnant women. Inclusion criteria were diabetes or prediabetes diagnosis, healthy subjects over age 30 and those who had participated in phase 1 of TTS and at least one of the other 3 phases. Healthy control patients were required to not have developed diabetes or prediabetes in any of the phases. Participants were excluded if they had thyroid disease, history of thyroid surgery or radioactive iodine intake, consumption of glucocorticoids, levothyroxine, methimazole, propylthiouracil or lithium or were pregnant at the entry phase.

The authors then concluded that the incidence of thyroid disease in type 2 diabetes or prediabetes was not higher than healthy controls.

The study did have limitations such as lack of data on the daily intake of iodine among the participants; the study should have repeated thyroid tests within 1 to 4 months and not clarified why some patients were taking levothyroxine.

Previous studies like the population-based prospective cohort study that included 8,452 participants (mean age 65 years) were followed for a mean of 7.9 years, in which 798 diabetes cases occurred. They found that higher TSH levels were associated with a higher diabetes risk. The risk of progression from prediabetes to diabetes was high with a low-normal thyroid function. The associated risk of developing diabetes was 1.09 times higher for every doubling of TSH levels. Within the normal range, the risk of diabetes was 1.16 time higher with higher TSH levels. The participants with prediabetes had an associated risk of developing diabetes 1.13 times higher for every doubling of TSH levels. The higher TSH levels and lower FT4 levels were associated with increased risk of diabetes. The study concluded that low and low-normal thyroid function were risk factors for incident diabetes, especially in those with prediabetes. The limitations in this study were the potential for confounders because it was an observational study. Another limitation was that the participants were mostly white and had an average age of 45 years and older2.

With these findings, there is still a need for future experimental studies with larger sample sizes and longer follow-up times to investigate whether or not type 2 diabetes is associated with thyroid dysfunction.

Practice Pearls:

  • In the new study, having diabetes was not a predictive factor for having thyroid dysfunction.
  • Thyroid function tests are usually done in high risk populations and may not be necessary to conduct thyroid tests in all patients unless they have TPOAb≥40 IU/ml or TSH>1.94 mU/L.
  • Previous conflicting studies indicate that thyroid dysfunction can be associated with diabetes, so further investigation is needed for better understanding and confirmation.

References:

Gholampour Dehaki M, Amouzegar A, Delshad H, Mehrabi Y, Tohidi M, Azizi F. Thyroid dysfunction in patients with impaired glucose metabolism: 11 year follow up from the Tehran Thyroid Study. PLoS ONE. 2017. 12(10): e0184808.

Chaker L, Ligthart S, Korevaar T, et al. Thyroid function and risk of type 2 diabetes: a population-based prospective cohort study. BMC Med. 2016; 14: 150. Doi: 10.1186/s12916-016-0693-4.

 

Jessica Quach, Doctor of Pharmacy Candidate: Class of 2018, GA-PCOM School of Pharmacy