Type 2 diabetes mellitus and thyroid dysfunction are among the most common endocrine disorders worldwide. Although each condition can develop independently, they frequently occur together because thyroid hormones influence glucose production, insulin sensitivity, lipid metabolism, body weight and energy expenditure.
Thyroid dysfunction may remain undiagnosed in a person with diabetes because many symptoms overlap. Fatigue, weight changes, muscle weakness, altered appetite and difficulty concentrating may be attributed to diabetes, ageing or medication rather than an underlying thyroid disorder.
This overlap matters clinically. An untreated thyroid disorder can make glucose levels more difficult to interpret, change insulin or medication requirements and potentially increase the overall cardiovascular and metabolic burden carried by a person with type 2 diabetes.
How Common Is Thyroid Dysfunction in Type 2 Diabetes?
The reported prevalence varies widely because studies use different populations, laboratory reference ranges, diagnostic criteria and screening methods. Age, sex, iodine intake, ethnicity and access to healthcare can also influence the results.
A 2024 systematic review and meta-analysis published in Systematic Reviews combined findings from 38 studies conducted between 2000 and 2022. It estimated that 20.24 percent of adults with type 2 diabetes had some form of thyroid dysfunction. In practical terms, the pooled result suggests that approximately one in five patients may be affected, although prevalence in an individual clinic or country can be substantially higher or lower.
Subclinical hypothyroidism was the most frequently identified disorder, with a pooled prevalence of 11.87 percent. Overt hypothyroidism was reported in 7.75 percent, subclinical hyperthyroidism in 2.49 percent and overt hyperthyroidism in 2.51 percent. These figures should not simply be added together because the included studies differed in design and did not all assess the subtypes in the same way.
The analysis also found higher prevalence estimates in studies from Asia and Africa. The authors acknowledged that relatively fewer studies were available from Europe, North America and Australia, limiting how confidently one pooled figure can be applied to every population.
Why the Two Conditions Frequently Occur Together
Thyroid hormones influence several organs involved in glucose regulation, including the liver, pancreas, skeletal muscle and adipose tissue. Changes in thyroid activity can therefore alter how glucose is produced, absorbed, stored and used.
Type 2 diabetes is strongly associated with insulin resistance, excess body weight, inflammation, dyslipidaemia and ageing. Several of these factors are also associated with thyroid dysfunction. The relationship is therefore not necessarily caused by one disease directly producing the other. In many patients, both conditions may emerge from overlapping metabolic risk factors.
A major review in Endocrine Reviews describes the association as bidirectional. Thyroid dysfunction can influence glucose metabolism, while insulin resistance, obesity and hyperinsulinaemia may affect thyroid structure and hormone regulation.
The 2024 meta-analysis identified several factors associated with thyroid dysfunction among people with type 2 diabetes. These included female sex, diabetes lasting longer than five years, a family history of thyroid disease, central obesity, smoking, HbA1c of at least seven percent, retinopathy and neuropathy. These findings describe associations rather than proving that any one factor directly causes thyroid disease.
How Hypothyroidism Can Affect Glycemic Control
Hypothyroidism occurs when the thyroid gland produces insufficient thyroid hormone. Overt hypothyroidism usually involves an elevated thyroid-stimulating hormone level and reduced free thyroxine, while subclinical hypothyroidism generally involves elevated TSH with free T4 remaining within the laboratory reference range.
Reduced thyroid activity slows several metabolic processes. Glucose absorption from the intestine may be delayed, hepatic glucose production may decline and insulin clearance can become slower. In a person using insulin or glucose-lowering medication, these changes may increase the risk of hypoglycaemia if treatment is not reassessed.
The relationship is not always straightforward because hypothyroidism is also associated with weight gain, abnormal lipids, reduced physical activity and impaired insulin sensitivity. These factors can contribute to worsening metabolic control over time even when immediate glucose production is reduced.
The Endocrine Society review on thyroid dysfunction and diabetes notes that patients with diabetes and hypothyroidism may require lower insulin doses. Continuing the previous insulin dose after thyroid function declines can produce symptomatic hypoglycaemia in some patients.
Symptoms such as unexplained low glucose readings, increasing fatigue, constipation, cold intolerance, dry skin, slow heart rate or unexpected weight gain should not automatically be attributed to diabetes alone.
How Hyperthyroidism Can Worsen Hyperglycemia
Hyperthyroidism creates the opposite hormonal state. Excess thyroid hormone increases metabolic activity and can raise glucose turnover throughout the body.
The liver may release more glucose, intestinal glucose absorption can increase and insulin can be cleared more rapidly. At the same time, peripheral insulin resistance may develop, meaning muscle and other tissues do not respond to insulin as effectively.
A recent review in Nature Reviews Endocrinology notes that hyperthyroidism can worsen glycemic control in type 2 diabetes by increasing hepatic glucose output and insulin resistance.
A person whose diabetes was previously stable may begin experiencing higher fasting glucose, stronger post-meal increases or an unexpected rise in medication requirements. Weight loss may occur despite increased appetite, while palpitations, sweating, tremor, anxiety, heat intolerance and frequent bowel movements may provide additional clues.
Treating hyperthyroidism can change glucose requirements again. Diabetes medication should therefore be monitored during treatment rather than assuming that the previous dose will remain appropriate after thyroid hormone levels normalise.
Thyroid Dysfunction Can Distort HbA1c Interpretation
HbA1c is widely used to estimate average glucose exposure over approximately two to three months. However, it depends not only on glucose concentration but also on the lifespan and turnover of red blood cells.
Overt hypothyroidism can slow red-cell turnover and produce an HbA1c result that appears higher than the person’s actual glucose measurements would suggest. Research published in Diabetes Care found that HbA1c decreased after thyroid hormone replacement in people with overt hypothyroidism even though fasting glucose and other direct measures of glucose control did not significantly change.
The American Thyroid Association’s summary of this evidence warns that hypothyroidism may falsely increase HbA1c and potentially contribute to an incorrect diagnosis of prediabetes or an exaggerated assessment of poor glycemic control.
This does not mean HbA1c should be ignored in every patient with thyroid disease. It means the result should be compared with fasting glucose, home meter readings, continuous glucose-monitoring data and clinical symptoms when the numbers do not agree.
Increasing diabetes medication solely because of a misleading HbA1c result could expose the patient to unnecessary hypoglycaemia.
Detecting Thyroid Dysfunction in a Patient With Diabetes
The initial laboratory investigation usually begins with TSH. When the result is abnormal, free T4 helps determine whether the condition is overt or subclinical and whether thyroid activity is reduced or excessive.
The American Thyroid Association’s hypothyroidism guidance explains that TSH is the principal screening test for primary hypothyroidism, while free T4 measures the amount of circulating hormone available to the body. T3 testing is not routinely required for diagnosing ordinary hypothyroidism but may have a role in evaluating suspected hyperthyroidism.
There is no single universally adopted screening schedule for every adult with type 2 diabetes. Evidence reviews have supported targeted or periodic testing, particularly when risk factors or symptoms are present, but professional recommendations have historically differed.
Testing becomes particularly relevant when a patient has unexplained deterioration in glucose control, recurrent hypoglycaemia, substantial weight change, abnormal cholesterol, atrial fibrillation, a goitre, a family history of thyroid disease or symptoms suggestive of altered thyroid function.
Why Integrated Management Matters
Diagnosing thyroid dysfunction does not replace normal diabetes care. Nutrition, physical activity, glucose monitoring and prescribed diabetes treatment remain necessary. However, restoring thyroid function may make glucose patterns more predictable and allow medication requirements to be reassessed more accurately.
Overt hypothyroidism is generally treated with levothyroxine. The American Thyroid Association states that blood testing is usually repeated six to eight weeks after starting treatment or changing the dose because thyroid hormone levels require time to stabilise.
Hyperthyroidism may require antithyroid medication, radioactive iodine, surgery or another approach depending on its cause and severity. Glucose levels should be monitored during treatment because insulin sensitivity and medication requirements can shift as the patient returns to a euthyroid state.
Thyroid dysfunction is common enough in type 2 diabetes to deserve clinical attention, but prevalence alone does not justify identical testing or treatment for every person. The strongest approach combines individual risk assessment, appropriate thyroid-function testing and careful interpretation of HbA1c alongside direct glucose measurements.
Recognising the interaction between these two endocrine disorders can prevent misinterpretation of glycemic control, reduce inappropriate medication adjustments and support more coordinated long-term management.
