Thyroid hormones act as a master dial for metabolism, and when that dial is turned too high or too low, insulin resistance often follows. Both hypothyroidism and hyperthyroidism disrupt blood sugar regulation, though through different routes: an underactive thyroid slows glucose uptake into cells and alters fat metabolism in ways that blunt insulin’s effects, while an overactive thyroid floods the bloodstream with glucose from the liver faster than insulin can clear it. The relationship is clinically meaningful enough that thyroid screening is a routine consideration in people with unexplained blood sugar problems, and the overlap between the two conditions creates some diagnostic traps worth knowing about.
How Thyroid Hormones Move Sugar Around Your Body
The active thyroid hormone T3 touches nearly every organ involved in blood sugar control. In the liver, T3 ramps up glucose production by promoting gluconeogenesis, the process of making new glucose from non-sugar building blocks. It also increases the breakdown of glycogen stores. When someone is hyperthyroid, the liver keeps pumping out glucose at an accelerated rate, and this sustained overproduction is driven mainly by gluconeogenesis rather than short-term glycogen breakdown.1Endocrine Practice. Excess Thyroid Hormone and Carbohydrate Metabolism At the same time, T3 can reduce signaling through a key molecule in the insulin pathway called Akt2, which normally tells the liver to store glucose as glycogen. The net effect is that too much thyroid hormone makes the liver act as though insulin is not really there.2PubMed Central. Why Can Insulin Resistance Be a Natural Consequence of Thyroid Dysfunction?
In skeletal muscle, which is the largest consumer of glucose in your body, T3 works in the opposite direction under normal conditions: it increases the production and translocation of GLUT4, the transporter protein that moves glucose from the bloodstream into muscle cells in response to insulin. Hypothyroid mice show decreased insulin-stimulated glucose uptake precisely because GLUT4 availability drops.3Journal of Endocrinology. Role of thyroid hormone in skeletal muscle physiology Cell culture studies confirm the same pattern: when thyroid hormone is removed, glucose uptake and the amount of GLUT4 at the cell surface both fall.4PubMed Central. Triiodothyronine Acutely Stimulates Glucose Transport into L6 Muscle Cells Without Increasing Surface GLUT4, GLUT1, or GLUT3 So hypothyroidism creates a situation where muscles are sluggish at absorbing sugar, while hyperthyroidism creates a situation where the liver dumps sugar faster than the body can use it. Both lead to insulin resistance, but from different ends of the equation.
What Thyroid Hormones Do to the Pancreas
Insulin-producing beta cells in the pancreas are themselves responsive to thyroid hormone. T3 promotes beta cell growth and enhances insulin secretion. In lab studies, T3 treatment stimulates beta cell proliferation through the MAPK/ERK signaling pathway and boosts insulin output.5PubMed. Triiodothyronine induces proliferation of pancreatic β-cells through the MAPK/ERK pathway Research in newborn rats shows that T3 supplementation during the first week of life increases pancreatic weight and beta cell proliferation, and isolated islets treated with T3 secrete more insulin in response to glucose.6PubMed Central. Pancreatic and Islet Development and Function: The Role of Thyroid Hormone T3 also appears to protect islet cells during stressful conditions: cultured rat islets treated with T3 showed improved survival and reduced core cell damage compared to untreated islets.7PubMed. The thyroid hormone T3 improves function and survival of rat pancreatic islets during in vitro culture
This matters because hypothyroidism does not just make tissues resistant to insulin; it may also reduce the pancreas’s ability to compensate by cranking out more of it. And in hyperthyroidism, the story has an added twist: despite the pancreas working harder, the body clears insulin from the bloodstream faster through increased kidney filtration and reduced pancreatic output in some settings.1Endocrine Practice. Excess Thyroid Hormone and Carbohydrate Metabolism The result is that blood sugar regulation gets squeezed from both sides.
Subclinical Thyroid Problems Still Affect Blood Sugar
You do not need a full-blown thyroid disorder to see metabolic effects. Subclinical hypothyroidism, where TSH is elevated but thyroid hormone levels are still in the normal range, is linked to higher insulin resistance even in people with normal blood sugar. A study comparing people with subclinical hypothyroidism to those with normal thyroid function found that the subclinical group had higher body mass index, higher waist-to-hip ratios, and higher scores on the HOMA-IR index, a standard measure of insulin resistance.8PubMed Central. Subclinical hypothyroidism increases insulin resistance in normoglycemic people Another study found that TSH levels in subclinical hypothyroidism positively correlated with both fasting insulin and HOMA-IR, meaning the higher the TSH climbed, the worse insulin resistance got.9PubMed Central. Study of Insulin Resistance in Subclinical Hypothyroidism
The picture gets more nuanced by sex. An analysis of U.S. national health survey data found that subclinical hypothyroidism correlated with higher insulin resistance in men, while subclinical hyperthyroidism showed a correlation in women.10PubMed Central. The association between thyroid function and insulin resistance as measured by the metabolic score for insulin resistance (METS-IR) The reasons for this sex difference are not fully worked out, but it is a reminder that thyroid effects on metabolism are not uniform across populations.
The HbA1c Trap
Here is a diagnostic pitfall that matters if you are monitoring blood sugar: hypothyroidism can falsely elevate HbA1c readings. HbA1c measures the percentage of hemoglobin in red blood cells that has glucose stuck to it, and it is supposed to reflect your average blood sugar over the past two to three months. But hypothyroidism slows red blood cell turnover, which means each red blood cell spends more time circulating and accumulates more glucose on its hemoglobin, even if your actual blood sugar is normal.
A study of hypothyroid patients found that while HbA1c appeared higher before treatment, this difference disappeared once the researchers adjusted for reticulocyte count, a marker of how quickly new red blood cells are being produced. In other words, the altered red blood cell lifespan, not genuinely worse blood sugar, was responsible for the higher HbA1c readings.11PubMed Central. Effects of thyroid status on glycated hemoglobin Separate research has confirmed that hypothyroidism may cause falsely elevated HbA1c independent of true glycemic status, driven by altered red blood cell dynamics and sometimes autoimmune-related blood count changes.12CME Journal Geriatric Medicine. Evaluation of HbA1c, Fasting Blood Glucose, Random Blood Glucose, Fructosamine, Glycated Albumin, Thyroid Autoantibodies, Antinuclear Antibodies, and Complete Blood Count Parameters in Patients with Hypothyroidism
If you have untreated or undertreated hypothyroidism, an HbA1c reading that looks prediabetic might be misleading. Fructosamine or glycated albumin tests, which reflect shorter-term blood sugar and are not affected by red blood cell lifespan, can provide a more accurate picture. This is worth bringing up with your doctor if both thyroid and blood sugar are being monitored.
Does Treating the Thyroid Fix Insulin Resistance?
The honest answer is: sometimes, and it depends on the direction of the thyroid problem. In hyperthyroidism, the evidence is encouraging. A study of hyperthyroid patients found that fasting glucose, insulin, and other markers were all significantly elevated compared to healthy controls before treatment. After just one month of antithyroid therapy, those levels dropped back to normal and stayed there through six months of follow-up.13PubMed. Effects of treatment of hyperthyroidism on glucose homeostasis, insulin secretion, and markers of bone turnover
For hypothyroidism, the results are more mixed. One study using the gold-standard clamp technique to measure insulin sensitivity in severely hypothyroid women found that glucose disposal and insulin sensitivity both improved significantly once they were on thyroid hormone replacement.14Clinical Chemistry and Laboratory Medicine. Insulin sensitivity and counter-regulatory hormones in hypothyroidism and during thyroid hormone replacement therapy But another study of patients with untreated primary hypothyroidism found that levothyroxine replacement did not improve HOMA-IR, even though it did improve their lipid profiles.15PubMed Central. Effects of levothyroxine replacement therapy on insulin resistance in patients with untreated primary hypothyroidism
The discrepancy likely comes down to the severity of hypothyroidism, how insulin resistance was measured, and whether other factors like obesity or inflammation were also at play. In severe hypothyroidism, restoring thyroid function seems to meaningfully improve insulin sensitivity. In milder cases, the thyroid contribution to insulin resistance may be smaller relative to body weight, diet, and other metabolic factors, so fixing the thyroid alone does not move the needle as clearly.
The Autoimmune Overlap
The most common cause of hypothyroidism in developed countries is Hashimoto’s thyroiditis, an autoimmune disease. And autoimmune thyroid disease shares significant genetic ground with type 1 diabetes. Both conditions are linked to the same immune-regulating gene variants, including those in the HLA region and in genes like CTLA4 and PTPN22 that control T-cell and B-cell behavior.16PubMed Central. Type 1 Diabetes and Autoimmune Thyroid Disease—The Genetic Link Genome-wide studies have identified cell cycle regulation, B-cell development, and CD40 signaling as major pathways that contribute to having both conditions simultaneously, a pattern classified as autoimmune polyglandular syndrome type 3 variant.17Journal of Autoimmunity. Genome wide identification of new genes and pathways in patients with both autoimmune thyroiditis and type 1 diabetes
This is a different pathway to blood sugar problems than the metabolic insulin resistance discussed so far. In type 1 diabetes, the immune system destroys beta cells outright; the problem is not resistance to insulin but absence of it. But the genetic overlap means that someone with autoimmune thyroid disease faces a meaningfully elevated risk of developing type 1 diabetes, and vice versa. Regular screening for both conditions in affected individuals is standard practice for this reason.
Pregnancy, Thyroid, and Gestational Diabetes
Pregnancy is a metabolic stress test, and thyroid status during pregnancy has real consequences for blood sugar. A meta-analysis pooling data from multiple studies found that pregnant women with subclinical hypothyroidism had roughly 34% higher odds of developing gestational diabetes compared to women with normal thyroid function.18PubMed. Maternal subclinical hypothyroidsm and gestational diabetes mellitus: a meta-analysis A more recent systematic review confirmed a similar risk estimate, with the odds ratio hovering around 1.34.19PubMed. The risk of developing gestational diabetes mellitus in maternal subclinical hypothyroidism: a systematic review and meta-analysis
Even when thyroid hormone levels are normal, the presence of thyroid autoimmunity, meaning the immune markers of Hashimoto’s without actual hormone deficiency, may carry risk. In women over 30, thyroid autoimmunity was associated with about 68% higher odds of gestational diabetes after adjusting for other risk factors.20PubMed Central. Association between thyroid autoimmunity and gestational diabetes mellitus in euthyroid women This suggests that the autoimmune process itself, perhaps through low-grade inflammation, may nudge insulin resistance higher during pregnancy even when the thyroid is still technically keeping up with hormone production.
Adipose Tissue, Adipokines, and the Fat Connection
Fat tissue is not a passive energy warehouse; it is an endocrine organ that talks back to the rest of the body through signaling molecules called adipokines. Thyroid hormones regulate both lipogenesis and lipolysis in fat cells and influence the expression of GLUT4 in brown adipose tissue.21PubMed Central. Adipose tissues and thyroid hormones When thyroid function goes awry, the adipokine profile shifts in ways that may amplify insulin resistance.
In hypothyroid patients, levels of resistin, an adipokine linked to chronic low-grade inflammation, tend to be elevated. Researchers have proposed resistin as a potential bridge between thyroid dysfunction and insulin resistance, and possibly as a biomarker for metabolic risk in thyroid patients.22Scientific Reports. Changes in profile of lipids and adipokines in patients with newly diagnosed hypothyroidism and hyperthyroidism Studies of primary hypothyroidism consistently find higher leptin and higher HOMA-IR values compared to controls.23PubMed Central. Variations of Adipokines and Insulin Resistance in Primary Hypothyroidism Interestingly, research in hypothyroid children found no significant correlation between HOMA-IR and either leptin or adiponectin, suggesting that in younger populations the increase in insulin resistance may operate through different mechanisms than the adipokine shifts seen in adults.24PubMed Central. Insulin resistance, leptin and adiponectin in lean and hypothyroid children and adolescents with obesity
A Gut Hormone Surprise
One of the more unexpected connections to emerge involves GLP-1, the gut hormone that has become famous through drugs like semaglutide. It turns out that thyroid hormone T3 in the liver regulates how much GLP-1 the gut produces, and it does so through bile acids. T3 suppresses a liver enzyme that shapes the bile acid pool, shifting it toward bile acids that block a receptor called FXR in the intestine. When FXR is suppressed, gut cells produce more GLP-1, which in turn stimulates insulin secretion and improves blood sugar control. In people with normal thyroid function, circulating T3 levels correlate with both plasma GLP-1 and fecal levels of the relevant bile acids.25Nature Communications. Hepatic thyroid hormone signalling modulates glucose homeostasis through the regulation of GLP-1 production via bile acid-mediated FXR antagonism This pathway means that hypothyroidism could impair blood sugar regulation not just through direct tissue effects but also by reducing one of the body’s most important insulin-boosting signals from the gut.
Metformin’s Effect on Thyroid Levels
If you take both metformin and thyroid medication, there is a pharmacological interaction worth knowing about. A meta-analysis found that metformin lowers TSH levels in people who are hypothyroid, but has no effect on TSH in people with normal thyroid function. It does not affect free T4 levels in either group.26Scientific Reports. The effect of metformin therapy on serum thyrotropin and free thyroxine concentrations in patients with type 2 diabetes: a meta-analysis A clinical trial in people with prediabetes showed a similar pattern: metformin only reduced TSH in participants whose baseline TSH was in the upper range of normal, not in those with lower starting values.27PubMed Central. Effect of metformin on thyroid stimulating hormone and thyroid volume in patients with prediabetes
The practical concern is that if you are on levothyroxine and start metformin, the TSH-lowering effect of metformin might make it look like your thyroid replacement dose is too high. A doctor who lowers the levothyroxine dose based on that reading could inadvertently underdose you. This is especially relevant for older patients, where the consequences of swinging between hypothyroid and hyperthyroid territory are more serious. If both medications are in play, thyroid labs should be interpreted with the metformin effect in mind.
Fatty Liver and Thyroid-Targeted Treatments
The thyroid-metabolism connection has opened a new front in drug development: thyroid hormone receptor agonists designed specifically for fatty liver disease. The liver expresses a particular type of thyroid hormone receptor called THR-beta, and activating it promotes fat burning, improves mitochondrial function, and reduces the fat accumulation that defines non-alcoholic fatty liver disease. Resmetirom, a selective THR-beta agonist, became the first drug approved for this condition after trials showed it outperformed placebo on two key measures: resolution of liver inflammation without worsening fibrosis in roughly a quarter to a third of patients, and improvement in fibrosis stage in about a quarter of patients.28Endocrinology and Metabolism. Thyroid Selective Agonists of Thyroid Hormone Receptor Beta: Promising Tools for the Treatment of Nonalcoholic Fatty Liver Disease
Fatty liver disease is itself tightly linked to insulin resistance; in many patients, the two are essentially different manifestations of the same metabolic problem. The fact that a drug targeting the thyroid hormone pathway can improve liver inflammation and fibrosis underscores just how central thyroid signaling is to metabolic health beyond the thyroid gland itself. It also suggests that the traditional view of thyroid disease and diabetes as separate conditions sharing an occasional patient understates how intertwined these systems really are.
Energy Metabolism in Muscle and Mitochondria
Thyroid hormones also shape how efficiently your mitochondria convert fuel into usable energy, and this has downstream effects on insulin sensitivity. In the muscle of people with resistance to thyroid hormone, a rare condition where tissues respond abnormally to thyroid hormones, the rate of mitochondrial fuel burning was about 75% higher than normal, yet the amount of ATP produced stayed the same. That gap means the mitochondria were running hot but wasting energy as heat rather than producing useful cellular fuel, a state called uncoupling.29PubMed Central. Resistance to thyroid hormone is associated with raised energy expenditure, muscle mitochondrial uncoupling, and hyperphagia
A key mediator of this process in normal physiology is an uncoupling protein called UCP3 in skeletal muscle, which T3 regulates. When UCP3 levels drop, insulin resistance develops alongside reduced fat burning and weakened signaling through pathways that normally help cells respond to insulin.30PubMed Central. Why Can Insulin Resistance Be a Natural Consequence of Thyroid Dysfunction? – Section: Direct Effects of Thyroid Hormones at the Peripheral Tissue Level The connection between mitochondrial efficiency and insulin sensitivity is an active area of research, but the basic observation is clear: thyroid hormones help set the metabolic rate in muscle, and when that rate is disrupted, the cell’s ability to handle glucose suffers.
Cardiovascular Risk and the Bigger Metabolic Picture
Insulin resistance rarely travels alone. When thyroid dysfunction adds insulin resistance to the mix, it also tends to worsen the lipid profile, raising LDL cholesterol, VLDL cholesterol, and triglycerides, and to impair blood vessel function through endothelial dysfunction.31PubMed Central. Metabolic Syndrome, Thyroid Dysfunction, and Cardiovascular Risk: The Triptych of Evil This cluster of problems, high blood sugar, abnormal lipids, and vascular damage, is essentially metabolic syndrome, and it is the reason that thyroid dysfunction carries cardiovascular risk that goes beyond just feeling tired or gaining weight. Addressing thyroid function is not a cosmetic concern or a matter of energy levels alone; it is a cardiovascular issue, particularly for people who already have other metabolic risk factors like obesity or a family history of diabetes.