A single person can absolutely experience both hyperthyroidism and hypothyroidism, sometimes swinging between the two over months or years, and in certain rare conditions, even showing features of both at the same time. The thyroid exists on a functional spectrum, and several well-documented mechanisms can push a person from one end to the other. Some of these transitions are a predictable part of a disease’s natural course, while others catch both patients and doctors off guard.
Autoimmune Thyroid Disease Is a Spectrum, Not Two Separate Boxes
The two most common autoimmune thyroid conditions, Hashimoto’s thyroiditis and Graves’ disease, are often taught as opposites: Hashimoto’s causes hypothyroidism, Graves’ causes hyperthyroidism. In reality, they sit on a shared autoimmune spectrum and can overlap or transition into one another.1Journal of the Endocrine Society. SAT-375 From Hypothyroidism to Hyperthyroidism: Interplay Between Hashimoto’s Thyroiditis and Graves’ Disease in a Case of Coexisting Autoimmune Thyroiditis Both involve the immune system targeting the thyroid gland, but which antibodies dominate at any given time determines whether the gland speeds up or slows down.
A classic example is hashitoxicosis, a transient hyperthyroid phase that occurs in some people with Hashimoto’s thyroiditis. As the immune system attacks thyroid tissue, stored thyroid hormones leak into the bloodstream, temporarily producing symptoms of hyperthyroidism like a racing heart, anxiety, and weight loss. This phase is typically self-limiting and resolves within weeks to months without specific treatment, after which the person moves into the expected hypothyroid state as the gland sustains enough damage.2Radiology Case Reports. Thyrotoxicosis due to Hashimoto’s disease triggered by radiofrequency ablation for low-risk papillary thyroid microcarcinoma: A case report – Section: Discussion Someone going through hashitoxicosis might first be told they are hyperthyroid, only to be reclassified as hypothyroid a few months later. Both diagnoses are correct for their moment in time.
Antibody Switching in Graves’ Disease
One of the more dramatic ways a person can oscillate between hyperthyroidism and hypothyroidism involves the antibodies produced in Graves’ disease. The immune system can produce two functionally opposite types of antibodies that target the same receptor on thyroid cells. Stimulating antibodies mimic the signal that tells the thyroid to produce more hormone, causing hyperthyroidism. Blocking antibodies sit on the same receptor but prevent it from receiving any signal at all, leading to hypothyroidism. When the balance between these two antibody types shifts over time, a person can alternate between overactive and underactive thyroid function.3PubMed Central. Alternating hyperthyroidism and hypothyroidism in Graves’ disease
This switching can make clinical management genuinely difficult. One case report described an elderly woman with Graves’ disease who was started on a medication to bring her thyroid levels down. Over the following months, she was admitted to the hospital multiple times with alternating hypothyroidism and hyperthyroidism, accompanied by worsening cardiac complications.4PubMed Central. Graves’ Disease in the Elderly: A Case of Fluctuating Hyperthyroidism and Hypothyroidism Resulting in Challenging Management Each swing required a different treatment approach, and the unpredictability of the antibody shifts meant that doses needed constant adjustment. This pattern is uncommon but well recognized, and it underscores why frequent monitoring of thyroid function tests matters in autoimmune thyroid disease.
Conditions That Naturally Move Through Both Phases
Some thyroid conditions follow a built-in trajectory that passes through both hyperthyroidism and hypothyroidism as part of their normal course. If you only catch the condition at one stage, you might get a diagnosis that seems to contradict what came before or what comes after.
Subacute thyroiditis, often triggered by a viral infection, is the textbook example. It follows a triphasic pattern: a period of thyrotoxicosis as the inflamed gland dumps stored hormones into the blood, followed by a hypothyroid phase as the gland’s reserves are depleted and it has not yet recovered, and then a gradual return to normal function.5Radiology Case Reports. Subacute thyroiditis – Section: Summary A person who sees a doctor during the first phase gets told they are hyperthyroid. If they return a few weeks later feeling sluggish and cold, blood work may now show hypothyroidism. Without context, it looks like two different diagnoses.
Postpartum thyroiditis follows a similar arc. It occurs within the first year after giving birth and typically presents as mild thyrotoxicosis followed by transient hypothyroidism. In one reported case, a woman who had been euthyroid (normal thyroid function) at delivery was found to be thyrotoxic about four and a half months postpartum; her thyroid function normalized on its own within a few weeks without drug therapy.6PubMed. A case of postpartum thyroiditis following SARS-CoV-2 infection In many cases, women pass through the hypothyroid phase after this and then recover fully, though some go on to develop permanent hypothyroidism. The key point is that both the hyperthyroid and hypothyroid phases are part of the same underlying condition, not two different diseases taking turns.
When Treatment for One Causes the Other
Treating hyperthyroidism carries a well-known risk of overshooting into hypothyroidism. Radioactive iodine therapy is one of the most common definitive treatments for an overactive thyroid, and it works by destroying enough thyroid tissue to reduce hormone output. The problem is that the destruction is difficult to calibrate precisely. In one study of fixed-dose radioactive iodine treatment for hyperthyroidism, the most common outcome was hypothyroidism, occurring in the vast majority of treated patients.7Endocrinology and Disorders. Efficacy of a single fixed 131I dose of radioactive iodine for the treatment of hyperthyroidism This is not considered a treatment failure; it is an expected trade-off. Hypothyroidism is easier to manage long-term with daily hormone replacement than hyperthyroidism is to control with ongoing medication. But it does mean that someone treated for hyperthyroidism is likely to spend the rest of their life managing hypothyroidism instead.
Anti-thyroid medications like carbimazole or methimazole can also tip the balance. If the dose is too high or if the underlying disease shifts on its own while the person is medicated, the thyroid can be suppressed into a hypothyroid state. The elderly Graves’ patient described earlier experienced exactly this: carbimazole brought her into hypothyroidism, the drug was stopped and levothyroxine started, and then she swung back again.4PubMed Central. Graves’ Disease in the Elderly: A Case of Fluctuating Hyperthyroidism and Hypothyroidism Resulting in Challenging Management Regular blood work is the only way to catch these swings before they cause serious symptoms.
On the other side of the equation, people treated for hypothyroidism with levothyroxine can end up overtreated into a hyperthyroid state. A large population-based study defined overtreatment as a TSH level suppressed below the normal range, indicating too much thyroid hormone in the system.8PubMed Central. Over- and Undertreatment With Levothyroxine Findings of the Population-Based Rhineland Study Symptoms of overtreatment mirror hyperthyroidism: palpitations, weight loss, irritability, and bone thinning over time. This is one of the most common ways everyday patients experience “both” conditions. They start hypothyroid, go on medication, and then their dose gradually drifts too high.
Drugs and Substances That Can Push the Thyroid Both Ways
Certain medications and chemical exposures can cause thyroid dysfunction in either direction, sometimes sequentially in the same person. Amiodarone, a widely used heart rhythm drug, is perhaps the most notorious. Each standard tablet delivers roughly 500 times the average daily requirement of iodine, and this massive iodine load can affect the thyroid through two separate mechanisms. In people with underlying thyroid nodules or latent Graves’ disease, the iodine overload can fuel excess hormone production. In people with previously normal glands, amiodarone and its breakdown products can directly damage thyroid cells, causing them to release stored hormones and eventually leading to hypothyroidism.9PubMed Central. Biphasic Amiodarone-Induced Thyroid Dysfunction: De Novo Hypothyroidism Followed by Delayed Type 2 Thyrotoxicosis: A Case Report Some patients experience both effects in sequence: hypothyroidism first, then a delayed swing into thyrotoxicosis as the drug’s destructive effects release accumulated hormones.
Even iodine itself, outside of medications, can trigger opposite effects in susceptible individuals. A case report described a teenager with congenital heart disease who developed hypothyroidism after exposure to iodinated contrast dye used in medical imaging, then developed hyperthyroidism after a subsequent exposure to the same type of contrast.10Hindawi / Case Reports in Endocrinology. Hypothyroidism and Hyperthyroidism in an Adolescent With Complex Congenital Heart Disease Exposed to Iodinated Contrast Media: Case Report The thyroid’s response to iodine depends on the state of the gland at the time, the person’s underlying genetics, and how much iodine they were getting before the exposure. The same substance causing opposite effects in the same person at different times illustrates how dynamic thyroid regulation really is.
When Different Tissues Experience Different Things at Once
Most discussions of “having both” assume a person swings between one state and the other over time. But there is a rare genetic condition in which a person can functionally experience symptoms of both hyperthyroidism and hypothyroidism simultaneously. Thyroid hormone resistance syndrome, caused by mutations in a thyroid hormone receptor, results in different tissues responding differently to the same circulating hormone levels. It occurs in roughly one in 40,000 live births.11PubMed Central. Thyroid hormone resistance and its management
In this condition, some tissues carry the mutated receptor and are effectively “deaf” to thyroid hormone, behaving as though they are hypothyroid. Other tissues that rely on the normal receptor respond to the elevated hormone levels by acting as though they are hyperthyroid. The result is a puzzling mix: a person might have a fast heart rate (a hyperthyroid symptom) and delayed growth or cognitive difficulties (hypothyroid symptoms) at the same time.12PubMed Central. Thyroid Hormone Resistance Syndrome: From Molecular Mechanisms to Its Potential Contribution to Hypertension Standard blood tests in these patients often show elevated thyroid hormones with a normal or high TSH, a combination that confuses providers who expect TSH to drop when hormones are high. Treatment is particularly tricky because addressing the hypothyroid symptoms in one set of tissues risks worsening the hyperthyroid symptoms in another.
Structural Overlap Within the Same Gland
The thyroid gland is not necessarily homogeneous. Different parts of the gland can behave differently, and this creates rare scenarios where features of both overactivity and underactivity coexist anatomically. A case was reported of a young woman with Hashimoto’s thyroiditis, which had rendered most of her thyroid underactive, who simultaneously had a single autonomously functioning (toxic) thyroid nodule producing excess hormone.13PubMed Central. Hashimoto’s thyroiditis presenting as a single toxic adenoma (A case report) One part of her gland was being destroyed by autoimmunity while another part was churning out hormones independently. The clinical picture depended on which process was dominant at the time.
An even rarer scenario involves the pituitary gland. The pituitary produces TSH, which tells the thyroid how much hormone to make. Normally, a damaged thyroid produces too little hormone, TSH rises to try to compensate, and the result is straightforward hypothyroidism. But in one reported case, a patient had both a TSH-secreting pituitary tumor and primary thyroid destruction at the same time. The pituitary was overproducing TSH while the thyroid was too damaged to respond adequately, creating lab results that looked contradictory: high TSH alongside low thyroid hormones, but for completely different reasons than simple hypothyroidism.14PubMed Central. Coexistence of TSH-secreting adenoma and primary hypothyroidism: a case report and review of literature Getting the diagnosis right in a case like this matters enormously because the treatment for a pituitary tumor is completely different from the treatment for a failing thyroid gland.
When Test Results Lie
Sometimes a person appears to have an impossible combination of thyroid dysfunction not because their thyroid is actually behaving strangely, but because something is interfering with the lab tests. High-dose biotin, a supplement many people take for hair and nail health, is one well-documented culprit. In one case, a patient taking high-dose biotin for multiple sclerosis produced thyroid lab results that looked exactly like Graves’ disease, complete with suppressed TSH and elevated free thyroid hormones. The patient had no symptoms of hyperthyroidism at all and a completely normal physical exam. When biotin was withheld for a week, the thyroid tests returned to normal.15PubMed Central. Effect of High-dose Biotin on Thyroid Function Tests: Case Report and Literature Review
This is worth knowing because biotin supplements are widely available and popular. The doses that cause lab interference are higher than what most people take for cosmetic purposes, but therapeutic doses used for neurological conditions regularly cross the threshold. If you take biotin and get confusing thyroid results, mentioning the supplement to your doctor can save you from unnecessary treatment or an incorrect diagnosis of thyroid disease you do not actually have.
Thyroid Antibodies That Cross the Placenta
The interplay between hyperthyroidism and hypothyroidism can even begin before birth. A mother with autoimmune thyroid disease produces antibodies that can cross the placenta and affect her baby’s thyroid function. If the mother’s dominant antibodies are the blocking type, the baby can be born with temporary congenital hypothyroidism even though the mother herself may have been managing Graves’ disease. In one reported case, a newborn developed hypothyroidism due to maternal blocking antibodies and required thyroid hormone replacement, but the treatment was successfully discontinued before the baby was seven months old as the maternal antibodies gradually cleared from the infant’s circulation.16PubMed Central. Congenital Hypothyroidism due to a Low Level of Maternal Thyrotropin Receptor-Blocking Antibodies
This creates an unusual situation where the mother and baby can have opposite thyroid states driven by the same underlying autoimmune process. The mother’s stimulating antibodies keep her hyperthyroid, while her blocking antibodies (present in smaller amounts but still capable of crossing the placenta) suppress her baby’s thyroid. Pediatricians who care for newborns of mothers with Graves’ disease screen for this specifically, because the condition is temporary but can cause developmental problems if it goes untreated during those critical early months.
Why Frequency of Testing Matters More Than People Realize
One practical takeaway from all of this is that thyroid disease is not always stable. The common understanding treats a thyroid diagnosis as a fixed label: you are hypothyroid, you take your pill, and that is the end of the story. But the mechanisms described throughout this article show that thyroid function can shift for many reasons, including changes in autoimmune activity, medication doses that drift out of range, new drug interactions, and even physiological events like pregnancy. The population-based study on levothyroxine treatment found measurable rates of both over- and undertreatment, meaning people on thyroid medication were walking around with levels outside the target range without necessarily knowing it.8PubMed Central. Over- and Undertreatment With Levothyroxine Findings of the Population-Based Rhineland Study
If you have a thyroid condition and your symptoms change, it is worth considering that your thyroid status may have shifted rather than assuming the symptoms are unrelated. Fatigue, mood changes, weight fluctuations, and heart rate changes can all reflect a thyroid that has drifted from one state toward the other. A simple blood draw is usually enough to check, and catching a shift early is far easier to manage than dealing with a severe swing after the fact.