Your thyroid gland weighs about 20 grams — roughly the same as four nickels. It sits at the base of your neck, shaped like a butterfly, and most people never think about it. But this small gland controls metabolic rate in virtually every cell in your body. Heart rate, body temperature, energy production, cholesterol metabolism, brain function, bone turnover, menstrual regularity, gut motility, mood — all regulated, in part, by two hormones this gland produces.
About 20 million Americans have some form of thyroid disease. Up to 60% of them are unaware of it. Women are 5 to 8 times more likely to develop thyroid problems than men, and the risk increases with age. The symptoms of thyroid dysfunction are maddeningly nonspecific — fatigue, weight changes, mood disturbances — which means they’re frequently attributed to aging, stress, depression, or “just being busy.” Thyroid disease is one of the most commonly missed diagnoses in primary care.
Key Takeaways
- Hypothyroidism (underactive thyroid) affects roughly 5% of the U.S. population; symptoms include fatigue, weight gain, cold intolerance, constipation, dry skin, and depression
- Hyperthyroidism (overactive thyroid) is less common but more acutely dangerous; symptoms include weight loss, rapid heartbeat, anxiety, tremor, heat intolerance, and diarrhea
- TSH (thyroid-stimulating hormone) is the primary screening test — elevated TSH suggests hypothyroidism, suppressed TSH suggests hyperthyroidism
- Hashimoto’s thyroiditis is the most common cause of hypothyroidism in developed countries; Graves’ disease is the most common cause of hyperthyroidism
- Subclinical thyroid disease (abnormal TSH with normal thyroid hormone levels) is common and controversial — not everyone with abnormal numbers needs treatment
How the Thyroid Works
The thyroid gland produces two main hormones: T4 (thyroxine) and T3 (triiodothyronine). T4 is produced in much larger quantities but is relatively inactive. It’s a prohormone — a precursor. Most T4 is converted to T3 in peripheral tissues (liver, kidneys, muscles) by deiodinase enzymes. T3 is the metabolically active form, roughly 3-5 times more potent than T4.
Thyroid hormone production is controlled by a feedback loop. The hypothalamus releases TRH (thyrotropin-releasing hormone), which tells the pituitary gland to release TSH (thyroid-stimulating hormone). TSH stimulates the thyroid to produce T4 and T3. When thyroid hormone levels are adequate, the pituitary reduces TSH. When they’re low, TSH rises — like a thermostat turning up the heat.
This is why TSH is the most useful screening test. It’s the body’s own assessment of whether thyroid hormone levels are adequate. A high TSH means the pituitary is shouting at the thyroid to work harder — the thyroid isn’t producing enough. A low TSH means the pituitary is backing off — there’s too much thyroid hormone in circulation.
Iodine is essential for thyroid hormone synthesis — it’s literally incorporated into the molecular structure of T3 and T4 (the numbers refer to iodine atoms). In the United States and most developed countries, iodine deficiency is rare due to iodized salt. Globally, however, iodine deficiency remains the most common cause of preventable hypothyroidism, affecting roughly 2 billion people.
Hypothyroidism: The Underactive Thyroid
Hypothyroidism means your thyroid isn’t producing enough hormone to meet your body’s needs. Everything slows down.
Symptoms
The symptom profile of hypothyroidism reads like a list of complaints that could mean almost anything, which is exactly the problem.
Fatigue is the most common symptom — reported by up to 90% of hypothyroid patients. Not ordinary tiredness, but a leaden, pervasive exhaustion that doesn’t resolve with rest. It’s similar to the fatigue of iron deficiency anemia, and the two conditions frequently coexist.
Weight gain — typically 5-15 pounds, mostly fluid retention rather than fat accumulation. Hypothyroidism reduces metabolic rate by roughly 10-15%, which translates to fewer calories burned per day. It’s enough to cause gradual weight gain but not enough to explain 50-pound gains — patients sometimes attribute massive weight gain to their thyroid when the thyroid contribution is actually modest.
Cold intolerance. Thyroid hormones drive thermogenesis. When they’re low, your body generates less heat. You’re the person who’s always cold in the office, always adding layers, always fighting over the thermostat.
Constipation. Gut motility slows. Transit time increases. Constipation is one of the earlier symptoms and sometimes the first one noticed.
Dry skin and hair. The skin becomes rough, pale, and sometimes yellowish (from impaired conversion of beta-carotene to vitamin A). Hair becomes coarse, dry, and brittle. Hair loss — particularly thinning of the outer third of the eyebrow — is a classic sign, though not everyone experiences it.
Depression and cognitive slowing. Hypothyroidism can produce a depressive syndrome that’s nearly indistinguishable from major depression: low mood, lack of motivation, difficulty concentrating, memory problems. Many patients are prescribed antidepressants before anyone checks a TSH. This is one of the strongest arguments for including thyroid function tests in any evaluation for new-onset depression.
Menstrual irregularities. Heavy or prolonged periods (menorrhagia) are common in hypothyroid women. Thyroid hormones influence sex hormone-binding globulin and estrogen metabolism. Hypothyroidism can also impair fertility and increase miscarriage risk.
Elevated cholesterol. Hypothyroidism slows LDL receptor function, causing LDL cholesterol to accumulate. If your cholesterol worsened without an obvious lifestyle change, hypothyroidism should be on the list of possible causes.
Muscle aches and joint pain. Diffuse myalgias that can mimic fibromyalgia.
Bradycardia. Heart rate slows, sometimes below 60 bpm.
Causes
Hashimoto’s thyroiditis accounts for roughly 90% of hypothyroidism in the U.S. It’s an autoimmune disease where the immune system attacks thyroid tissue, progressively destroying it over months to years. Antibodies against thyroid peroxidase (anti-TPO) and thyroglobulin (anti-TG) are present in 95% of cases and are detectable in blood tests. Hashimoto’s runs strongly in families and is associated with other autoimmune conditions (type 1 diabetes, celiac disease, vitiligo, rheumatoid arthritis).
Post-treatment hypothyroidism follows radioactive iodine therapy or surgical removal of the thyroid for hyperthyroidism, thyroid cancer, or large goiters. This is expected and permanent.
Medications. Lithium, amiodarone, interferon-alpha, and certain cancer immunotherapy drugs can all cause hypothyroidism.
Subacute thyroiditis. A viral-triggered inflammation of the thyroid that causes a transient hyperthyroid phase (stored hormone leaking out) followed by hypothyroidism. It usually resolves on its own over several months.
Treatment
Levothyroxine (Synthroid, Levoxyl, generic) is the standard treatment. It’s synthetic T4, chemically identical to what your thyroid produces. The dose is titrated based on TSH — typically started at 1.6 mcg per kilogram of body weight and adjusted every 6-8 weeks until TSH normalizes.
Levothyroxine should be taken on an empty stomach, 30-60 minutes before food. Coffee, calcium supplements, iron supplements, and antacids all interfere with absorption and should be separated by at least 4 hours. Consistency in timing and brand matters — switching between generic manufacturers can sometimes cause TSH fluctuations due to minor differences in bioavailability.
The T3 controversy. Some patients on levothyroxine report persistent symptoms — fatigue, brain fog, mood issues — despite normal TSH levels. Some practitioners add liothyronine (T3) or prescribe desiccated thyroid (Armour Thyroid, NP Thyroid), which contains both T4 and T3 from porcine thyroid glands. The American Thyroid Association’s guidelines recommend against routine combination therapy, citing inconsistent evidence of benefit and potential risks of T3 excess. However, some endocrinologists offer a trial of combination therapy for patients who remain symptomatic. This remains one of the more contentious areas in thyroid medicine, with strong opinions on both sides.
What’s less contentious: if you’re on levothyroxine, your TSH is normal, and you still feel terrible, consider other causes. Vitamin D deficiency, iron deficiency, sleep disorders, and depression can all coexist with treated hypothyroidism and produce identical symptoms.
Hyperthyroidism: The Overactive Thyroid
Hyperthyroidism means your thyroid is producing too much hormone. Everything speeds up.
Symptoms
Unintentional weight loss despite normal or increased appetite. Metabolic rate ramps up, sometimes by 60-100% in severe cases. Patients may lose 10-20 pounds over weeks.
Rapid or irregular heartbeat. Tachycardia (resting heart rate above 100), palpitations, and atrial fibrillation are common and can be the presenting symptom. Thyroid storm — a rare, life-threatening exacerbation — can cause heart failure.
Anxiety, irritability, and insomnia. The nervous system is overstimulated. Patients often describe feeling “wired,” unable to relax, with a sense of internal tremor. This can be misdiagnosed as an anxiety disorder, particularly in younger women.
Tremor. A fine tremor in the hands, noticeable when holding a cup or extending the fingers.
Heat intolerance and excessive sweating. The opposite of hypothyroid cold intolerance. You’re the person throwing windows open in winter.
Frequent bowel movements or diarrhea. Gut motility increases. Some patients develop malabsorption from too-rapid transit.
Menstrual changes. Lighter, less frequent periods or amenorrhea.
Eye changes (specific to Graves’ disease). Graves’ ophthalmopathy causes eye bulging (proptosis), dry irritated eyes, double vision, and in severe cases, vision loss. This is caused by autoimmune inflammation of the muscles and fat behind the eyes and can occur independently of thyroid hormone levels — sometimes worsening even after thyroid function is treated.
Causes
Graves’ disease accounts for 60-80% of hyperthyroidism. It’s autoimmune — TSH receptor antibodies stimulate the thyroid to overproduce hormones continuously, bypassing the normal feedback loop. Like Hashimoto’s, it’s more common in women and runs in families. Some patients progress from Graves’ to eventual burnout hypothyroidism, while others cycle between the two.
Toxic multinodular goiter. One or more thyroid nodules autonomously produce excess hormone, independent of TSH regulation. More common in older adults and in regions with historical iodine deficiency.
Subacute thyroiditis. The hyperthyroid phase of thyroid inflammation — stored hormone dumps into the bloodstream. Self-limited but can be quite symptomatic.
Excessive iodine or thyroid hormone intake. Supplements containing high-dose iodine (particularly kelp supplements), the heart medication amiodarone, and intentional or accidental excess levothyroxine can all cause hyperthyroidism.
Treatment
Antithyroid drugs (methimazole, propylthiouracil/PTU) block thyroid hormone synthesis. Methimazole is preferred for most patients due to fewer side effects. Treatment typically lasts 12-18 months, after which about 30-50% of Graves’ disease patients achieve remission. The rest relapse and need definitive treatment.
Radioactive iodine (RAI) ablation. A single oral dose of radioactive I-131 destroys thyroid tissue over several weeks. Effective and widely used, but results in permanent hypothyroidism requiring lifelong levothyroxine. Contraindicated in pregnancy and not ideal for patients with significant Graves’ eye disease (can transiently worsen it).
Surgery (thyroidectomy). Removes part or all of the thyroid. Preferred in cases of large goiters, suspicious nodules, severe eye disease, or patient preference. Also results in permanent hypothyroidism if total thyroidectomy is performed.
Beta-blockers (propranolol, atenolol) are used for symptom control while waiting for definitive treatment. They don’t affect thyroid hormone levels but reduce heart rate, tremor, and anxiety.
Subclinical Thyroid Disease
This is where things get complicated. Subclinical hypothyroidism means TSH is elevated (usually 4.5-10 mIU/L) but free T4 is normal. Subclinical hyperthyroidism means TSH is suppressed but free T4 and T3 are normal.
Subclinical hypothyroidism is common — affecting roughly 5-10% of the population, higher in older adults. Whether to treat it is debated. The argument for treatment: elevated TSH may progress to overt hypothyroidism (about 2-5% per year, higher if anti-TPO antibodies are present), and some patients feel better on levothyroxine. The argument against: many patients have no symptoms, treatment commits them to lifelong medication, and overtreatment (suppressing TSH too low) carries risks, particularly atrial fibrillation and osteoporosis in older adults.
Most guidelines suggest treating when TSH is above 10 mIU/L. Between 4.5 and 10, the decision is individualized — considering symptoms, antibody status, age, and cardiovascular risk factors. In patients over 70-80, mild TSH elevation may be protective and should not be reflexively treated.
Thyroid Nodules: When to Worry
Thyroid nodules are extremely common — found in up to 65% of adults on ultrasound. The vast majority are benign. About 5-10% are malignant.
Features that increase concern: nodule larger than 1 cm, irregular margins, microcalcifications, increased blood flow within the nodule, and rapid growth. Nodules with these features on ultrasound typically undergo fine-needle aspiration biopsy.
Thyroid cancer, when it occurs, is usually highly treatable. Papillary thyroid cancer — the most common type, representing 80% of cases — has a 5-year survival rate above 98%. This doesn’t mean thyroid cancer is trivial, but it does mean that finding a thyroid nodule should not cause panic.
When to See a Doctor
Get your thyroid checked (TSH test) if you experience:
- New-onset fatigue that’s persistent and unexplained
- Unexplained weight changes (gain or loss) despite stable diet and activity
- Cold or heat intolerance that’s new or worsening
- Changes in menstrual pattern
- New depression, anxiety, or cognitive changes
- Family history of thyroid disease or other autoimmune conditions
- A visible or palpable lump in your neck
- Previous radiation exposure to the head or neck
For women, thyroid testing should be part of routine evaluation during pregnancy planning and early pregnancy. Uncontrolled hypothyroidism during pregnancy increases risk of miscarriage, preeclampsia, and impaired fetal neurodevelopment.
If you’re already on thyroid medication, get your TSH checked at least annually, and 6-8 weeks after any dose change. Symptoms of over-replacement (suppressed TSH) — palpitations, anxiety, insomnia, weight loss — warrant dose reduction.
Frequently Asked Questions
Can thyroid problems cause anxiety and depression?
Yes, and this is one of the most important clinical overlaps in medicine. Hypothyroidism can produce a syndrome essentially identical to major depression. Hyperthyroidism can mimic generalized anxiety disorder or panic disorder. Multiple studies have shown that thyroid function testing should be standard in the evaluation of new mental health symptoms. Treatment of the thyroid condition resolves the psychiatric symptoms in a significant proportion of cases.
Should I avoid gluten if I have Hashimoto’s?
The relationship between Hashimoto’s and celiac disease is real — they coexist at a higher rate than chance would predict, and screening for celiac disease is recommended in Hashimoto’s patients with GI symptoms. Whether a gluten-free diet benefits Hashimoto’s patients without celiac disease is less clear. Some small studies suggest modest reductions in thyroid antibodies, but the evidence isn’t strong enough for a blanket recommendation. If you have Hashimoto’s and GI symptoms, get tested for celiac disease. If celiac is negative, a trial gluten-free diet is low risk but shouldn’t replace standard treatment.
Are thyroid supplements safe?
Supplements marketed for “thyroid support” typically contain iodine, selenium, zinc, and sometimes desiccated thyroid tissue. The iodine content can be wildly variable — a 2013 study in Thyroid found that 9 of 10 commercially available thyroid supplements contained measurable thyroid hormone, despite not listing it as an ingredient. Excess iodine can actually worsen both hypothyroidism (in Hashimoto’s) and hyperthyroidism. Selenium supplementation has modest evidence for reducing thyroid antibodies, but the optimal dose and long-term safety aren’t established. Approach thyroid supplements with real caution, and never substitute them for prescribed medication.
How does thyroid disease affect fertility?
Both hypothyroidism and hyperthyroidism can impair fertility. Hypothyroidism disrupts ovulation through effects on sex hormone-binding globulin, estrogen metabolism, and prolactin levels. Hyperthyroidism can cause anovulatory cycles. The good news: once thyroid function is normalized with treatment, fertility typically improves. The American Thyroid Association recommends maintaining TSH between 0.5-2.5 mIU/L (lower end of normal) for women trying to conceive.
Can stress cause thyroid problems?
Stress alone doesn’t cause thyroid disease, but there’s evidence it can trigger autoimmune thyroid conditions in genetically susceptible individuals. Graves’ disease, in particular, has been associated with preceding major life stressors in retrospective studies. The mechanism likely involves stress-induced immune dysregulation. Once autoimmune thyroid disease is established, stress may exacerbate it. Managing stress won’t cure thyroid disease, but it may reduce flare severity.
