Thyroid Disorders: Hypothyroidism and Hyperthyroidism Symptoms, Causes and Treatment
- Dr. Kimryn Rathmell
- Aug 4
- 8 min read
The thyroid gland is small — a butterfly-shaped structure sitting at the front of your neck, weighing less than an ounce — but its influence extends to nearly every cell and organ in the body. Thyroid hormones regulate metabolism, heart rate, body temperature, mood, bone density, fertility, and the function of the brain, gut, and muscles. When thyroid hormone production goes wrong in either direction, the consequences are wide-ranging and frequently misattributed to other conditions.
According to the NIDDK, approximately 4.6% of Americans aged 12 and older have hypothyroidism — an underactive thyroid producing too little hormone. An additional 1.2% have hyperthyroidism — an overactive thyroid producing too much. Combined, thyroid disorders affect tens of millions of Americans, with women affected at approximately 5–8 times the rate of men. Yet thyroid disorders are frequently missed: most cases of hypothyroidism are mild with few obvious symptoms, and the fatigue, weight gain, and depression of hypothyroidism are easily — and often mistakenly — attributed to stress, ageing, or mental health conditions.
This complete guide covers both major thyroid disorder types, their causes, full symptom profiles, how they are diagnosed, and all treatment options.
The NIDDK provides comprehensive patient information on hypothyroidism at: https://www.niddk.nih.gov/health-information/endocrine-diseases/hypothyroidism
How the Thyroid Works — The HPT Axis
Understanding thyroid disorders requires understanding the hormonal control system:
The hypothalamic-pituitary-thyroid (HPT) axis is a feedback loop:
1. The hypothalamus releases TRH (thyrotropin-releasing hormone)
2. TRH signals the pituitary gland to release TSH (thyroid-stimulating hormone)
3. TSH signals the thyroid gland to produce T4 (thyroxine) and T3 (triiodothyronine)
4. Rising T4 and T3 levels feed back to suppress TRH and TSH — the classic negative feedback loop
T4 is the main thyroid hormone produced by the gland but is relatively inactive. It is converted peripherally — primarily in the liver, kidneys, and muscles — to T3, the biologically active form that enters cells and drives metabolic activity. Understanding this conversion step is important: some patients feel better on combination T4/T3 therapy than T4 alone, because their conversion is suboptimal.
TSH is the single most important diagnostic test: a high TSH means the pituitary is working harder to stimulate a sluggish thyroid (hypothyroidism); a low TSH means the pituitary is suppressing in response to excess thyroid hormone (hyperthyroidism).
Hypothyroidism — Underactive Thyroid
Hypothyroidism occurs when the thyroid gland produces insufficient T4 and T3. Every metabolic process in the body slows down as a result.
Causes of hypothyroidism:
Hashimoto's thyroiditis (most common cause in developed countries):
An autoimmune condition in which the immune system produces antibodies (anti-TPO and anti-thyroglobulin antibodies) that attack and progressively destroy thyroid tissue. Hashimoto's may cause fluctuating symptoms before settling into permanent hypothyroidism. It runs strongly in families and is far more common in women. Many patients have Hashimoto's antibodies for years before TSH becomes abnormal.
Other causes:
Thyroid surgery — partial or total thyroidectomy for thyroid cancer, nodules, or hyperthyroidism
Radioactive iodine treatment — used to treat hyperthyroidism; frequently results in permanent hypothyroidism
Medications — lithium, amiodarone, interferon-alpha, and immune checkpoint inhibitors (cancer immunotherapy) all commonly cause hypothyroidism
Iodine deficiency — the leading cause globally in developing regions; rare in the USA due to iodised salt
Congenital hypothyroidism — present at birth; screened in newborns in all US states
Pituitary or hypothalamic disorders — secondary hypothyroidism (low TSH + low T4)
Symptoms of hypothyroidism:
Symptoms develop gradually and are often attributed to other causes:
Fatigue and sluggishness — the most common symptom; often profound
Weight gain — despite no change in diet; from reduced metabolic rate
Cold intolerance — feeling cold when others are comfortable
Constipation
Dry skin and brittle nails
Hair loss and hair thinning — diffuse; may involve outer third of eyebrows
Slowed heart rate (bradycardia)
Depression — hypothyroidism is a frequently missed medical cause of depression
Cognitive slowing — poor memory, difficulty concentrating ("brain fog")
Muscle aches, weakness, and cramps
Heavy or irregular menstrual periods
Elevated cholesterol — TSH raises LDL; hypothyroidism is a reversible cause of high cholesterol
Peripheral oedema — particularly around the eyes and hands
In severe untreated hypothyroidism: myxoedema coma — a rare, life-threatening emergency
Diagnosis:
TSH elevated (above 4.0 mIU/L) — primary hypothyroidism
Free T4 below normal — confirms clinically significant hypothyroidism
Anti-TPO antibodies — confirms Hashimoto's thyroiditis as the cause
Subclinical hypothyroidism: TSH mildly elevated (4–10 mIU/L) with normal Free T4 and few or no symptoms — management is individualised
Treatment of hypothyroidism:
Levothyroxine (synthetic T4) is the standard treatment — replacing the missing thyroid hormone directly. It is one of the most commonly prescribed medicines in the USA. Key points:
Take on an empty stomach 30–60 minutes before food and other medications (many supplements and medicines reduce absorption)
Calcium, iron, and certain antacids must be separated by at least 4 hours
TSH is monitored 6–8 weeks after starting or changing dose, then annually once stable
Some patients do not feel fully well on T4 alone — combination T4/T3 (liothyronine) therapy is an option discussed with a specialist
Treatment is lifelong in the vast majority of cases
Subclinical hypothyroidism: treatment is recommended when TSH is above 10, when pregnant, or when symptoms are significant
Hyperthyroidism — Overactive Thyroid
Hyperthyroidism occurs when the thyroid produces excess T4 and T3, accelerating metabolic processes throughout the body.
Causes of hyperthyroidism:
Graves' disease (most common cause — approximately 70–80% of hyperthyroidism):
An autoimmune condition in which TSH-receptor antibodies (TRAb) stimulate the thyroid to produce excess hormone continuously — bypassing the normal feedback mechanism. Graves' disease also frequently causes Graves' ophthalmopathy (eye disease) — proptosis (bulging eyes), eye irritation, and in severe cases, vision problems. It is the only cause of hyperthyroidism associated with eye disease. Affects approximately 1% of the population; far more common in women; strong genetic component.
Other causes:
Toxic multinodular goitre — multiple autonomous thyroid nodules producing excess hormone independently; more common in older adults and in iodine-deficient regions
Toxic adenoma — a single autonomous nodule overproducing hormone
Thyroiditis — inflammation releasing stored hormone (postpartum thyroiditis, subacute thyroiditis, Hashitoxicosis); typically transient and self-limiting
Excess iodine intake — including contrast dye from medical imaging (Jod-Basedow phenomenon)
Excess levothyroxine dosing — iatrogenic (medicine-induced) hyperthyroidism
Symptoms of hyperthyroidism:
Symptoms reflect an accelerated metabolic state:
Heat intolerance and excessive sweating
Weight loss despite increased appetite
Rapid or irregular heartbeat (palpitations) — including atrial fibrillation, which is common in older hyperthyroid patients
Anxiety, irritability, and nervousness
Fine tremor of the hands
Fatigue and muscle weakness — particularly proximal muscles (difficulty climbing stairs)
Frequent bowel movements or diarrhoea
Insomnia and sleep disturbance
Menstrual irregularity or light periods
Reduced fertility
Hair thinning
Warm, moist skin
Graves' ophthalmopathy — in Graves' disease specifically: eye bulging, dryness, irritation, double vision
Thyroid storm — rare but life-threatening emergency of extreme hyperthyroidism (fever, extreme tachycardia, confusion)
Diagnosis:
TSH suppressed (below 0.4 mIU/L) — primary hyperthyroidism
Free T4 and Free T3 elevated — confirms excess hormone
TSH-receptor antibodies (TRAb) — positive in Graves' disease
Thyroid uptake and scan — distinguishes Graves' (diffuse uptake) from nodular disease (focal uptake) and thyroiditis (near-zero uptake)
Full guidance on thyroid diagnostic tests is available from the NIDDK at: https://www.niddk.nih.gov/health-information/diagnostic-tests/thyroid
Treatment of hyperthyroidism:
Antithyroid medicines (thionamides):
Methimazole (Tapazole) — first-line in the USA; blocks thyroid hormone synthesis; used for 12–18 months; remission achieved in approximately 40–50% of Graves' disease patients
Propylthiouracil (PTU) — preferred in first trimester of pregnancy; blocks T4-to-T3 conversion in addition to synthesis
Beta-blockers (propranolol, atenolol) — used alongside antithyroid medicines to rapidly control symptoms (palpitations, tremor, anxiety) while hormone levels normalise
Radioactive iodine (RAI):
Radioactive iodine-131 is absorbed by thyroid cells and destroys them — permanently reducing thyroid activity. The most commonly used definitive treatment in the USA for Graves' disease. Results in permanent hypothyroidism in the majority of patients — requiring lifelong levothyroxine. Not used in pregnancy.
Thyroid surgery (thyroidectomy):
Total or near-total thyroidectomy — chosen when goitre is large, when cancer is suspected, in pregnancy when medicines fail, or by patient preference. Provides the most immediate and definitive resolution. Results in permanent hypothyroidism requiring lifelong levothyroxine.
For our complete guide to depression — hypothyroidism is one of the most important reversible medical causes: [Depression: Symptoms, Causes and Treatment]
For our guide to hair loss — thyroid disorders are a leading reversible cause of diffuse hair shedding: [What Causes Hair Loss? Types, Causes and Treatments]
For our guide to high cholesterol — hypothyroidism is a reversible cause of elevated LDL: [High Cholesterol: Symptoms, Causes and Treatment]
NIDDK comprehensive patient information on hyperthyroidism is available at: https://www.niddk.nih.gov/health-information/endocrine-diseases/hyperthyroidism
Hypothyroidism vs Hyperthyroidism — Side-by-Side Comparison
Feature | Hypothyroidism (Underactive) | Hyperthyroidism (Overactive) |
Metabolism | Slowed | Accelerated |
Weight | Gain | Loss despite appetite |
Heart rate | Slow (bradycardia) | Fast (tachycardia, palpitations) |
Temperature tolerance | Cold intolerance | Heat intolerance |
Bowel habits | Constipation | Frequent/loose stools |
Energy | Fatigue, sluggishness | Fatigue + anxiety/restlessness |
Mood | Depression, brain fog | Anxiety, irritability, nervousness |
Skin and hair | Dry, puffy, hair loss | Warm, moist, hair thinning |
Menstrual periods | Heavy and irregular | Light or absent |
Cholesterol | Elevated LDL | Low cholesterol |
TSH | High | Low (suppressed) |
Most common cause | Hashimoto's thyroiditis | Graves' disease |
Thyroid Medicines Comparison
Medicine | Used For | Mechanism | Key Notes |
Levothyroxine (T4) | Hypothyroidism | Replaces missing T4; converted to T3 peripherally | Take fasting; most common thyroid Rx in USA |
Liothyronine (T3) | Add-on for T4 non-responders | Active T3 directly; shorter half-life | Specialist use; not first-line alone |
Methimazole | Hyperthyroidism (Graves') | Blocks thyroid hormone synthesis | First-line antithyroid medicine in USA |
Propylthiouracil (PTU) | Hyperthyroidism in pregnancy | Blocks synthesis + T4→T3 conversion | Preferred in first trimester only |
Propranolol (beta-blocker) | Hyperthyroidism symptom control | Reduces heart rate, tremor, anxiety | Adjunct — not definitive treatment |
Radioactive iodine (RAI) | Definitive Graves'/toxic nodule Rx | Destroys thyroid cells permanently | Results in hypothyroidism; not in pregnancy |
Frequently Asked Questions
Can thyroid disorders cause depression and anxiety?
Yes — and this is one of the most clinically important links in medicine. Hypothyroidism is a well-established reversible medical cause of depression: slowed metabolism affects brain serotonin and noradrenaline function, producing symptoms identical to major depression. Hyperthyroidism commonly causes anxiety, nervousness, and in severe cases panic attacks. Any patient presenting with new depression or anxiety should have thyroid function tested — a simple TSH blood test — before concluding the cause is purely psychological.
Can thyroid disorders cause weight problems?
Yes in both directions. Hypothyroidism slows metabolic rate, causing weight gain that does not respond to diet and exercise until thyroid hormone is replaced. Hyperthyroidism accelerates metabolism, causing weight loss despite increased appetite. After treating hyperthyroidism — particularly with radioactive iodine — patients often gain weight as metabolism normalises and hypothyroidism sets in. Weight changes with thyroid disorders are driven by physiology, not willpower.
How often should I have my thyroid checked?
Adults with no risk factors do not require routine thyroid screening, though the American Thyroid Association recommends testing from age 35 every 5 years. People with higher risk — women over 60, family history of thyroid disease, personal history of autoimmune conditions, previous thyroid disease or treatment, pregnancy — should be tested more regularly. Anyone with symptoms suggestive of thyroid dysfunction should be tested regardless of age.
Is hypothyroidism permanent?
Most cases of hypothyroidism — particularly Hashimoto's thyroiditis — are permanent and require lifelong levothyroxine. However, some causes are reversible: hypothyroidism from medication (lithium, amiodarone) may improve if the medicine is stopped; postpartum thyroiditis often resolves within 12 months; subclinical hypothyroidism does not always progress to overt disease. A trial of dose reduction or stopping treatment is appropriate in selected cases under close medical monitoring.
Can I get pregnant with a thyroid disorder?
Yes — but thyroid disorders require careful management during pregnancy. Both untreated hypothyroidism and hyperthyroidism increase risks of miscarriage, preterm birth, and complications. Levothyroxine requirements typically increase by 25–30% during pregnancy — women on levothyroxine should have TSH checked as soon as pregnancy is confirmed and regularly throughout. Methimazole is generally avoided in the first trimester (PTU preferred). Radioactive iodine is absolutely contraindicated in pregnancy.
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