Hypothyroidism means the thyroid makes too little hormone and hyperthyroidism means it makes too much, and on a blood test the headline number moves backwards: TSH, the signal sent to the thyroid by the pituitary (a pea-sized gland under the brain), climbs when thyroid hormone is short and drops when it is in excess. An underactive thyroid is also the more common of the two; in a large US survey it turned up about three and a half times as often (4.6% of people versus 1.3%) 2.
Underactive or overactive: the two side by side
Why TSH runs backwards
The thyroid works like a heating system with a thermostat in the brain. TSH (thyroid-stimulating hormone) is the thermostat's call for heat. When the thyroid falls short, the pituitary shouts louder and TSH rises; when the thyroid overproduces, the pituitary goes quiet and TSH falls. That is why TSH is the most sensitive single screening test for a primary thyroid problem 1.
How the thyroid feedback loop sets TSH
Symptoms: slowed down vs sped up
Many of these thyroid symptoms are non-specific: fatigue, hair loss, mood swings, always feeling cold and a missed period have dozens of causes, including anaemia, anxiety and perimenopause. A few thyroid symptoms are more telling: heat intolerance with palpitations and a fine tremor, Graves' eye changes, or a visibly swollen neck. Weight change sits in the middle; most people with hypothyroidism gain only a few kilograms, much of it fluid, while unexplained weight loss with a racing heart is a stronger hint of an overactive thyroid.
Age blurs the picture. Older adults with hyperthyroidism often lack the jittery symptoms and present with weight loss, tiredness or atrial fibrillation (an irregular, often fast heart rhythm that raises stroke risk) instead, sometimes called apathetic hyperthyroidism 4. Because the symptom lists overlap with so much else, a blood test decides which thyroid problem, if any, is present.
What causes each one
In countries with enough iodine in the diet, including the US, the most common cause of hypothyroidism is Hashimoto's thyroiditis, an autoimmune attack that slowly wears the gland down and usually shows up as TPO (thyroid peroxidase) antibodies; worldwide, iodine deficiency still leads 3. Other causes include radioactive iodine or surgery for an earlier overactive thyroid, and medicines such as amiodarone and lithium. Hashimoto's disease is the autoimmune cause, not a separate thyroid state: it is one route into hypothyroidism.
Graves' disease causes roughly 70 to 80% of hyperthyroidism in iodine-sufficient regions: antibodies called TRAb lock onto the TSH receptor and drive the thyroid flat out, and can also cause eye changes and, rarely, thickened skin on the shins 4. Toxic multinodular goitre and single toxic nodules come next, mostly in older adults. Thyroiditis (painless, postpartum or the painful subacute kind) leaks stored hormone without the gland overproducing, which is why doctors use the wider word thyrotoxicosis, meaning too much thyroid hormone in the blood from any source, and reserve hyperthyroidism for a gland that is making too much 1. Taking more levothyroxine than the body needs causes thyrotoxicosis too.
Excess iodine, including from kelp and iodine supplements, can tip a vulnerable thyroid either way, into an overactive or an underactive state 13. Both autoimmune causes affect women several times more often than men 34.
Reading the blood test: the patterns that tell them apart
| TSH | Free T4 | Free T3 | What the pattern usually means |
|---|---|---|---|
| High | Low | Low or normal | Overt primary hypothyroidism |
| High | Normal | Normal | Subclinical hypothyroidism |
| Low (often below 0.1) | High | High | Overt hyperthyroidism |
| Low | Normal | High | T3 toxicosis, an early or mild overactive thyroid |
| Low | Normal | Normal | Subclinical hyperthyroidism |
| Low or normal | Low | Low | Central (pituitary) hypothyroidism, rare: needs a doctor's review |
In mild hyperthyroidism free T3 can rise before free T4 does, so the ATA advises measuring free T3 as well as free T4 when TSH is low 1. Antibodies then name the cause: TPO antibodies, found in about 11% of the US population in NHANES III and strongly linked to hypothyroidism 2, point to Hashimoto's, while TRAb (or the closely related TSI) points to Graves' 1.
Where an example TSH of 5 sitsmIU/L
Markers to read together
Subclinical: TSH is off but free T4 is normal
Subclinical means TSH sits outside the range while free T4 and free T3 stay normal. Subclinical hypothyroidism made up most of the 4.6% hypothyroid share in NHANES III 2. TSH drifts with illness, season and lab, so a single odd TSH is usually rechecked in 6 to 12 weeks before anyone acts. In pregnancy, or when planning one, an abnormal TSH is reviewed promptly rather than waiting 3. For subclinical hypothyroidism, treatment is generally considered once TSH reaches 10 mIU/L or above; below that, age, symptoms, antibodies and pregnancy plans guide the decision 3.
Levothyroxine provided no apparent symptomatic benefits in older persons with subclinical hypothyroidism.Stott DJ et al., TRUST trial, N Engl J Med 2017 5
The TRUST trial randomised adults aged 65 and over with a mean TSH of about 6.4 to levothyroxine or placebo, and after about a year tiredness and hypothyroid symptoms were no better on the hormone 5. Subclinical hyperthyroidism runs the other way on risk: a TSH below 0.1 mIU/L is linked to more atrial fibrillation and fractures, and European Thyroid Association guidance favours treatment, particularly in people 65 and older, a decision made with a thyroid specialist 6.
Treatment: replace vs slow down
Hypothyroidism is treated by replacing the missing hormone with daily levothyroxine, a synthetic T4. The dose is adjusted by TSH, rechecked 6 to 8 weeks after any change and then usually once a year when stable, and most people stay on it for life 3.
Replacing thyroid hormone
Hyperthyroidism is treated by slowing hormone output. For Graves' disease the options are an antithyroid drug (methimazole, or propylthiouracil in the first trimester of pregnancy), radioactive iodine, or surgery, with a beta blocker often used meanwhile to calm the heart rate and tremor 1. Radioactive iodine and surgery usually leave the person permanently hypothyroid, so many people treated for an overactive thyroid end up on levothyroxine 1. Thyroiditis often needs only symptom control while it settles. For mild Graves' eye disease, a 2011 randomised trial found that six months of selenium improved quality of life and slowed eye changes compared with placebo 7. The trial ran in Europe, where selenium intake is lower than in the US, so the benefit may not carry over; eye and thyroid specialists use it for that specific problem under their care, too much selenium is harmful, and it does not treat an over- or underactive thyroid itself.
Can one turn into the other?
Yes, over time, though never both at once. Painless, postpartum and subacute thyroiditis typically run an overactive phase as stored hormone leaks out, then an underactive phase, before most people recover, so the same person can test overactive and then underactive within a few months 1. Early Hashimoto's can briefly leak hormone in the same way. Too much levothyroxine pushes labs into the overactive range, and Graves' treatment with radioactive iodine or surgery commonly ends in lifelong hypothyroidism 1.
Which is more serious if left alone?
Neither is harmless, and both respond well to treatment. Untreated hypothyroidism raises LDL cholesterol, which is why thyroid function is often checked in people with newly found high cholesterol, and it is linked to heart strain, infertility and pregnancy complications 3. Untreated hyperthyroidism brings atrial fibrillation, which raises stroke risk, and bone loss that leads to osteoporosis and fractures 46. For more on each, see hypothyroidism and hyperthyroidism.
If you have a report in hand, our guide to reading your thyroid panel walks through every line.
Questions
Is hypothyroidism or hyperthyroidism more common?
Hypothyroidism is more common. In the US NHANES III survey, 4.6% of people had hypothyroidism (mostly subclinical) and 1.3% had hyperthyroidism, so an underactive thyroid turned up about three and a half times as often.
Does hypothyroidism cause weight gain or weight loss?
Hypothyroidism tends to cause modest weight gain, usually a few kilograms and much of it fluid. Hyperthyroidism tends to cause weight loss, often with a bigger appetite. Large or rapid weight changes usually have other causes alongside the thyroid.
Can hypothyroidism turn into hyperthyroidism?
A person can move between the two over time. Thyroiditis (painless, postpartum or subacute) usually runs an overactive phase then an underactive one; too much levothyroxine pushes labs into the overactive range; and Graves' treatment with radioactive iodine or surgery often ends in permanent hypothyroidism. Nobody has both at the same moment.
Which is worse, hypothyroidism or hyperthyroidism?
Both cause harm when untreated. Untreated hypothyroidism raises LDL cholesterol and is linked to heart strain and pregnancy problems; untreated hyperthyroidism raises the risk of atrial fibrillation and fractures. Both respond well to treatment once diagnosed.
Should I take iodine or thyroid support supplements?
Not without a clinician's advice. In iodine-sufficient countries like the US, extra iodine can push the thyroid either way, and biotin in many supplements can distort thyroid test results so they mimic Graves' disease. See supplements that affect blood tests.
Citations
- Ross DS et al. 2016 American Thyroid Association Guidelines for Diagnosis and Management of Hyperthyroidism and Other Causes of Thyrotoxicosis. Thyroid 2016;26(10):1343-1421
- Hollowell JG et al. Serum TSH, T4, and thyroid antibodies in the United States population (1988 to 1994): NHANES III. J Clin Endocrinol Metab 2002;87(2):489-499
- Chaker L, Bianco AC, Jonklaas J, Peeters RP. Hypothyroidism. Lancet 2017;390:1550-1562
- De Leo S, Lee SY, Braverman LE. Hyperthyroidism. Lancet 2016;388:906-918
- Stott DJ et al. Thyroid Hormone Therapy for Older Adults with Subclinical Hypothyroidism (TRUST). N Engl J Med 2017;376:2534-2544
- Biondi B et al. 2015 European Thyroid Association Guidelines on Diagnosis and Treatment of Endogenous Subclinical Hyperthyroidism. Eur Thyroid J 2015;4:149-163
- Marcocci C et al. Selenium and the course of mild Graves' orbitopathy. N Engl J Med 2011;364:1920-1931
Educational context only, not medical advice or a diagnosis. Always discuss your results with a clinician.