Underactive Thyroid: Symptoms, Causes & How It's Treated
Your Thyroid Is Probably
the Reason You're Exhausted
Fatigue, weight gain, hair falling out, feeling cold at room temperature. Millions of people live with these symptoms for years before anyone checks their thyroid. Here's what's happening — and what to actually do about it.
What the Thyroid Actually Does
The thyroid is a butterfly-shaped gland sitting at the front of your neck, just below your Adam's apple. It produces 2 hormones — thyroxine (T4) and triiodothyronine (T3) — that travel through the bloodstream and reach essentially every cell in the body.
These hormones control your metabolic rate. How fast your heart beats, how quickly you convert food to energy, how warm you run, how fast your bowels move, how sharp your thinking is. The thyroid is the body's thermostat, and when it runs slow, everything runs slow with it.
Hypothyroidism doesn't feel like one thing. It feels like getting older too fast, being tired in ways that sleep doesn't fix, and gaining weight while doing everything right.
The thyroid is regulated by a hormone called TSH (thyroid-stimulating hormone), produced by the pituitary gland. When thyroid hormone levels drop, the pituitary pumps out more TSH to try to kick the thyroid into gear. A high TSH is the classic signal that the thyroid isn't keeping up.
Symptoms: The Full Picture
Hypothyroidism rarely announces itself dramatically. It creeps in over months, sometimes years. Each symptom on its own seems explainable — stress, age, poor sleep — which is exactly why it gets missed.
The classic symptoms doctors teach in textbooks are fatigue, weight gain, cold intolerance, constipation, and dry skin. But the full list is longer, and some of the entries are genuinely surprising.
The ones that catch people off-guard: elevated LDL cholesterol (hypothyroidism slows cholesterol clearance), puffiness around the eyes and face, irregular or heavy periods, reduced libido, carpal tunnel syndrome, and hoarse voice from a swollen thyroid pressing on the larynx.
In severe, long-untreated hypothyroidism, the skin becomes doughy and thickened — a condition called myxoedema. The most dangerous endpoint is myxoedema coma: coma, extreme hypothermia (temperature dropping to 24–32°C), seizures, and respiratory depression. It's rare, but it's fatal without rapid treatment.
Go to the emergency department if you or someone else has severe confusion, extreme cold sensitivity, loss of consciousness, or a markedly swollen neck with breathing difficulty. These aren't typical hypothyroid presentations — they suggest a serious complication.
What Causes Hypothyroidism
The cause determines the treatment approach, the prognosis, and whether the condition is permanent or reversible.
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1Hashimoto Thyroiditis (most common in iodine-sufficient countries) An autoimmune disease where your immune system produces antibodies — specifically anti-TPO and anti-thyroglobulin — that attack and gradually destroy thyroid tissue. It runs in families. Associated with other autoimmune conditions: type 1 diabetes, coeliac disease, rheumatoid arthritis, lupus. Eventually leads to permanent hypothyroidism in most patients.
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2Iodine Deficiency (most common cause globally) Iodine is the raw material for thyroid hormone. Without enough of it, the gland can't produce T3 or T4. Salt iodisation has largely eliminated this in developed countries, but it remains widespread in South and Central Asia, sub-Saharan Africa, and parts of Eastern Europe.
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3Thyroid Surgery or Radioactive Iodine Treatment Removing the thyroid (thyroidectomy) causes immediate, permanent hypothyroidism. Partial removal has a 15–30% chance of causing insufficiency. Radioactive iodine used to treat hyperthyroidism or Graves disease causes permanent hypothyroidism in 3–6 months in most cases.
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4Medications Amiodarone (a heart drug) causes hypothyroidism in roughly 20% of patients on it. Lithium, interferon alpha, interleukin-2, and some cancer immunotherapy drugs can also suppress thyroid function. If you're on any of these long-term, your doctor should be monitoring your TSH periodically.
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5Postpartum Thyroiditis About 5–10% of women develop thyroiditis in the 2–12 months after delivery. It often starts with a brief hyperthyroid phase, followed by hypothyroidism. Most cases resolve within a year, but women with anti-TPO antibodies are at higher risk of it becoming permanent.
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6Central (Pituitary or Hypothalamic) Hypothyroidism Rare. Occurs when the pituitary doesn't produce enough TSH (secondary) or the hypothalamus doesn't produce enough TRH (tertiary). In this case, the thyroid itself is fine — it's just not being told to work. TSH-based screening can miss this, so free T4 must be checked when central disease is suspected.
Understanding Your TSH, T4 and T3
A blood test is the only way to confirm hypothyroidism. Symptoms alone aren't enough — too many other conditions mimic them. The standard first test is a serum TSH, which the AACE and ATA both recommend as the single best screening test for primary thyroid dysfunction.
| TSH Level | Free T4 | Interpretation | Action |
|---|---|---|---|
| 0.4–4.5 mIU/L | Normal | Normal thyroid function | No treatment needed; look for other causes of symptoms |
| 4.5–10 mIU/L | Normal | Subclinical hypothyroidism | Depends on symptoms, antibodies, and cardiovascular risk |
| >10 mIU/L | Normal | Subclinical (treat in most cases) | ATA/AACE recommend levothyroxine in patients ≤70 years |
| High | Low | Clinical hypothyroidism | Levothyroxine required |
| Low | Low | Central hypothyroidism | Refer to endocrinology; pituitary workup needed |
What about T3?
Routinely testing T3 is not recommended by current guidelines. Most of the T3 in circulation comes from T4 being converted in peripheral tissues, so T3 levels don't add much to the diagnosis. The exception is when central hypothyroidism is suspected, or when symptoms persist despite a normal TSH on levothyroxine treatment.
Anti-TPO antibodies
Testing for thyroid peroxidase antibodies (anti-TPO) doesn't diagnose hypothyroidism, but a positive result confirms Hashimoto as the cause and predicts whether subclinical hypothyroidism will progress to overt disease. Positive anti-TPO plus elevated TSH is a strong indication to start treatment earlier rather than waiting.
TSH isn't reliable in acutely ill patients. Non-thyroidal illness syndrome (euthyroid sick syndrome) can produce abnormal TSH readings in people with perfectly normal thyroid function. Treat the underlying illness first, then recheck thyroid function when the patient has recovered. This is a common source of unnecessary thyroid diagnoses in inpatient settings.
Subclinical Hypothyroidism: Should It Be Treated?
Subclinical hypothyroidism means your TSH is elevated but your free T4 is still normal. Up to 8% of the general population has it. It's more common in women, and prevalence rises with age — roughly 7–14% of people over 60 have TSH above the upper limit of normal.
The debate around treatment is genuine and ongoing. Here's the current consensus:
Treat when TSH is above 10 mIU/L in patients aged 70 or under, or when anti-TPO antibodies are elevated. The risk of progression to overt hypothyroidism is 2–6% per year in this group, and cardiovascular risk — including coronary artery disease and heart failure — is meaningfully elevated.
Watch and wait when TSH is mildly elevated (4.5–10 mIU/L) without symptoms, without positive antibodies, and in older adults. TSH levels in the elderly naturally shift upward, and overtreating elderly patients with levothyroxine carries real risks — atrial fibrillation, bone loss, and over-replacement symptoms.
Some people genuinely feel unwell with a TSH between 2.5 and 4.5 mIU/L. The current normal range was established using population averages that included people with subclinical thyroid disease. When data from anti-TPO-positive individuals are excluded from that calculation, the upper limit of a truly healthy TSH drops closer to 2.5 mIU/L. This is contested in guidelines, but it's worth discussing with your doctor if your symptoms are present and your TSH sits in the upper half of "normal."
Levothyroxine, Dosing, and When T3 Gets Added
The standard treatment for hypothyroidism is levothyroxine (L-T4) — a synthetic version of the T4 your thyroid can't make enough of. It's taken once daily on an empty stomach, 30–60 minutes before food. Taken correctly, it has a long half-life (7 days) and maintains stable blood levels throughout the day.
| Patient Type | Starting Dose | Notes |
|---|---|---|
| Young/healthy adult | 100 mcg or 1.7 mcg/kg/day | Full replacement can be started immediately |
| Elderly or cardiac disease | 12.5–25 mcg/day | Dose increased slowly every 6 weeks to avoid cardiac stress |
| Subclinical hypothyroidism | 25–50 mcg/day | Lower starting dose; titrate to TSH target |
| Pregnancy | Increase dose by ~30% | Take 9 doses per week instead of 7 (extra dose twice weekly) |
Dose adjustments happen every 6–8 weeks based on TSH levels, until the TSH stabilises within the normal range. The goal is a TSH between 0.5 and 2.5 mIU/L for most patients. In pregnancy, the target tightens to below 2.5 mIU/L in the first trimester.
Things that interfere with levothyroxine absorption
Calcium supplements, iron tablets, magnesium antacids, and some cholesterol drugs (cholestyramine) all bind levothyroxine in the gut and reduce absorption. Take them at least 4 hours apart. Certain anticonvulsants (phenobarbital, carbamazepine) and rifampicin increase levothyroxine metabolism — the dose may need to go up if these are started.
What if symptoms don't resolve on levothyroxine alone?
About 10–15% of patients on adequate levothyroxine still feel symptomatic despite normal TSH. For these patients, some endocrinologists add liothyronine (L-T3) alongside the T4. The evidence for combination therapy isn't definitive — guidelines don't yet recommend it as standard — but it's a reasonable conversation to have with a specialist if symptoms persist after 6 months of well-dosed T4 therapy.
Stick to one brand of levothyroxine. Different brands and generics use slightly different inactive ingredients that can affect absorption. Switching between them can create fluctuations in TSH, which are then chased with dose adjustments. If your pharmacy switches you to a generic, mention it to your doctor and recheck TSH in 6–8 weeks.
Thyroid and Pregnancy
Thyroid disease in pregnancy deserves its own section because the stakes are high and the management is different.
Thyroid hormone is critical for fetal brain development, particularly in the first trimester before the fetal thyroid is functional (around week 10–12). Uncontrolled hypothyroidism during early pregnancy is associated with miscarriage, pre-eclampsia, placental abruption, preterm delivery, and impaired cognitive development in the baby.
Women with known hypothyroidism who become pregnant should increase their levothyroxine dose by approximately 30% immediately — ideally before conception is confirmed, as soon as pregnancy is planned. The practical advice from the ATA: take an extra tablet twice per week as soon as the pregnancy test is positive, then see your endocrinologist within the first few weeks.
The TSH target in pregnancy is tighter than normal: below 2.5 mIU/L in the first trimester, below 3.0 mIU/L in the second and third. Check TSH every 4 weeks in the first half of pregnancy, then every 6–8 weeks in the second half.
Women with anti-TPO antibodies who have normal thyroid function (TSH below 2.5 mIU/L) are still at elevated risk and should be monitored throughout pregnancy. Postpartum thyroiditis is also more common in anti-TPO-positive women.
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