Common symptoms
- Persistent fatigue and needing more sleep than usual
- Brain fog, slowed thinking, and word-finding difficulty
- Cold intolerance, especially hands and feet
- Weight gain or difficulty losing weight
- Dry skin, brittle nails, and hair thinning or shedding
- Constipation
- Low mood or depression
- Heavier or irregular menstrual periods
- Muscle aches, cramps, and joint stiffness
- Hoarseness and puffiness around the eyes
Which labs matter
| Test | What it shows |
|---|---|
| TSH | Raised when the pituitary is pushing an underactive thyroid; the primary diagnostic and dose-monitoring test. |
| Free T4 | Low in overt hypothyroidism, normal in subclinical hypothyroidism. |
| Free T3 | Optional context about how much T4 is being converted to the active hormone. |
| TPO antibodies | Positive in Hashimoto's thyroiditis, the most common cause of hypothyroidism in iodine-sufficient countries. |
| Ferritin, vitamin B12, vitamin D | Common co-existing deficiencies that mimic or worsen hypothyroid symptoms. |
What causes hypothyroidism
In countries with adequate iodine intake, the leading cause is Hashimoto's thyroiditis, an autoimmune process in which the immune system gradually damages thyroid tissue. Other causes include thyroid surgery, radioactive iodine treatment, radiotherapy to the neck, certain medications such as amiodarone and lithium, and — less commonly — pituitary disease, which produces central hypothyroidism.
Iodine deficiency remains the leading cause worldwide. Excess iodine, including from high-dose supplements, can also trigger hypothyroidism in susceptible people.
Overt vs subclinical hypothyroidism
Overt hypothyroidism means a raised TSH with a low Free T4, and treatment is standard. Subclinical hypothyroidism means a raised TSH with a Free T4 still inside the reference range.
For subclinical hypothyroidism, guidelines generally support treatment when TSH is persistently above 10 mIU/L. Between the upper reference limit and 10, treatment is individualised, taking account of symptoms, TPO antibody status, cardiovascular risk, age, and pregnancy plans.
How treatment and dosing work
Levothyroxine, a synthetic form of T4, is the standard treatment. It is taken once daily on an empty stomach — typically 30–60 minutes before breakfast, or at bedtime at least three to four hours after eating — because food, coffee, and several supplements reduce absorption.
After a dose change, TSH is rechecked after six to eight weeks, because it takes that long for levels to stabilise. Absorption problems, not dosing errors, explain many cases of stubbornly high TSH: calcium, iron, magnesium, multivitamins, proton pump inhibitors, and bile acid binders all interfere and should be separated by about four hours.
Combination T4/T3 therapy is considered by some clinicians for patients who remain symptomatic on adequate levothyroxine, but guidelines treat it as an option for selected patients rather than a first-line approach.
What to track between appointments
The hardest part of hypothyroidism management is that symptoms change slowly and memory is unreliable. A short daily record of energy, brain fog, mood, and sleep, plus whether the dose was taken and how far it was separated from food and supplements, gives your clinician something far more useful than a general impression.
- Daily energy, brain fog, mood, and sleep ratings
- Medication taken, at what time, and the gap before food or coffee
- Supplements that interfere: calcium, iron, magnesium, multivitamins
- Every lab result with its date and reference range
- New symptoms that started after a dose change