Hypothyroidism: The Signs, the Test and When to Treat
Key takeaways
- Hypothyroidism symptoms are vague and overlapping, so diagnosis depends on blood tests rather than how you feel.
- TSH works in reverse: a high reading suggests an underactive thyroid, and one raised result should be repeated after two to three months.
- Overt hypothyroidism is treated with levothyroxine, while subclinical cases are increasingly monitored rather than medicated.
- Trials found little benefit from levothyroxine for subclinical hypothyroidism, particularly in people aged 65 and over.
- Absorption matters: take levothyroxine on an empty stomach and separate it from calcium, iron, antacids and coffee.
An underactive thyroid produces symptoms that could belong to almost anything: tiredness, weight gain, feeling cold, low mood, constipation, foggy thinking. That vagueness cuts both ways. Some people spend years being told they are simply run down, while others are put on lifelong medication for a borderline blood test that might have corrected itself. Here is how the diagnosis actually works and what the evidence says about treating it.
Hypothyroidism: the short answer
The thyroid sets the pace of metabolism. When it produces too little hormone, everything slows: heart rate, digestion, temperature regulation and mental processing. The diagnosis rests on blood tests rather than symptoms, because the symptoms overlap with so many other conditions. The main test is TSH, and it works in reverse: a high TSH means the pituitary is pushing a sluggish thyroid harder.
Treatment is straightforward when the thyroid has clearly failed: levothyroxine, a replacement for the hormone you are not making, taken daily. The genuinely debated area is subclinical hypothyroidism, where TSH is raised but thyroid hormone itself is still normal. Here the evidence has moved toward treating less, not more. A BMJ rapid recommendation found levothyroxine provided no meaningful benefit for adults aged 65 and over, and a similar lack of benefit in younger adults, which is why repeat testing before starting treatment has become standard.
The signs worth noticing
Individually these symptoms mean little; together, and especially if they are new, they are worth testing for. The classic picture includes persistent fatigue that sleep does not fix, feeling cold when others are comfortable, unexplained weight gain of a few kilograms, constipation, dry skin and coarse or thinning hair, low mood and slowed thinking.
Less obvious features include a slower heart rate, puffiness around the eyes and face, hoarseness, muscle aches and weakness, heavier or irregular periods, reduced fertility, and raised cholesterol on a routine blood test. A visible swelling at the base of the neck, a goitre, can occur with Hashimoto's thyroiditis, the autoimmune condition that causes most hypothyroidism where iodine intake is adequate. Hashimoto's affects somewhere around 5 to 10 percent of the population in global estimates, and is between two and seven times more common in women.
The test, and what the numbers mean
Testing starts with TSH. If it is raised, free T4 is measured to see whether thyroid hormone output has actually fallen. Thyroid peroxidase antibodies are often added, because their presence indicates autoimmune thyroiditis and a higher chance of progressing to overt disease, at a rate of roughly 5 percent a year in people with Hashimoto's.
One raised TSH is not a diagnosis. Levels fluctuate with illness, stress, time of day and recent infection, and a meaningful proportion drift back to normal on retesting. Guidance generally recommends repeating the test after around two to three months before making any decision. Thresholds also differ internationally: UK guidance uses a TSH of 10 or above as the clear treatment trigger, while some other guidelines set reference values lower. The practical advice is to ask for the actual numbers rather than accepting the word normal, since the clinically interesting range sits just above the top of the reference interval.
When treatment helps, and when it does not
Overt hypothyroidism, meaning a high TSH with a low free T4, is treated with levothyroxine. Most people feel considerably better within weeks, and the dose is adjusted by repeat blood testing every six to eight weeks until stable.
Subclinical hypothyroidism is where judgement comes in. Current guidance generally favours treating when TSH is 10 or higher on two occasions three months apart, and considering it in younger people with clear symptoms or raised cardiovascular risk. Pregnancy is the clear exception, where treatment thresholds are lower and testing matters, since thyroid hormone is essential for fetal brain development. For a TSH between about 4 and 10 with no symptoms, most guidance now favours monitoring rather than medication, because trials have not shown that treatment improves symptoms, weight or cardiovascular outcomes in that group.
Taking levothyroxine properly
Absorption is the most common reason someone stays symptomatic on an apparently adequate dose, and it is fixable.
- Take it on an empty stomach, usually 30 to 60 minutes before breakfast, or at bedtime at least three hours after eating.
- Separate it from calcium and iron supplements by at least four hours, since both block absorption.
- Do the same with antacids, proton pump inhibitors and some cholesterol medications.
- Keep the timing consistent, including the relationship to coffee, which also reduces absorption.
- Stick to the same brand where possible, since formulations can differ slightly.
- Retest after any dose change, or after starting oestrogen, which can raise requirements.
Avoid thyroid support supplements. Products containing iodine can trigger thyroid dysfunction in susceptible people, and glandular extracts contain variable, unregulated amounts of actual hormone. If your thyroid is underactive, the treatment is a measured dose adjusted by blood tests.
Expect the dose to take a few months to settle. Most people notice energy and mental clarity improving within two to four weeks of reaching the right level, while skin, hair and weight changes lag further behind. Weight loss from treatment is usually modest, often a few kilograms of retained fluid rather than fat, which is worth knowing because disappointment on that front leads some people to abandon an otherwise well-managed treatment.
When to see a doctor
See a doctor if you have persistent fatigue, unexplained weight gain, cold intolerance, constipation, dry skin or low mood, and ask specifically for thyroid function tests alongside a full blood count, ferritin and vitamin B12, since those causes overlap. Ask for testing if you have a family history of thyroid or autoimmune disease, or if you are planning pregnancy.
Seek prompt assessment for a lump or swelling in the neck, difficulty swallowing or a persistently hoarse voice, and urgent care for extreme drowsiness, confusion or very low body temperature in someone with known hypothyroidism, which is rare but serious. If you already take levothyroxine and start hormone therapy or a new medication, ask whether your levels need rechecking. This is general information rather than medical advice, and dose decisions belong with your doctor.
Frequently asked questions
What are the first signs of hypothyroidism?
Persistent fatigue, feeling cold, unexplained weight gain, constipation, dry skin, hair thinning, low mood and slowed thinking. None is specific, which is why testing matters.
What TSH level means hypothyroidism?
A raised TSH with low free T4 indicates overt hypothyroidism. A raised TSH with normal T4 is subclinical, and UK guidance treats at 10 or above on two occasions three months apart.
Should subclinical hypothyroidism be treated?
Often not. Trials found little benefit from levothyroxine in this group, especially over 65. Treatment is more defensible at TSH of 10 or above, in pregnancy, or with clear symptoms.
How should levothyroxine be taken?
On an empty stomach, 30 to 60 minutes before breakfast or at bedtime well after eating, separated from calcium, iron, antacids and coffee, with consistent timing.
Do thyroid support supplements work?
They are best avoided. Iodine can trigger thyroid problems in susceptible people, and glandular products contain variable amounts of actual hormone.
Read next
- Thyroid or Menopause? The Symptoms That Overlap, and How to Tell
Sorting midlife symptoms between two common causes.
- Blood Test Results Explained: A Plain-English Guide to the Common Numbers
Reading your own results, including thyroid tests.
- Brain Fog: What Is Actually Causing It, and What Helps
Other treatable causes of the same symptoms.
This article is for general informational purposes only and is not medical advice. Always consult a qualified healthcare professional about your individual situation, and never start or stop a medication without your doctor. See our Medical Disclaimer.