How is hypothyroidism diagnosed? Which doctor to see, which blood tests come first, and what a smartwatch can't show

You have felt worn out for weeks, your skin is dry, and someone suggests getting your thyroid checked. The next questions are who to see and what happens there. Until then, note your symptoms on paper or in Sam's symptom diary, which feeds a monthly report you create for the previous month, with your symptom days and top five symptoms.

This guide covers the steps before a diagnosis. It explains who to see, which symptoms matter, how testing works and what a smartwatch can't do. For what TSH, free T4 and free T3 mean once your results are back, see TSH results explained. For what you can keep in Sam with an underactive thyroid, see the hypothyroidism overview.

Which doctor should you see first for a thyroid check?

Start with your GP or primary care doctor. The NHS says to see a GP if you think you might have an underactive thyroid. The British Thyroid Foundation puts it the same way: book an appointment and ask for a blood test.

Specialists come in when the picture is less clear. A referral guide for GPs in one NHS region (the Scottish Borders) lists situations such as these for a referral to endocrinology; other regions and countries differ:

• an underactive thyroid during pregnancy or within 12 months after it
• doubt about the diagnosis
• neck pain, feeling generally unwell, or earlier symptoms of an overactive thyroid
• a suspected problem with the pituitary gland
• an underactive thyroid linked to certain long-term medicines

The German family medicine guideline (DEGAM, 2023) adds two more. It refers to an endocrinologist when TSH and free T4 are both raised, since another cause may be behind it. It also refers when TSH does not come down during ongoing treatment. Your doctor decides when.

If you are waiting for a referral and your symptoms get worse or new ones appear, contact your GP sooner. For the emergency signs at the end of this guide, call for help straight away.

Which symptoms are a reason to ask for a thyroid test?

An underactive thyroid usually develops slowly, according to the NHS. The NHS and IQWiG list symptoms such as these:

• extreme tiredness
• feeling cold more than usual
• weight gain
• constipation
• trouble concentrating or thinking clearly
• low mood
• dry skin, dry hair or hair loss
• a croaky or hoarse voice
• heavy or irregular periods
• a low pulse

The British Thyroid Foundation also names a swelling in the neck as a reason for a blood test.

There is a catch. Many of these symptoms can have other causes, as the NHS and the American Thyroid Association point out. The German family medicine guideline goes further. It finds single symptoms, and combinations of them, too unreliable to confirm a diagnosis. So the blood test decides, not a checklist. Your doctor decides with you whether a test makes sense.

Testing people without symptoms is a different question. The US Preventive Services Task Force looked at screening adults who have no symptoms and are not pregnant. In 2015 it found too little evidence to weigh the benefits and harms. The German guideline advises against routine TSH screening in adults without symptoms. If you have a known thyroid condition, are pregnant or are planning a pregnancy, ask your doctor what applies to you.

What helps the conversation is a few weeks of notes: which symptoms, since when, on how many days, how strong. In Sam you log fatigue, dry skin, hair loss or constipation day by day, each as mild, moderate or severe. Feeling cold is not in the catalog, so you add it under "Other". The monthly report covers the previous month and needs, among other things, notifications and Apple Health access. For what a useful diary holds, see symptom diary: paper template or app.

What happens when your doctor tests your thyroid?

The steps below are based on the German family medicine guideline for adults with a raised TSH and on IQWiG information. Your doctor decides which of them you need and in what order.

1. Questions: your doctor asks about your symptoms and about things that make an underactive thyroid more likely (next section).
2. Examination: depending on your symptoms, your doctor examines you, partly to look for other causes. IQWiG describes feeling the thyroid in your neck.
3. TSH: the blood test usually starts with TSH, a hormone from the pituitary gland. It rises when the thyroid makes too little hormone.
4. Confirmation: after a first raised TSH, your doctor decides from the value and your history whether to repeat the test first or to look further straight away. The guideline uses 10 mU/L as one of its thresholds. It is a rule for doctors, not a way to judge your own result. If you are pregnant or planning a pregnancy, raise a high TSH with your doctor right away; the guideline has separate recommendations for pregnancy.
5. Free T4: next, free T4 is measured once. The British Thyroid Foundation also describes adding free T4 when TSH is above the range. It shows whether thyroid hormone is actually low.
6. Antibodies: if TSH is raised but free T4 is in range, TPO antibodies may be checked once, to look for Hashimoto's.

The German guideline advises against a routine thyroid ultrasound for a raised TSH. All the medical societies involved agreed. The patient representative did not, out of concern that a cancerous nodule might be missed. The guideline says that concern belongs in the conversation with your GP. It does not cover a swollen thyroid (goiter) or nodules: if you feel a swelling or lump in your neck, tell your doctor.

The guideline says a TSH value should not be read on its own. Your age, free T4, symptoms, BMI, medicines and general health all count. How to read the numbers on your report is covered in TSH results explained. If your symptoms get worse or new ones appear before a repeat test, contact your doctor sooner. For the emergency signs at the end of this guide, call for help straight away.

What will your doctor ask about?

The German guideline lists factors that make an underactive thyroid more likely and asks GPs to check them once TSH is raised. You can note your answers in advance:

• a known thyroid condition or earlier thyroid surgery
• an autoimmune thyroid condition or an underactive thyroid in a parent, sibling or child
• radiation to the head or neck, or radioactive iodine treatment
• other autoimmune conditions, such as type 1 diabetes, celiac disease or vitiligo
• depression or dementia, anemia, heart disease or high cholesterol
• long-term medicines, since some can cause an underactive thyroid

The NHS says your GP may also ask about thyroid problems in your family and about a recent pregnancy. Add supplements to your medicine list too. The British Thyroid Foundation asks you to mention anything that might affect the readings, especially supplements with iodine or biotin. For general preparation, see the doctor appointment checklist for a chronic condition.

Does a routine blood count include thyroid tests?

No. A full blood count (FBC), called a complete blood count (CBC) in the US, measures your blood cells. MedlinePlus lists red and white blood cells, platelets, hemoglobin, hematocrit and the average size of your red cells. Thyroid hormones are not on that list. TSH is a thyroid function test of its own, so it has to be requested.

A normal blood count therefore says nothing about your thyroid. Your lab report shows whether TSH was measured. The German guideline notes that a raised TSH mostly turns up by chance, when it was measured alongside other values. What hemoglobin and ferritin say about your iron is explained in iron deficiency: ferritin and hemoglobin.

How is Hashimoto's diagnosed, and by whom?

Hashimoto's thyroiditis is an autoimmune inflammation of the thyroid. The German guideline calls it the most common cause of an underactive thyroid. The first stop is the same: your GP or primary care doctor. The tests are TSH and free T4. If TSH is raised and free T4 is in range, TPO antibodies may be checked once. For how Hashimoto's and hypothyroidism differ, see hypothyroidism vs Hashimoto's.

Raised antibodies alone are not enough for the diagnosis, the German guideline says. On ultrasound, a thyroid with Hashimoto's can look darker than usual (hypoechoic). The guideline says this does not change the treatment decision. If your doctor wants more tests, you may be referred to an endocrinologist.

Can an Apple Watch show an underactive thyroid?

No. An Apple Watch does not measure thyroid hormones. The American Thyroid Association says a blood test is the only way to know for sure. What the watch records is your heart rate, including at rest.

A low pulse is on the IQWiG list of symptoms. But a slow heart rate has many possible causes. The Cleveland Clinic names an underactive thyroid among them, next to heart conditions, certain medicines, sleep apnea and age-related changes. It also notes that athletes naturally have a lower heart rate and that heart rate drops a bit during sleep.

So a low resting heart rate on your watch says nothing about your thyroid on its own. If it is unusual for you, or you have symptoms, raise it with your doctor. A slow pulse together with chest pain, palpitations, trouble breathing, dizziness or fainting is an emergency.

Sam reads your resting heart rate from Apple Health. You see it in the same app as your symptom diary, a few taps away. Sam does not measure it. What a low resting heart rate means for you is a question for your doctor.

When is it an emergency, not a GP appointment?

In rare cases, a severe untreated underactive thyroid leads to a life-threatening condition called myxoedema coma. The NHS describes it as an emergency that needs hospital treatment.

Call your local emergency number straight away (112 in the EU, 999 in the UK, 911 in the US) for severe trouble breathing, chest pain, passing out, confusion or being very hard to wake. The same goes for very strong drowsiness or an unusually low body temperature, and for a slow pulse together with chest pain, palpitations, trouble breathing, dizziness or fainting. Do not wait for it to pass. For anything urgent but not life-threatening when your practice is closed, use the out-of-hours service where you live.

Low mood is one of the listed symptoms of an underactive thyroid. If you have thoughts of harming yourself, call your local emergency number or a crisis line right away. In the UK and Ireland, Samaritans answer on 116 123, day or night. In the US, you can call or text 988, the Suicide & Crisis Lifeline, at any hour.

Log symptoms in Sam before your thyroid appointment

You arrive at the first appointment with weeks of notes, not a rough guess from memory.

• Instead of a paper note: you log fatigue, dry skin, hair loss or constipation each day, as mild, moderate or severe. Anything missing goes under "Other".
• Instead of memory: you create the monthly report yourself, for the previous month. The PDF shows your symptom days and five most frequent symptoms.
• Instead of Apple Health alone: resting heart rate and sleep from Apple Health sit in the same app, a few taps from your diary.
• Instead of a folder: add the report to the Health Vault. With your consent, Sam reads your lab report for TSH, free T4 and free T3. Rows can be mismatched, so check every value.

Sam doesn't measure thyroid levels or interpret a result medically. More in the hypothyroidism overview.

Try Sam for free

Sam is a wellness companion, not a medical device. The app does not diagnose, treat, cure or prevent any disease or health condition. Always consult qualified professionals for medical decisions.

Related reading

• TSH Results Explained: Hypothyroidism and Your Doctor Visit
• Symptom Diary: Paper Template or App?
• Doctor Appointment Checklist for a Chronic Condition
• Iron Deficiency: Ferritin and Hemoglobin Explained
• Hypothyroidism vs Hashimoto's: What Is the Difference?

Sources

Updated: October 2026

• NHS: Underactive thyroid (hypothyroidism) (last reviewed 28 April 2025)
• British Thyroid Foundation: Thyroid function tests (updated 8 October 2026)
• American Thyroid Association: Hypothyroidism
• IQWiG (informedhealth.org): Underactive thyroid (hypothyroidism) (updated 28 May 2024)
• DEGAM: S2k guideline Erhöhter TSH-Wert in der Hausarztpraxis, long version (PDF) (AWMF 053-046, April 2023, in German; recommendations 4.2.1, 5.1.1, 5.2.1, 5.3.1, 5.4.1.1, 5.4.1.3, 5.4.3.1, 5.4.4.1, 5.4.4.2, 6.2.11)
• US Preventive Services Task Force: Thyroid dysfunction: screening (final recommendation, 24 March 2015)
• NHS Borders Ref Help: Endocrinology: thyroid (referral guidance for GPs, next review June 2027)
• MedlinePlus: Complete blood count (CBC) (last updated 15 October 2024)
• Cleveland Clinic: Bradycardia (last updated 7 November 2025)
• Apple Support: Monitor your heart rate with Apple Watch
• Acharya R et al.: Myxedema Coma: A Forgotten Medical Emergency With a Precipitous Onset, Cureus 2020;12(9):e10478
• MedlinePlus: Recognizing medical emergencies (reviewed 8 January 2025)
• Samaritans: Contact a Samaritan
• 988 Suicide & Crisis Lifeline: 988lifeline.org