Hypothyroidism: Symptoms, Causes and Which Tests to Have

Hypothyroidism (an underactive thyroid) is one of the most common endocrine disorders. It affects several per cent of adults, with women five to eight times more likely to develop it than men. Mild, subclinical forms are even more common. Hypothyroidism is also easy to overlook because its symptoms are non-specific: fatigue, weight gain, feeling cold, dry skin and low mood. Many people attribute these symptoms to stress or ageing for years. Yet diagnosis begins with a simple, inexpensive blood test, and treatment is effective and generally well tolerated.

What does the thyroid do, and what happens when it becomes underactive?

The thyroid gland produces thyroxine (T4) and triiodothyronine (T3), hormones that regulate metabolism in virtually every cell of the body. Their production is controlled by the pituitary gland through thyroid-stimulating hormone (TSH). When thyroid hormone levels are too low, the pituitary gland releases more TSH to stimulate the thyroid. This is why TSH rises in hypothyroidism, while FT4 (free thyroxine) falls or, in the early stages, remains within the reference range. Metabolism slows down: the heart beats more slowly, bowel movements become less frequent, the skin and hair become dry, the body retains fluid and cholesterol levels rise.

Most common causes

• Hashimoto’s thyroiditis (chronic autoimmune thyroiditis): the most common cause in Poland. The immune system gradually damages the thyroid gland. Anti-TPO and anti-TG antibodies are markers of the condition.
• Previous treatment for hyperthyroidism with radioactive iodine or thyroid surgery.
• Medicines: amiodarone, lithium, interferon and certain cancer treatments.
• Iodine deficiency or excess, and, less commonly, congenital thyroid disorders and pituitary disease (central hypothyroidism, which may be associated with low TSH).
• Postpartum period: temporary postpartum thyroiditis, which can develop into permanent hypothyroidism in some women.

Symptoms that should prompt thyroid testing

No single symptom is exclusive to hypothyroidism. Several of the following symptoms usually occur together and develop gradually:

• persistent fatigue, sleepiness, sluggishness and difficulty concentrating,
• weight gain despite an unchanged diet, swelling (particularly of the face and eyelids),
• feeling cold, cold hands and feet,
• dry, pale skin, brittle hair and nails, hair loss and thinning eyebrows,
• constipation,
• low mood, apathy and sometimes depression-like symptoms,
• in women: heavy or irregular periods, difficulty conceiving and miscarriage,
• a slow heart rate, raised cholesterol, a hoarse voice, and muscle and joint pain.

Tests: where to start

TSH

Thyroid-stimulating hormone (TSH) – laboratory test is the first-line investigation. A normal TSH level in someone without symptoms virtually excludes primary hypothyroidism. The reference range in most laboratories is approximately 0.3–4.2 mIU/l, although it depends on the testing method, age and pregnancy (lower reference values apply during the first trimester). An elevated TSH result should be followed by an FT4 test.

FT4 and FT3

Elevated TSH together with low FT4 indicates overt hypothyroidism. Elevated TSH with normal FT4 indicates subclinical hypothyroidism, which does not always require treatment but does require monitoring. Free triiodothyronine (FT3) – laboratory test is less important in diagnosing hypothyroidism; it is mainly used when investigating hyperthyroidism and in certain specific circumstances.

Anti-TPO and anti-TG

Antibodies against thyroid peroxidase (anti-TPO) and thyroglobulin (anti-TG) help confirm an autoimmune cause, namely Hashimoto’s thyroiditis. High antibody levels in someone whose TSH is still normal indicate an increased risk of developing hypothyroidism in subsequent years and justify annual TSH monitoring.

Thyroid ultrasound

Although not a blood test, thyroid ultrasound provides additional diagnostic information. In Hashimoto’s thyroiditis, the gland typically has a heterogeneous, hypoechoic appearance. Ultrasound can also detect thyroid nodules that require further assessment.

Additional tests

Lipid profile (cholesterol) – laboratory test (hypothyroidism raises LDL cholesterol), full blood count and Ferritin – laboratory test (Hashimoto’s thyroiditis often coexists with anaemia and iron deficiency, while iron deficiency can impair thyroid hormone function), Vitamin B12 – laboratory test (autoimmune gastritis may coexist with Hashimoto’s thyroiditis), 25-hydroxyvitamin D (25 OH) – vitamin D test and Glucose – laboratory test. TSH testing is essential for women planning pregnancy because untreated hypothyroidism increases the risk of miscarriage and can affect the baby’s development.

Interpreting results: what common patterns mean

A single, mildly elevated TSH result should be repeated after a few weeks because TSH may rise temporarily following an infection, with insufficient sleep, during recovery from illness or as a result of certain medicines. Reference ranges and treatment decisions should be assessed by a doctor. The table is intended only to help you understand which questions to ask during your appointment.

Treatment and monitoring

Standard treatment is levothyroxine, a synthetic form of T4, taken in the morning on an empty stomach, 30–60 minutes before breakfast, with appropriate intervals between doses and iron supplements, calcium supplements and proton pump inhibitors. The dose is adjusted gradually, with TSH checked 6–8 weeks after each change. Once TSH has stabilised, monitoring is usually performed every 6–12 months. Symptoms generally improve within a few weeks. Weight loss is usually modest and mainly reflects the loss of retained fluid, so levothyroxine is not a weight-loss medicine.

Preparing for thyroid blood tests

Blood samples are best collected in the morning, between 7:00 and 10:00, because TSH follows a circadian rhythm. People taking levothyroxine should take their tablet after blood collection. High doses of biotin (found in hair and nail supplements) can interfere with thyroid hormone test results and should be stopped 2–3 days before testing. Fasting is not strictly necessary for TSH testing alone, but if a lipid profile or glucose test is also being performed, you should avoid food for 12 hours.

Frequently asked questions

Can hypothyroidism be cured?

Hashimoto’s thyroiditis usually requires lifelong thyroid hormone replacement, but with the correct dose, people can lead normal lives. Temporary forms, such as postpartum or drug-induced hypothyroidism, may resolve.

How often should I check TSH if I have positive anti-TPO antibodies but normal TSH?

Usually once a year, and always before a planned pregnancy and during early pregnancy.

Is a TSH result alone sufficient for diagnosis?

For initial screening, yes. A complete diagnosis and treatment decision require FT4 testing, usually antibody testing, and a clinical assessment by a doctor.

Sources

• NICE, Thyroid disease: assessment and management (NG145), 2019, updated 2023. nice.org.uk
• Jonklaas J. et al., American Thyroid Association, Thyroid, Guidelines for the Treatment of Hypothyroidism, 2014. journals.sagepub.com
• Pearce SHS et al., European Thyroid Association, 2013 ETA Guideline: Management of Subclinical Hypothyroidism, 2013. karger.com
• AOTMiT (Polish Agency for Health Technology Assessment and Tariff System), Prevention of Thyroid Diseases: Report on Recommended Medical Technologies, 2022. aotm.gov.pl