TSH, T3 and T4: normal thyroid test levels

The thyroid is a small gland with an outsized impact: it regulates metabolism, weight, body temperature, mood and energy. That is why assessing its function with a blood test is one of the most frequently ordered studies in the clinical laboratory.

What a thyroid profile includes

A basic thyroid profile includes three main markers:

• TSH (thyrotropin): a hormone made by the pituitary gland that stimulates the thyroid. It is the most sensitive marker for detecting early thyroid dysfunction.
• Free T4 (free thyroxine): the biologically active fraction of the most abundant thyroid hormone. It reflects the gland's actual output.
• Free T3 (free triiodothyronine): the active form of thyroid hormone. It is ordered in selected cases, above all when hyperthyroidism or T3 thyrotoxicosis is suspected.

In specific situations, the profile is expanded with antithyroid antibodies:

• Anti-TPO (thyroid peroxidase antibodies): a marker of autoimmune thyroiditis (Hashimoto's, the leading cause of hypothyroidism worldwide).
• Anti-Tg (thyroglobulin antibodies): useful alongside TPO in thyroid autoimmunity.
• TRAb (TSH-receptor antibodies): used to diagnose Graves' disease.

Reference values (for guidance)

These are the reference values used at Laboratorio Aclimu (in adults). They are indicative: the exact range can vary with age and is printed on your own report, so it is best to read it there.

Parameter	Reference range	What it reflects
TSH	0.35 – 4.94 µIU/mL	The most sensitive marker; rises in hypothyroidism and falls in hyperthyroidism
Free T4	0.7 – 1.48 ng/dL	Actual hormone output; defines whether the dysfunction is overt or subclinical
Free T3	2 – 4.4 pg/mL	The most active form; useful above all when hyperthyroidism is suspected
Anti-TPO	Up to 5.61 IU/mL (negative)	A raised level points to autoimmune thyroiditis (Hashimoto's)

Ranges can vary with age and with the measurement method. In pregnancy, trimester-specific TSH ranges apply — stricter in the first trimester, because demand for thyroid hormone rises from the earliest weeks. In older adults, the upper limit of TSH tends to be somewhat higher as a normal, physiological change.

Hypothyroidism vs. hyperthyroidism

The thyroid can be underactive or overactive, and the two conditions are almost mirror images. Understanding them helps connect your symptoms with what the test shows.

In hypothyroidism the gland produces too little hormone and metabolism slows down. The most common cause here is Hashimoto's thyroiditis (autoimmune). Typical symptoms include fatigue, weight gain, intolerance to cold, dry skin, hair loss, constipation, cramps and, in many cases, low mood. In the lab this shows up as a high TSH, with free T4 either low (overt) or still normal (subclinical).

In hyperthyroidism the opposite happens: there is too much hormone and metabolism speeds up. The most common cause is Graves' disease. It brings palpitations, weight loss despite a good appetite, intolerance to heat, sweating, a fine hand tremor, anxiety and insomnia. The test shows a low (suppressed) TSH, with raised free T4 and/or free T3. Measuring TRAb helps confirm an autoimmune origin.

When is a thyroid profile worth ordering?

A thyroid profile is not a test to repeat without reason, but there are situations where it genuinely helps:

• When symptoms suggest hypo- or hyperthyroidism (those described above).
• As part of a routine check from age 35, especially in women, including at least TSH.
• When planning a pregnancy and during the first trimester, because thyroid function affects fertility and fetal development.
• With a family history of thyroid disease, or a personal history of another autoimmune condition (type 1 diabetes, coeliac disease, vitiligo).
• To monitor treatment with levothyroxine or an already diagnosed case of hyperthyroidism.
• When cholesterol is persistently high with no clear cause, since hypothyroidism can be behind it.

If you are putting together your annual check-up, it is worth adding thyroid function to your general panel; around that age, a women's check-up typically already includes a baseline TSH.

How the most common patterns are read

Combining TSH and free T4 gives the initial diagnosis:

• Overt hypothyroidism: high TSH + low free T4.
• Subclinical hypothyroidism: high TSH + normal free T4.
• Overt hyperthyroidism: low (suppressed) TSH + high free T4 and/or free T3.
• Subclinical hyperthyroidism: low TSH + normal free T4 and T3.
• Pituitary dysfunction (a central cause): low or normal TSH with low free T4 (a referral to an endocrinologist is essential).

Symptoms that warrant checking your thyroid

Several common symptoms justify a thyroid profile:

• Fatigue or a sense of "not performing".
• Weight changes with no apparent cause.
• Hair loss, dry skin, brittle nails.
• Feeling excessively cold or hot.
• Mood disturbances, anxiety or depression.
• Menstrual changes or difficulty getting pregnant.
• Palpitations, a fine tremor, insomnia.
• Goitre (a visible or palpable enlargement of the thyroid).

Thyroid testing is also recommended when planning a pregnancy, during the first trimester, with a family history of thyroid disease, and as part of preventive check-ups from age 35 (especially in women).

How to prepare for the test

A thyroid profile does not require strict fasting, although for standardization we usually ask for about 4 hours of fasting. If you take levothyroxine, it is best to have your blood drawn before your morning dose so the T4 level is not overestimated. Tell the laboratory if you are taking corticosteroids, biotin, amiodarone or anticonvulsants, as these can interfere with the results.

Thyroid testing at Laboratorio Aclimu

At Laboratorio Aclimu we run the full thyroid profile and the autoimmune markers using high-sensitivity methods. Thyroid function can be added to a general check-up in a single blood draw, or booked as part of a dedicated thyroid check-up. Book your appointment with an order from your primary care doctor, endocrinologist or gynaecologist; results are available within 24 to 48 business hours through the online results portal.

This article was reviewed by the biochemistry team at Laboratorio Aclimu. The information is for general guidance only and does not replace a consultation with your doctor or health professional.

Frequently asked questions

What does a high TSH mean?

A high TSH means the pituitary is pushing the thyroid harder because it senses too little circulating hormone. Most often this reflects hypothyroidism: if free T4 is also low, it is overt hypothyroidism; if free T4 is still normal, it is called subclinical hypothyroidism. Interpreting the result and deciding whether to treat always rests with your doctor.

Do I need to fast for a thyroid profile?

No strict fasting is required. For standardization we usually ask for about 4 hours of fasting, but you can drink water normally. If you take levothyroxine, it is best to have your blood drawn before your morning dose so the T4 level is not overestimated.

How often should I have my thyroid checked?

In healthy people with no symptoms, including TSH in a routine check every one to two years from age 35 is enough, especially for women. If you are being treated for hypothyroidism, checks are more frequent: after a dose change TSH is usually repeated at 6 to 8 weeks, then once or twice a year once your levels are stable.

Can biotin affect thyroid test results?

Yes. Biotin (vitamin B7), found in many skin, hair and nail supplements, can interfere with the measurement methods and produce falsely high or low TSH, T4 and T3 values. It is recommended to stop it at least 48 to 72 hours before the blood draw, and to always tell the lab if you are taking it.

What is the difference between hypothyroidism and hyperthyroidism?

In hypothyroidism the thyroid makes too little hormone and metabolism slows down: fatigue, weight gain, feeling cold, dry skin and constipation. In hyperthyroidism it makes too much and metabolism speeds up: palpitations, weight loss, feeling hot, a fine tremor and insomnia. The thyroid profile, combining TSH with free T4 and T3, tells the two apart.

References

1. ATA (Jonklaas J et al.). Guidelines for the treatment of hypothyroidism: prepared by the American Thyroid Association task force on thyroid hormone replacement (2014). Available at: pubmed.ncbi.nlm.nih.gov
2. ATA (Ross DS et al.). 2016 American Thyroid Association Guidelines for Diagnosis and Management of Hyperthyroidism and Other Causes of Thyrotoxicosis (2016). Available at: pubmed.ncbi.nlm.nih.gov
3. ATA (Alexander EK et al.). 2017 Guidelines of the American Thyroid Association for the Diagnosis and Management of Thyroid Disease During Pregnancy and the Postpartum (2017). Available at: pubmed.ncbi.nlm.nih.gov
4. Clin Chem Lab Med (Piketty ML et al.). High-dose biotin therapy leading to false biochemical endocrine profiles: validation of a simple method to overcome biotin interference (2017). Available at: pubmed.ncbi.nlm.nih.gov