Hypothyroidism: Levothyroxine, Dosing, and Follow Up Tests

You have felt tired, cold, and foggy for months. Then a blood test showed a high TSH, and you left the appointment with a prescription for levothyroxine. Or you have taken it for years and wonder why your dose keeps changing. This guide explains what the tests mean, how levothyroxine works, and what follow-up to expect.

What hypothyroidism is

The thyroid is a small gland in the front of the neck. It makes hormones that help control how your body uses energy. In hypothyroidism (underactive thyroid), the gland makes too little hormone [2].

Common symptoms include [2]:

• Tiredness
• Feeling cold
• Weight gain
• Constipation
• Dry skin and hair
• Muscle aches
• Low mood or trouble concentrating
• Heavier or irregular periods

Symptoms often come on slowly and can be mild.

Common causes

• Hashimoto's thyroiditis, an autoimmune condition where the immune system attacks the thyroid. This is the most common cause in Canada and the US [2].
• Thyroid surgery or radioactive iodine treatment
• Some medications, such as lithium and amiodarone
• Inflammation of the thyroid after pregnancy

What your doctor looks at

The blood tests

• TSH (thyroid-stimulating hormone) is made by the pituitary gland in the brain. When thyroid hormone is low, TSH rises to push the thyroid to work harder. TSH is the main screening and follow-up test.
• Free T4 is the main hormone the thyroid makes. It shows how much active hormone is in your blood.

Typical patterns:

• High TSH and low free T4: overt hypothyroidism, which is usually treated
• High TSH and normal free T4: subclinical hypothyroidism, which may or may not need treatment
• Normal TSH: thyroid function is usually normal

Reference ranges vary by lab. TSH also rises naturally with age.

Screening

The Canadian Task Force on Preventive Health Care recommends against routine screening for thyroid problems in adults without symptoms who are not pregnant [6]. It advises clinicians to test people who have symptoms or risk factors [6]. In the US, the USPSTF found too little evidence to recommend for or against screening adults without symptoms.

Starting levothyroxine

Levothyroxine is a man-made form of T4 [3]. It is the standard treatment recommended by the American Thyroid Association (ATA) [1]. Key points from the ATA guideline [1]:

• The full replacement dose is often based on body weight, about 1.6 micrograms per kilogram per day for many adults.
• Older adults and people with heart disease usually start at a low dose and increase slowly.
• The dose is adjusted based on TSH results.
• Combination therapy with T3 (liothyronine) is not routinely recommended.

NICE (UK) guidance also advises against routinely offering liothyronine [5].

Follow-up tests

• The ATA suggests checking TSH about 4 to 8 weeks after starting or changing the dose [1]. Many clinicians check at about 6 to 8 weeks.
• Once the dose is stable, TSH is often checked once a year [5].
• Extra checks may be needed after a brand switch, weight change, pregnancy, or new medication.

Subclinical hypothyroidism

Many people with a mildly high TSH and normal free T4 do not benefit from treatment. In the TRUST trial, 737 adults aged 65 and older with subclinical hypothyroidism took levothyroxine or a placebo [4]. After one year, levothyroxine did not improve symptoms or tiredness compared with placebo [4]. NICE suggests considering treatment when TSH is 10 or higher on two tests 3 months apart, and taking a more cautious approach for smaller rises [5].

Pregnancy

Thyroid hormone needs rise early in pregnancy. If you take levothyroxine and become pregnant, contact your clinician right away, since your dose often needs to go up [1].

How to take levothyroxine

The drug label and ATA guidance stress consistency [1] [3]:

• Take it on an empty stomach with water, 30 to 60 minutes before breakfast, or at bedtime at least 3 hours after your last meal.
• Take it at the same time each day.
• Separate it by at least 4 hours from iron, calcium, antacids, and some other medications and supplements.
• Coffee, soy, and high-fibre foods can lower absorption if taken at the same time.
• If you miss a dose, ask your pharmacist what to do.

Some supplements, such as biotin, can interfere with thyroid blood tests. Tell your clinician what you take, and ask whether to stop biotin for a few days before testing.

Risks of too much

Taking too much levothyroxine can cause a fast heartbeat, anxiety, trouble sleeping, and, over time, a higher risk of atrial fibrillation and bone loss, especially in older adults [1]. The FDA label carries a boxed warning that levothyroxine should not be used for weight loss [3].

Questions to ask your clinician

• What were my TSH and free T4 results, and what do they mean?
• Do I need treatment, or should we recheck first?
• What is my target TSH range?
• When is my next blood test?
• Do any of my medications or supplements affect levothyroxine?
• What should I do if I become pregnant?
• What symptoms might mean my dose is too high or too low?

When to get care quickly

Call 911 or go to the emergency department if you have chest pain, a very fast or irregular heartbeat, fainting, or severe shortness of breath. Very rarely, severe untreated hypothyroidism can cause confusion, extreme drowsiness, and a low body temperature, which also needs emergency care.

Bottom line

Hypothyroidism is common and usually easy to treat with levothyroxine. TSH guides the dose, and it is usually rechecked about 6 to 8 weeks after any change, then once a year when stable. Take your pill the same way every day, away from iron and calcium. Mildly high TSH often does not need treatment, especially in older adults.

Primary AI can show you the guideline and study sources behind this topic in plain language, so you can bring better questions to your appointment.