Hypothyroidism: Symptoms, Causes, Diagnosis, Treatment & Prevention

If you have been feeling unusually tired, gaining weight despite no change in diet, or constantly feeling cold even when everyone else is comfortable, the cause may not be stress or ageing — it could be your thyroid. Cross-sectional studies from Bangladesh suggest thyroid dysfunction affects roughly 10–20% of the general population, with hypothyroidism prevalence measured at around 7.0% in Dhaka and 4.97% in Khulna in regional surveys. Despite this, thyroid disease remains widely underdiagnosed because its symptoms overlap so closely with everyday fatigue, depression, and normal ageing. This guide explains what hypothyroidism actually is, why it happens, how doctors diagnose and treat it, and when you should get tested.

What Is Hypothyroidism?

The thyroid is a small, butterfly-shaped gland at the base of your neck. It produces two hormones — thyroxine (T4) and triiodothyronine (T3) — that control how fast your body's cells use energy. Hypothyroidism means the thyroid is not producing enough of these hormones, so the body's metabolism slows down. This is different from hyperthyroidism, where the thyroid produces too much hormone and speeds the body up.

Left untreated, hypothyroidism can affect the heart, mental health, fertility, pregnancy outcomes, and — in newborns — brain development. The good news is that it is one of the most manageable chronic conditions once correctly diagnosed.

What Causes Hypothyroidism?

• Iodine deficiency — globally, this is the single most common cause of hypothyroidism and goiter. Bangladesh made major progress after introducing universal salt iodization: goiter prevalence fell from about 55% in 1993 to roughly 7% by 2005. However, iodine deficiency has not disappeared — surveys have found insufficient iodine intake in close to 40% of school-age children and around 42% of non-pregnant, non-lactating women, meaning the risk has not gone away, especially outside major cities.
• Hashimoto's thyroiditis (autoimmune) — in iodine-sufficient regions and increasingly in Bangladeshi studies, this is the leading cause. Here, the immune system mistakenly produces antibodies — thyroid peroxidase antibody (TPOAb) and thyroglobulin antibody (TgAb) — that attack and gradually destroy thyroid tissue. Bangladeshi epidemiological data attributes roughly a quarter of thyroid disease cases to autoimmune causes. It is far more common in women and can develop slowly over years.
• Postpartum thyroiditis — temporary thyroid inflammation after childbirth, noted as a recognised pattern in Bangladeshi clinical research; it can cause a hypothyroid phase weeks to months after delivery.
• Other causes — surgical removal of the thyroid, radiation treatment to the neck, certain medicines (such as lithium or amiodarone), and, rarely, pituitary gland problems that reduce TSH production.

Symptoms: What to Watch For

Because thyroid hormone affects nearly every organ system, symptoms are wide-ranging and often develop gradually — which is why many people dismiss them for months or years. Common symptoms include:

• Persistent fatigue and low energy, even after adequate sleep
• Unexplained weight gain or difficulty losing weight
• Feeling unusually cold (cold intolerance)
• Constipation
• Dry skin, brittle nails, hair thinning or loss (including outer eyebrows)
• Puffiness in the face, especially around the eyes
• Hoarse voice
• Muscle aches, joint pain, and stiffness
• Depression, low mood, and "brain fog" — slowed thinking or forgetfulness
• Heavy, prolonged, or irregular menstrual periods
• A visible swelling in the front of the neck (goiter)
• Slow heart rate

Because fatigue, mood changes, and menstrual irregularity are also common in depression, anemia, perimenopause, and simple overwork, hypothyroidism is frequently missed without a blood test. If several of these symptoms occur together and persist for weeks, it is worth getting checked rather than assuming it is "just stress."

Hypothyroidism vs. Hyperthyroidism: Know the Difference

These two conditions are opposites, and confusing them can lead to the wrong treatment path:

• Hypothyroidism (underactive): body slows down — fatigue, weight gain, cold intolerance, constipation, depression, slow heart rate.
• Hyperthyroidism (overactive): body speeds up — anxiety, unintentional weight loss, heat intolerance and sweating, insomnia, palpitations, tremor.

Both can cause tiredness and mood changes, and both can cause a goiter, so the symptom overlap makes it impossible to tell them apart without a blood test.

Diagnosis: What Tests Are Used

A blood test is the only reliable way to confirm hypothyroidism — symptoms alone cannot diagnose it.

• TSH (Thyroid-Stimulating Hormone) is the primary and most sensitive screening test. TSH is released by the pituitary gland to signal the thyroid to work harder; when the thyroid is underactive, TSH rises. Most laboratories consider roughly 0.4–4.5 mIU/L as the normal adult range, though the exact reference range varies slightly between labs — your doctor will interpret your result against the range printed on your specific report.
• Free T4 is checked when TSH is abnormal, to see how much active hormone is actually circulating and to distinguish overt from subclinical disease.
• TPO antibody (anti-TPO) test helps confirm whether Hashimoto's thyroiditis is the underlying cause, especially useful when TSH is only mildly elevated.
• Thyroid ultrasound may be done if there is a visible or palpable goiter or nodule, or to look for the characteristic patchy, low-echo appearance seen in autoimmune thyroiditis.

Subclinical hypothyroidism refers to a mildly elevated TSH with a normal Free T4 and few or no symptoms — a borderline stage that some patients need to monitor rather than treat immediately, depending on their doctor's assessment of age, symptoms, and antibody status.

Treatment: Levothyroxine and Lifelong Management

Hypothyroidism is treated with levothyroxine, a synthetic form of T4 hormone that replaces what the thyroid can no longer produce adequately. It is available in Bangladesh under several brand names, including Thyrox, Eltroxin, and Thyrin, among others, in various microgram strengths. For most people, this is a lifelong daily medication, not a short course.

• The starting dose and pace of adjustment depend heavily on age and heart health — younger, otherwise healthy adults are usually started on a higher weight-based dose, while older adults or those with heart disease are started low and increased gradually to avoid straining the heart.
• The medicine should be taken on an empty stomach, generally 30–60 minutes before breakfast, with a full glass of water.
• Calcium supplements, iron supplements, and antacids can interfere with absorption and should be taken at least 4 hours apart from levothyroxine.
• Because the hormone takes weeks to reach a stable level in the blood, TSH is usually rechecked about 6–8 weeks after starting or changing the dose — not sooner. Once the dose is stable, monitoring typically continues on a periodic (often annual) basis.
• Never adjust the dose or stop the medicine on your own based on how you feel — dose changes should always be guided by blood test results, decided with your doctor.

This article provides general information only, not an individual treatment plan. Exact dosing must be determined by a physician based on your test results, weight, age, and any coexisting conditions.

Special Situations

Pregnancy

Thyroid hormone is essential for the baby's brain and nervous system development, particularly in the first trimester, when the fetus depends entirely on the mother's thyroid hormone supply. Untreated or poorly controlled hypothyroidism during pregnancy has been linked to a higher risk of miscarriage, cesarean delivery, and postpartum bleeding. Women who are pregnant, trying to conceive, or have a personal or family history of thyroid disease should discuss thyroid testing with their doctor early in pregnancy or before conception.

Postpartum Period

Some women develop postpartum thyroiditis — thyroid inflammation appearing weeks to months after delivery, which can cause a temporary overactive phase followed by an underactive phase. Persistent fatigue or mood changes after childbirth that don't improve deserve a thyroid check rather than being attributed only to the demands of a newborn.

Children and Newborns

Congenital hypothyroidism (a baby born with an underactive thyroid) requires early detection and treatment, since thyroid hormone is critical for a newborn's brain development in the first weeks of life. Any unusual sluggishness, poor feeding, prolonged jaundice, or slow growth in an infant should be discussed with a pediatrician.

Elderly Patients

Symptoms like fatigue, constipation, and slowed thinking are easy to mistake for normal ageing in older adults. Because levothyroxine can affect heart rhythm, elderly patients and those with existing heart disease are usually started on a lower dose with slower, carefully monitored increases.

When to See a Doctor

Consider getting a thyroid function test if you have:

• Persistent, unexplained fatigue lasting more than a few weeks
• Unexplained weight gain along with cold intolerance or constipation
• A visible swelling or lump in the front of your neck
• Heavy or irregular periods with no other clear cause
• Difficulty conceiving, or you are planning a pregnancy and have risk factors for thyroid disease
• A family history of thyroid or other autoimmune disease

A simple TSH blood test, available at most diagnostic centers in Bangladesh, is usually the first and most cost-effective step.

Prevention

• Use iodized salt consistently — Bangladesh's salt iodization program has dramatically reduced goiter rates nationally, but household usage still varies, and switching to non-iodized salt reverses this protection.
• Do not over-supplement iodine — excessive iodine intake (from unregulated supplements) can also disrupt thyroid function; more is not automatically better.
• Know your family history — autoimmune thyroid disease cannot be entirely prevented, but knowing you're at higher risk means earlier testing and earlier treatment before symptoms become severe.
• Don't ignore vague, persistent symptoms — early detection through a simple blood test prevents years of unexplained fatigue, weight struggles, and, in pregnancy, more serious complications.

Sources: Guidelines and patient information from the American Thyroid Association (ATA), the American Association of Clinical Endocrinology (AACE), the American Academy of Family Physicians (AAFP), and peer-reviewed epidemiological studies on thyroid disease prevalence in Bangladesh.