Hypothyroidism — Causes, Diagnosis and Treatment

Tiredness, weight gain, feeling cold when everyone else is comfortable, dry skin — these are some of the most common complaints I hear in clinic, and they are easy to dismiss as "just stress." Sometimes, though, the real culprit is an underactive thyroid.

Hypothyroidism is common, easy to diagnose with a simple blood test, and very treatable. The key is thinking of it in the first place.

What is hypothyroidism?

The thyroid is a small butterfly-shaped gland at the front of the neck. It makes thyroid hormones that set the pace of your metabolism, affecting heart rate, body temperature, digestion, mood, and energy. In hypothyroidism, the gland does not make enough of these hormones, and the body's processes slow down.

It is more common in women and becomes more frequent with age.

What causes it?

The most common cause in iodine-sufficient areas is Hashimoto's thyroiditis, an autoimmune condition in which the immune system gradually damages the thyroid. Other causes include:

• Thyroid surgery or radioactive iodine treatment (for example, after treating hyperthyroidism)
• Medications such as lithium and amiodarone
• Severe iodine deficiency, which remains an important cause worldwide and can lead to goiter (an enlarged thyroid)
• Pituitary or hypothalamic disease, known as central hypothyroidism
• Congenital thyroid problems in newborns

Symptoms to watch for

Symptoms usually develop slowly, which is why many people do not notice them for months or even years. Typical features include:

• Fatigue and sluggishness
• Weight gain and constipation
• Cold intolerance
• Dry skin, brittle nails, and hair thinning
• Low mood or poor concentration
• Heavy or irregular periods
• Muscle aches, a hoarse voice, and a visible goiter

None of these is specific on its own, so the diagnosis always needs laboratory confirmation.

How is it diagnosed?

Diagnosis rests on two blood tests: TSH (thyroid-stimulating hormone) and free T4.

• Overt primary hypothyroidism: high TSH with a low free T4.
• Subclinical hypothyroidism: high TSH with a normal free T4.
• Central hypothyroidism: low or inappropriately normal TSH with a low free T4, which points toward the pituitary.

If Hashimoto's is suspected, anti-thyroid peroxidase (anti-TPO) antibodies can support the diagnosis. An ultrasound is not needed routinely but may be used if there is a goiter or a nodule.

Treatment with levothyroxine

The standard treatment is levothyroxine, a synthetic form of T4 taken once daily. In young, otherwise healthy adults, a full replacement dose is often around 1.6 mcg/kg/day. In older patients or those with heart disease, we start lower and increase slowly.

Some practical points that make a real difference:

• Take it on an empty stomach, ideally 30–60 minutes before breakfast, with water.
• Keep it at least 4 hours apart from calcium and iron supplements, which reduce absorption.
• Take it at the same time every day.
• Tell your doctor right away if you become pregnant, as the dose often needs to be increased.

Follow-up

TSH is rechecked about 6–8 weeks after starting treatment or after any dose change, and the dose is adjusted until TSH sits in the target range. Once stable, testing every 6–12 months is usually enough.

Too little levothyroxine leaves symptoms uncontrolled, while too much can cause palpitations, atrial fibrillation, and bone loss over time. Regular monitoring protects against both. For most people, treatment is lifelong, and with the right dose they live completely normal lives.

Seek urgent care for severe lethargy, confusion, a very low body temperature, or slow breathing in someone with known hypothyroidism. These can signal myxedema coma, a medical emergency.