Overview.
Hypothyroidism occurs when your thyroid gland produces insufficient thyroid hormone, slowing your metabolism and affecting nearly every organ system. Affecting 5% of the population (with another 5% undiagnosed), it's especially common in women over 60. The most common cause is Hashimoto's thyroiditis—an autoimmune condition detectable years before clinical hypothyroidism develops.
The thyroid gland produces hormones (T4 and T3) that regulate your metabolic rate—how fast your cells burn energy. In hypothyroidism, insufficient hormone production slows everything: heart rate, digestion, mental processing, and body temperature regulation. The pituitary gland senses low thyroid hormones and releases TSH to stimulate production, which is why elevated TSH is the hallmark of primary hypothyroidism.
Prevalence: About 5% of Americans have diagnosed hypothyroidism, with an estimated additional 5% undiagnosed. Women are 5-8 times more likely to develop it than men. Risk increases with age—over 10% of women over 60 have hypothyroidism. It's more common in those with other autoimmune conditions or family history.
Medical name: Primary Hypothyroidism
Symptoms.
Early warnings
- Increased sensitivity to cold
- Unexplained fatigue
- Constipation
- Dry skin
- Weight gain without dietary changes
- Muscle weakness
- Elevated cholesterol
- Irregular or heavy menstrual periods
Classic symptoms
- Fatigue and sluggishness
- Cold intolerance
- Weight gain
- Constipation
- Dry skin and hair
- Hair loss (including outer third of eyebrows)
- Depression and cognitive slowness
- Puffy face and fluid retention
- Muscle aches and stiffness
- Bradycardia (slow heart rate)
- Hoarseness
Progression
Hypothyroidism often develops gradually. Subclinical hypothyroidism (elevated TSH, normal T4) may precede overt hypothyroidism by years. Hashimoto's thyroiditis causes progressive destruction—antibodies can be positive 5-10 years before TSH rises. Untreated severe hypothyroidism can lead to myxedema, a life-threatening condition.
Risk factors.
- Female sex
- Age over 60
- Family history of thyroid disease
- Other autoimmune conditions (Type 1 diabetes, rheumatoid arthritis, celiac)
- Previous thyroid surgery or radiation
- Iodine deficiency or excess
- Certain medications (lithium, amiodarone)
- Pregnancy or postpartum period
- History of thyroid inflammation
Lab interpretation.
Key biomarkers
- TSH — Elevated (>4.5-5.0 mIU/L); higher values indicate more severe hypothyroidism (primary)
- Free T4 — Low or low-normal; clearly low in overt hypothyroidism (primary)
- Free T3 — May be low or preserved initially (body prioritizes T3 production) (secondary)
- TPO Antibodies — Elevated in Hashimoto's thyroiditis (>35 IU/mL) (secondary)
- LDL Cholesterol — Often elevated due to reduced LDL receptor activity (supportive)
Diagnostic criteria
- Overt hypothyroidism: TSH >4.5 mIU/L with low Free T4
- Subclinical hypothyroidism: TSH 4.5-10 mIU/L with normal Free T4
- Hashimoto's thyroiditis: Positive TPO antibodies (with or without elevated TSH)
- Central hypothyroidism (rare): Low TSH with low Free T4
Recommended panels
When & next steps.
When to test
- Symptoms of hypothyroidism (fatigue, cold intolerance, weight gain)
- Family history of thyroid disease
- Other autoimmune conditions
- Age 35+, especially women (some guidelines recommend screening)
- Elevated cholesterol without clear cause
- Pregnancy planning or early pregnancy
- Difficulty losing weight despite diet and exercise
- New onset depression or cognitive changes
If suspected
- Order full thyroid panel: TSH, Free T4, and TPO antibodies
- Request Free T3 if poor conversion is suspected
- Review medications that can affect thyroid (biotin, steroids, amiodarone)
- Consider thyroid ultrasound if nodules suspected
If confirmed
- Levothyroxine is the standard treatment for overt hypothyroidism
- Start low and increase gradually (especially in elderly or heart disease)
- Take medication on empty stomach, 30-60 min before breakfast
- Recheck TSH 6-8 weeks after starting or changing dose
- Target TSH typically 0.5-2.5 mIU/L (discuss with provider)
- Annual monitoring once stable
FAQs.
Can hypothyroidism be cured?
Hashimoto's (the most common cause) is not curable, but hypothyroidism is easily treated with thyroid hormone replacement. With proper dosing, you can feel completely normal. Some cases (postpartum thyroiditis, subacute thyroiditis) may resolve on their own.
Why might I still have symptoms despite 'normal' labs?
The 'normal' TSH range is wide (0.4-4.5). Your optimal TSH may be lower. Also, some people convert T4 to T3 poorly, so Free T3 should be checked. Additionally, symptoms can have other causes that coexist with treated hypothyroidism.
Should I take T3 (Cytomel) in addition to T4?
Most people do well on T4 alone because the body converts it to T3. Some patients with persistent symptoms may benefit from combination therapy, but evidence is mixed. Discuss with an endocrinologist if you're not responding to T4 alone.
Can diet and supplements treat hypothyroidism?
Diet cannot replace thyroid hormone in true hypothyroidism. However, ensuring adequate selenium, zinc, and iodine supports thyroid function. Some foods (soy, cruciferous vegetables) can affect thyroid hormone absorption—take medication separately.