Thyroid Hormones (T3/T4)

What It Is

Thyroid hormones are the metabolic regulators of the body—they control how fast or slow nearly every cell operates. They affect energy production, body temperature, heart rate, digestion, brain function, weight, mood, and more. When thyroid hormones are optimal, you feel energized, clear-headed, warm, and metabolically balanced. When they're off, everything can feel sluggish, heavy, foggy, and cold—or occasionally too revved up.

Where they're produced:

• Thyroid gland → a butterfly-shaped gland at the base of the neck
• Regulated by the hypothalamic-pituitary-thyroid (HPT) axis

The thyroid hormone family:

T4 (Thyroxine):

• The storage form of thyroid hormone
• Produced in large quantities by the thyroid gland
• Relatively inactive (has some activity, but much less than T3)
• Must be converted to T3 to be fully active

T3 (Triiodothyronine):

• The active form of thyroid hormone
• About 3-4 times more potent than T4
• Produced in smaller amounts by the thyroid gland (about 20% of T3)
• Mostly produced by conversion of T4 to T3 in the liver, kidneys, and other tissues (about 80% of T3)
• This is the hormone that binds to thyroid receptors and produces effects

Reverse T3 (rT3):

• An inactive form of T3
• Produced when the body converts T4 to rT3 instead of active T3
• Acts as a "brake" on metabolism (blocks active T3 from binding to receptors)
• Rises during chronic stress, illness, calorie restriction, or inflammation → adaptive metabolic slowing

TSH (Thyroid-Stimulating Hormone):

• Produced by the pituitary gland (brain)
• Signals the thyroid gland to produce more T4 and T3
• High TSH → thyroid is underactive (hypothyroidism); brain is trying to stimulate more thyroid hormone production
• Low TSH → thyroid is overactive (hyperthyroidism); brain is trying to reduce thyroid hormone production

Primary functions:

• Regulates metabolism → controls how efficiently cells produce and use energy
• Controls body temperature → thyroid hormones regulate heat production (why hypothyroidism causes cold intolerance)
• Affects heart rate and cardiovascular function → thyroid hormones increase heart rate, cardiac output, blood flow
• Supports brain function → cognition, memory, focus, processing speed, mood
• Regulates digestion and gut motility → thyroid hormones affect how quickly food moves through the digestive tract
• Influences weight and body composition → thyroid hormones affect metabolic rate, fat storage, muscle mass
• Supports hair, skin, and nail health → thyroid hormones affect growth and regeneration of these tissues
• Affects menstrual cycle and fertility → thyroid dysfunction can cause irregular periods, heavy bleeding, or infertility
• Regulates cholesterol metabolism → low thyroid → high cholesterol

Why It Matters During Perimenopause/Menopause

Thyroid issues become much more common during perimenopause and menopause. The reasons are interconnected:

1. Estrogen affects thyroid function:

• Estrogen increases thyroid-binding globulin (TBG), which binds thyroid hormones and reduces the amount of free (active) thyroid hormone available
• When estrogen fluctuates (perimenopause), TBG fluctuates → thyroid hormone availability becomes erratic
• When estrogen declines (menopause), TBG declines → more free thyroid hormone becomes available (but thyroid gland function may also decline with age)

2. Autoimmune thyroid disease becomes more common with age:

• Hashimoto's thyroiditis (autoimmune hypothyroidism) is the most common cause of hypothyroidism in women
• Prevalence increases with age, peaking in the 50s-60s (perimenopause/menopause years)
• Women are 5-10 times more likely than men to develop Hashimoto's
• Hormonal fluctuations during perimenopause may trigger or worsen autoimmune thyroid dysfunction

3. Symptoms of hypothyroidism overlap with perimenopause/menopause:

• Fatigue, weight gain, brain fog, mood changes, hair thinning, cold intolerance, constipation, heavy periods
• This means thyroid dysfunction is often missed—symptoms are attributed to "just menopause" when thyroid testing could reveal a treatable condition

4. Stress, inflammation, and HPA axis dysfunction affect thyroid conversion:

• Chronic stress (common during perimenopause) → high cortisol → reduced conversion of T4 to active T3 → more conversion to reverse T3 (inactive)
• Result: "Functional hypothyroidism"—TSH and T4 look normal, but T3 is low, reverse T3 is high → symptoms of hypothyroidism despite "normal" labs

The pattern:

Early perimenopause:

• Estrogen fluctuations → TBG fluctuates → free thyroid hormone availability is erratic
• Stress and cortisol dysregulation → reduced T4-to-T3 conversion → symptoms of low thyroid (fatigue, weight gain, brain fog) even if labs are "normal"
• Autoimmune thyroid disease may surface or worsen

Mid-perimenopause:

• Thyroid symptoms often intensify (fatigue, weight gain, hair thinning, cold intolerance, brain fog, mood changes)
• Overlap with perimenopausal symptoms makes diagnosis challenging
• Many women are told "your thyroid is fine" based on TSH alone, when more comprehensive testing (free T3, free T4, reverse T3, thyroid antibodies) would reveal dysfunction

Late perimenopause and menopause:

• Estrogen stabilizes at low levels → TBG stabilizes → free thyroid hormone availability becomes more predictable
• However, thyroid gland function may decline with age → hypothyroidism becomes more common
• For women with existing thyroid conditions, medication needs may change during this transition

Post-menopause:

• Thyroid dysfunction (especially hypothyroidism) remains common
• Regular monitoring is important, especially for women with autoimmune thyroid disease or family history of thyroid issues

How It Works

Mechanism of action:

Thyroid hormones work by:

1. T4 is converted to active T3 (in liver, kidneys, gut, and other tissues) via the enzyme 5'-deiodinase
2. T3 enters cells and binds to thyroid hormone receptors (TR-alpha and TR-beta) in the cell nucleus
3. T3-receptor complex binds to DNA and affects gene expression (turning genes on or off)
4. Result: Changes in cellular metabolism, energy production, protein synthesis, and function

The HPT axis (thyroid regulation):

1. Hypothalamus (brain) releases TRH (thyrotropin-releasing hormone)
2. Pituitary gland (brain) responds to TRH → releases TSH (thyroid-stimulating hormone)
3. Thyroid gland responds to TSH → produces and releases T4 and T3
4. T4 is converted to T3 in tissues throughout the body
5. T3 feeds back to brain → tells hypothalamus and pituitary to reduce TRH and TSH (negative feedback loop)

When thyroid function is disrupted:

Hypothyroidism (underactive thyroid):

• Thyroid gland doesn't produce enough T4/T3 → TSH rises (brain is trying to stimulate more thyroid hormone production)
• Result: Metabolism slows, energy declines, weight increases, cognition slows, body temperature drops

Hyperthyroidism (overactive thyroid):

• Thyroid gland produces too much T4/T3 → TSH drops (brain is trying to reduce thyroid hormone production)
• Result: Metabolism speeds up, heart rate increases, weight loss, anxiety, heat intolerance, insomnia

Functional hypothyroidism (poor T4-to-T3 conversion):

• TSH and T4 are normal, but T3 is low and/or reverse T3 is high
• Result: Symptoms of hypothyroidism despite "normal" labs
• Common during chronic stress, inflammation, nutrient deficiencies (selenium, zinc, iron)

Thyroid hormone's relationship with other hormones:

Thyroid + Estrogen:

• Estrogen increases TBG (thyroid-binding globulin), which binds T4 and T3 → reduces free (active) thyroid hormone
• When estrogen is high (early perimenopause, estrogen dominance), free thyroid hormone may be lower → hypothyroid symptoms
• When estrogen fluctuates (perimenopause), free thyroid hormone fluctuates → erratic symptoms
• When estrogen is low (menopause), TBG is lower → more free thyroid hormone available (but thyroid production may also decline)

Thyroid + Progesterone:

• Progesterone supports T4-to-T3 conversion (some evidence suggests progesterone enhances thyroid function)
• When progesterone is low (perimenopause), T3 production may be less efficient

Thyroid + Cortisol:

• Chronic high cortisol (stress, HPA axis dysfunction) reduces T4-to-T3 conversion and increases reverse T3
• Result: Functional hypothyroidism—normal TSH/T4, but low T3, high reverse T3 → symptoms of low thyroid
• Addressing cortisol dysregulation often improves thyroid function

Thyroid + Insulin:

• Hypothyroidism → slower metabolism → insulin resistance → weight gain, difficulty losing weight, increased diabetes risk
• Hyperthyroidism → faster metabolism → increased insulin sensitivity (can cause hypoglycemia if severe)

Thyroid + Testosterone:

• Low thyroid can reduce SHBG (sex hormone-binding globulin) → more free testosterone available (can worsen acne, hair growth in women with PCOS)
• Severe hypothyroidism can suppress ovarian function → lower testosterone

What It Looks Like

When Optimal (Healthy Thyroid Function)

Physical:

• Stable energy throughout the day
• Healthy weight, easy to maintain
• Normal body temperature (warm hands and feet, no cold intolerance)
• Regular, predictable menstrual cycles (if premenopausal)
• Healthy digestion (regular bowel movements, no bloating or constipation)
• Healthy hair, skin, nails (hair is thick, skin is moist, nails are strong)

Cognitive:

• Clear thinking, sharp focus
• Good memory and recall
• Fast processing speed
• Mental stamina throughout the day

Emotional:

• Stable mood
• Motivated, engaged, interested in life
• Calm, not anxious or agitated

When Low (Hypothyroidism)

Hypothyroidism is the most common thyroid disorder, especially in women over 40.

Physical:

• Fatigue → exhaustion, low energy, difficulty waking, need for naps
• Weight gain (or difficulty losing weight) → even with healthy diet and exercise
• Cold intolerance → always cold, cold hands and feet, need layers when others are comfortable
• Constipation → slow gut motility
• Hair thinning → hair falls out more, eyebrows thin (especially outer third), hair is dry and brittle
• Skin changes → dry, rough, flaky skin; sometimes yellowish tint (from poor conversion of beta-carotene)
• Brittle nails → slow-growing, ridged, breaking easily
• Puffy face, fluid retention → especially around eyes
• Heavy or irregular periods (if premenopausal)
• Muscle aches, joint pain → unexplained soreness, stiffness
• Slow heart rate (bradycardia)
• High cholesterol → especially LDL

Cognitive:

• Brain fog → difficulty concentrating, slow thinking, "fuzzy" head
• Memory problems → forgetfulness, difficulty retaining information
• Slow processing speed → takes longer to understand, respond, make decisions

Emotional:

• Depression → low mood, hopelessness, reduced interest in activities
• Low motivation → apathy, difficulty initiating tasks
• Mood swings → emotional flatness or irritability

When High (Hyperthyroidism)

Hyperthyroidism is less common than hypothyroidism but can occur (especially with Graves' disease, an autoimmune condition).

Physical:

• Weight loss (despite normal or increased appetite)
• Rapid or irregular heart rate (tachycardia, palpitations, atrial fibrillation)
• Heat intolerance, excessive sweating → always too warm
• Tremor → shaky hands, internal trembling
• Diarrhea or frequent bowel movements → fast gut motility
• Hair thinning → hair becomes fine, falls out
• Muscle weakness → especially in thighs, arms
• Menstrual changes → light periods or absent periods (if premenopausal)
• Eye changes (with Graves' disease) → bulging eyes, eye irritation, double vision

Cognitive:

• Difficulty concentrating → mind races, hard to focus on one thing
• Restlessness → can't sit still, always need to be doing something

Emotional:

• Anxiety → nervousness, worry, panic
• Irritability → short fuse, low tolerance
• Insomnia → difficulty falling asleep, staying asleep, feeling wired
• Mood swings → emotional volatility

When Conversion Is Poor (Functional Hypothyroidism: Low T3, High Reverse T3)

Symptoms are similar to hypothyroidism, but labs may show "normal" TSH and T4:

• Fatigue, weight gain, cold intolerance, brain fog, mood changes, hair thinning
• This pattern is common during chronic stress, inflammation, nutrient deficiencies, or HPA axis dysfunction

Phase Impact

Baseline (Regular Cycle, Pre-Perimenopause): Thyroid function is typically stable. Estrogen and progesterone support healthy thyroid hormone availability and conversion. Women with existing thyroid conditions (hypothyroidism, Hashimoto's) may be well-managed on medication. Symptoms are predictable and manageable.

Electric Cougar (Early Perimenopause): Estrogen fluctuations begin → TBG fluctuates → free thyroid hormone availability becomes erratic. Stress and cortisol dysregulation (from progesterone decline and estrogen swings) may reduce T4-to-T3 conversion. Symptoms of hypothyroidism may surface or worsen (fatigue, weight gain, brain fog, hair thinning). Autoimmune thyroid disease may be triggered or worsen. For women on thyroid medication, dose may need adjustment.

Wild Tide (Mid-Perimenopause): Thyroid symptoms often intensify and overlap with perimenopausal symptoms, making it hard to distinguish what's thyroid and what's sex hormones. Weight gain, fatigue, brain fog, hair thinning, cold intolerance, mood changes, and heavy periods can all be thyroid-related, hormone-related, or both. Comprehensive thyroid testing (TSH, free T3, free T4, reverse T3, antibodies) is essential. Many women are told "your thyroid is fine" based on TSH alone when more detailed testing would reveal dysfunction.

Henapause (Late Perimenopause, 7-11 Months Without Period): Estrogen fluctuations calm → TBG becomes more stable → free thyroid hormone availability stabilizes. However, thyroid gland function may decline with age. Hypothyroidism may develop or worsen. For women on thyroid medication, dose may need adjustment as estrogen stabilizes. Chronic stress and cortisol dysregulation may continue to impair T4-to-T3 conversion.

The Pause (Menopause, 12+ Months Without Period): Estrogen is low and stable → TBG is low and stable → free thyroid hormone availability is more predictable. However, thyroid gland function often declines with age. Hypothyroidism becomes increasingly common. Regular thyroid monitoring is important, especially for women with autoimmune thyroid disease or family history. For women on thyroid medication, dose may need adjustment.

Phoenix Phase (Early Post-Menopause, 2-10 Years After Last Period): Thyroid function is generally stable for women without thyroid disease. For those with hypothyroidism, medication needs may have stabilized after the hormonal fluctuations of perimenopause. However, autoimmune thyroid disease can progress, so ongoing monitoring is important. Addressing stress, inflammation, and nutrient status supports optimal thyroid function.

Golden Sovereignty (Established Post-Menopause, 7+ Years After Last Period): Thyroid function may remain stable or decline with age. Hypothyroidism prevalence continues to increase. Regular screening (every 1-2 years, or more often if symptomatic) is recommended. Optimal thyroid function supports energy, metabolism, cognition, cardiovascular health, and quality of life.

Testing & Optimization

When to Test

Thyroid testing is recommended if you have symptoms of thyroid dysfunction, family history of thyroid disease, or autoimmune conditions.

When testing makes sense:

• Fatigue, weight gain, cold intolerance, brain fog, mood changes, hair thinning (symptoms of hypothyroidism)
• Anxiety, weight loss, heat intolerance, rapid heart rate, insomnia (symptoms of hyperthyroidism)
• Irregular or heavy periods (thyroid dysfunction can affect menstrual cycle)
• Family history of thyroid disease or autoimmune conditions
• Personal history of autoimmune disease (one autoimmune condition increases risk of others)
• Before starting HRT (to establish baseline)
• Periodically during perimenopause/menopause (thyroid issues are common during this transition)

What tests to request:

Standard thyroid panel (often insufficient):

• TSH → screening test; elevated in hypothyroidism, low in hyperthyroidism
  • Normal range: 0.5-4.5 mIU/L (varies by lab)
  • Optimal range: 0.5-2.5 mIU/L (many functional practitioners consider TSH above 2.5 a sign of suboptimal thyroid function)

Comprehensive thyroid panel (more informative):

• TSH → screening
• Free T4 → storage hormone; should be in mid-to-upper normal range
• Free T3 → active hormone; most important for symptom relief; should be in mid-to-upper normal range
• Reverse T3 → inactive form; elevated in chronic stress, inflammation, nutrient deficiencies; high reverse T3 blocks active T3
• Thyroid antibodies:
  • TPO antibodies (thyroid peroxidase) → elevated in Hashimoto's thyroiditis (autoimmune hypothyroidism)
  • Thyroglobulin antibodies → also elevated in Hashimoto's
  • TSI antibodies (thyroid-stimulating immunoglobulin) → elevated in Graves' disease (autoimmune hyperthyroidism)

Why comprehensive testing matters:

• TSH alone can miss thyroid dysfunction → you can have "normal" TSH but low free T3 (functional hypothyroidism)
• Free T3 is the most important number → it's the active hormone that produces effects; if free T3 is low, you'll have symptoms, even if TSH and T4 are normal
• Reverse T3 reveals conversion problems → high reverse T3 (relative to free T3) indicates poor T4-to-T3 conversion (common in stress, inflammation)
• Antibodies detect autoimmune thyroid disease early → you can have elevated antibodies for years before TSH becomes abnormal; early detection allows for intervention

Best timing for testing:

• Morning, fasting (TSH is highest in the morning)
• If on thyroid medication, test before taking your dose (to get accurate levels)

Optimization Strategies

1. Thyroid Hormone Replacement (Most Effective for Hypothyroidism)

Types of thyroid medication:

Levothyroxine (synthetic T4):

• Brand names: Synthroid, Levoxyl, Tirosint
• Most commonly prescribed
• Provides T4 only → body converts T4 to active T3
• Works well for many women, especially if T4-to-T3 conversion is healthy
• May not work well if conversion is poor (chronic stress, inflammation, nutrient deficiencies) → you may need T3 added

Liothyronine (synthetic T3):

• Brand name: Cytomel
• Provides active T3 directly (bypasses conversion)
• Used alone or added to T4 therapy if symptoms persist despite normal TSH on T4 alone
• Shorter half-life than T4 → often taken twice daily

Combination T4/T3 therapy:

• Some practitioners prescribe both T4 and T3 (either as separate medications or compounded together)
• May provide better symptom relief than T4 alone, especially for women with conversion issues

Natural Desiccated Thyroid (NDT):

• Brand names: Armour Thyroid, Nature-Throid, WP Thyroid
• Derived from porcine (pig) thyroid glands
• Contains both T4 and T3 (in roughly 4:1 ratio, plus T2, T1, and calcitonin)
• Some women feel better on NDT than synthetic T4, especially if they have conversion issues
• Dosing can be trickier (ratios of T4:T3 are fixed, may not match individual needs)

What works best is individual:

• Some women do great on T4 alone
• Others need T3 added
• Some prefer NDT
• It often takes trial and error to find the right medication and dose

Monitoring:

• Retest TSH, free T4, free T3 6-8 weeks after starting or adjusting dose
• Goal: TSH in optimal range (0.5-2.5), free T3 in mid-to-upper normal range, symptoms resolved
• Continue monitoring every 6-12 months (or more often if symptomatic)

2. Support T4-to-T3 Conversion

Nutrient sufficiency is critical for thyroid function:

Selenium:

• Required for conversion of T4 to T3 (enzyme 5'-deiodinase is selenium-dependent)
• Deficiency impairs conversion → low T3, high reverse T3
• Dosing: 200 mcg daily (from food or supplement)
• Food sources: Brazil nuts (1-2 per day), fish, eggs, poultry

Zinc:

• Supports thyroid hormone production and conversion
• Dosing: 15-30 mg daily
• Food sources: Oysters, red meat, poultry, beans, nuts

Iron:

• Required for thyroid hormone production (thyroid peroxidase enzyme is iron-dependent)
• Deficiency (common in women) impairs thyroid function
• Test ferritin (iron storage); optimal level: 70-90 ng/mL
• Dosing: Based on ferritin level; work with clinician
• Food sources: Red meat, poultry, fish, beans, spinach (animal sources are better absorbed)

Iodine:

• Required for thyroid hormone production (T4 has 4 iodine molecules; T3 has 3)
• Deficiency impairs thyroid function
• Caution: Too much iodine can worsen autoimmune thyroid disease (Hashimoto's, Graves')
• Dosing: Most people get enough from iodized salt; supplementation should be guided by testing
• Food sources: Iodized salt, seaweed, fish, dairy

Vitamin D:

• Supports immune function and may reduce autoimmune thyroid disease progression
• Dosing: 1000-4000 IU daily (or enough to achieve blood level of 40-60 ng/mL)

B vitamins (especially B12):

• Support energy production and thyroid function
• Dosing: B-complex or B12 (500-1000 mcg daily)

3. Reduce Stress and Support HPA Axis

Chronic stress and high cortisol reduce T4-to-T3 conversion:

• Stress management: Meditation, breathwork, therapy, boundaries, rest
• Adaptogenic herbs: Ashwagandha (supports thyroid and adrenals), rhodiola, holy basil
• Sleep: Prioritize 7-9 hours per night
• Blood sugar stability: Eat protein and healthy fats at every meal

4. Reduce Inflammation

Inflammation impairs thyroid function and worsens autoimmune thyroid disease:

• Anti-inflammatory diet: Whole foods, healthy fats (omega-3s), limit processed foods, sugar, refined carbs
• Eliminate food sensitivities: Gluten is a common trigger for Hashimoto's (consider trial elimination)
• Gut health: Heal leaky gut, support microbiome (probiotics, fiber, fermented foods)
• Omega-3 fatty acids: 1000-2000 mg EPA+DHA daily

5. Avoid Thyroid Disruptors

Certain foods, medications, and environmental toxins interfere with thyroid function:

Goitrogens (in large amounts, when raw):

• Cruciferous vegetables (broccoli, kale, Brussels sprouts, cabbage)
• Soy (especially processed soy)
• Note: Cooking deactivates most goitrogens; moderate amounts are fine for most people

Medications:

• Biotin (high-dose supplements) → can interfere with thyroid lab tests (false results)
• Estrogen (HRT or birth control) → increases TBG, may require thyroid dose adjustment
• Calcium, iron, magnesium supplements → can bind to thyroid medication and reduce absorption (take 4 hours apart)

Environmental toxins:

• Fluoride, chlorine, bromide → can displace iodine in the thyroid
• BPA, phthalates, pesticides → endocrine disruptors that affect thyroid function
• Reduce exposure: Filter water, choose organic when possible, avoid plastic food containers

6. Lifestyle

Exercise:

• Moderate exercise supports thyroid function and metabolism
• Caution: Excessive exercise can suppress thyroid (if already hypothyroid, avoid overtraining)

Sleep:

• Thyroid hormones regulate circadian rhythm; poor sleep disrupts thyroid function
• Prioritize consistent sleep/wake times, sleep hygiene

When to Review with Clinician

You should discuss thyroid if:

• Symptoms of hypothyroidism (fatigue, weight gain, cold intolerance, brain fog, hair thinning, constipation, depression)
• Symptoms of hyperthyroidism (anxiety, weight loss, rapid heart rate, heat intolerance, insomnia, tremor)
• Family history of thyroid disease or autoimmune conditions
• Personal history of autoimmune disease
• Heavy or irregular periods (thyroid dysfunction can affect menstrual cycle)
• High cholesterol (hypothyroidism can raise cholesterol)
• Currently on thyroid medication and symptoms persist (may need dose adjustment or addition of T3)
• Interested in comprehensive thyroid testing (TSH, free T3, free T4, reverse T3, antibodies)

Red flags requiring medical attention:

• Severe hyperthyroidism: Rapid heart rate (>120 bpm), chest pain, severe anxiety, unintentional weight loss, bulging eyes (possible Graves' disease or thyroid storm—can be life-threatening)
• Severe hypothyroidism: Extreme fatigue, confusion, low body temperature, slow heart rate, swelling (possible myxedema coma—rare but life-threatening)
• Thyroid nodule or goiter (enlarged thyroid) → requires evaluation to rule out cancer

Related Terms

• cortisol
• estrogen
• progesterone
• insulin
• fatigue
• weight-gain
• brain-fog
• hair-thinning
• cold-intolerance
• hashimotos-thyroiditis
• hypothyroidism
• hyperthyroidism