Residency · Residency · Endocrinology

Hypothyroidism - Hashimoto Thyroiditis

Classification and Epidemiology

Classification

Hypothyroidism is classified based on the anatomical level of dysfunction. Primary hypothyroidism, accounting for approximately 95% of cases, results from failure of the thyroid gland itself and is characterized by elevated TSH with low free T4. Central hypothyroidism, encompassing both secondary (pituitary) and tertiary (hypothalamic) etiologies, accounts for the remaining 5% and presents with low or inappropriately normal TSH in the setting of low free T4. Subclinical hypothyroidism, defined as an elevated TSH with a normal free T4, has a prevalence of 4 to 10% in the adult population, while overt hypothyroidism, with elevated TSH and frankly low free T4, affects 0.3 to 3.7% of adults. A rare but instructive entity, consumptive hypothyroidism, occurs when massive infantile hemangiomas express high levels of type 3 deiodinase, which inactivates T4 and T3 faster than the thyroid can produce them.

Epidemiology

Autoimmune thyroid disease demonstrates a striking female predominance, with a female-to-male ratio of 5 to 8:1. The prevalence of hypothyroidism increases with age, reaching 10 to 15% of elderly women with elevated TSH. Geographic variation exists, with higher prevalence in iodine-sufficient and iodine-excess regions, reflecting the thyrotoxic and immunostimulatory effects of iodine on susceptible thyroid glands. Data from the NHANES III survey in the United States documented overt hypothyroidism in 0.3%, subclinical hypothyroidism in 4.3%, and positive anti-TPO antibodies in 11.3% of the general population.

Hashimoto Thyroiditis (Chronic Lymphocytic Thyroiditis)

Pathophysiology

Hashimoto thyroiditis is an organ-specific autoimmune disease characterized by progressive immune-mediated destruction of thyroid follicular cells. The pathological process involves both cellular and humoral immune mechanisms. CD4+ and CD8+ T lymphocytes infiltrate the thyroid parenchyma, with CD8+ cytotoxic T cells directly attacking follicular cells and CD4+ helper T cells orchestrating the immune response through a Th1-dominant cytokine milieu rich in interferon-gamma, TNF-alpha, and IL-1. The inflammatory infiltrate often organizes into germinal centers within the thyroid, reflecting active local immune activation.

Anti-TPO antibodies, present in more than 90% of patients, are directly cytotoxic to thyroid cells through complement fixation and antibody-dependent cell-mediated cytotoxicity (ADCC). Anti-thyroglobulin antibodies, present in 60 to 80% of patients, are less specific but contribute to the autoimmune process. In a subset of patients, blocking TSH receptor antibodies are present and contribute to thyroid atrophy.

Genetic susceptibility involves multiple loci, including HLA-DR3, HLA-DR4, and HLA-DR5 associations, as well as polymorphisms in CTLA-4 (a critical negative regulator of T cell activation), PTPN22 (a protein tyrosine phosphatase involved in T cell signaling), and thyroglobulin gene variants. Environmental triggers play an important role in disease initiation in genetically susceptible individuals. Iodine excess is the most well-established environmental factor, likely through increased thyroglobulin immunogenicity and direct thyroid cell toxicity. Other environmental factors include selenium deficiency, viral infections (hepatitis C virus has been implicated), psychological stress, radiation exposure, and medications including interferon-alpha, IL-2, lithium, amiodarone, and immune checkpoint inhibitors.

Clinical Variants

Hashimoto thyroiditis manifests in several distinct clinical variants. Goitrous Hashimoto, the most common presentation, features a firm, diffuse, nontender goiter with a lobulated or "bosselated" surface texture. Atrophic thyroiditis presents without goiter, with the thyroid becoming progressively atrophic, often in association with blocking TSH receptor antibodies, and is more common in elderly patients.

Hashitoxicosis represents a transient thyrotoxic phase occurring at disease onset, caused by destructive release of stored thyroid hormone from inflamed follicles. It is distinguished from Graves disease by low radioactive iodine uptake and is self-limited, typically resolving within weeks to months before progressing to the hypothyroid phase. The fibrous variant is characterized by dense fibrosis producing a hard gland that may clinically mimic malignancy or Riedel thyroiditis. IgG4-related thyroiditis is a recently recognized subset characterized by IgG4-positive plasma cell infiltration, associated with systemic IgG4 disease, and tends to show more prominent fibrosis and more rapid progression.

Associations

Hashimoto thyroiditis frequently coexists with other autoimmune conditions, reflecting a shared genetic predisposition to immune dysregulation. Type 1 diabetes is among the most common associations, with 15 to 30% of patients having thyroid antibodies. Other associated conditions include celiac disease, vitiligo, Addison disease, pernicious anemia, rheumatoid arthritis, and systemic lupus erythematosus. The combination of Hashimoto thyroiditis with Addison disease and type 1 diabetes constitutes autoimmune polyglandular syndrome type 2 (APS-2, or Schmidt syndrome). Chromosomal disorders, particularly Turner syndrome (in which 50% develop autoimmune thyroiditis) and Down syndrome (15 to 20%), carry elevated risk.

A critically important association is the 60- to 80-fold increased risk of primary thyroid lymphoma in patients with longstanding Hashimoto thyroiditis. Thyroid lymphoma should be suspected when a patient with known Hashimoto thyroiditis presents with a rapidly enlarging goiter, particularly one that causes compressive symptoms. Most thyroid lymphomas are diffuse large B-cell lymphoma (DLBCL) or mucosa-associated lymphoid tissue (MALT) lymphoma.

<image>A histopathological illustration comparing normal thyroid tissue with Hashimoto thyroiditis. Left panel: normal thyroid showing uniform follicles filled with pink colloid, lined by cuboidal follicular epithelium. Right panel: Hashimoto thyroiditis showing dense lymphocytic infiltration with germinal centers, Hurthle cell (oncocytic) change of follicular cells with abundant eosinophilic granular cytoplasm, follicular destruction and atrophy, and interstitial fibrosis. Label key features including germinal centers, Hurthle cells, lymphocytic infiltrate, and residual follicles. Use H&E staining appearance with clean medical illustration style.</image>

Other Causes of Hypothyroidism

Primary Causes

Beyond autoimmune thyroiditis, numerous other conditions can cause primary hypothyroidism. Iodine deficiency remains the most common cause of hypothyroidism worldwide, producing goiter and, in severe cases, endemic cretinism. Conversely, iodine excess can cause hypothyroidism through the Wolff-Chaikoff effect in susceptible glands, including those already affected by Hashimoto thyroiditis or previously treated with radioactive iodine.

Post-ablative hypothyroidism predictably follows radioactive iodine therapy, with 50 to 80% of patients treated for Graves disease developing hypothyroidism within the first year, and it is essentially universal after thyroidectomy. External beam radiation to the head and neck for lymphoma or head and neck cancer causes hypothyroidism in a dose-dependent fashion that may take years to manifest.

Drug-induced hypothyroidism represents an important and growing category. Amiodarone, which contains 37% iodine by weight (delivering approximately 75 mg of iodine per 200 mg tablet, far exceeding the daily recommended intake), can cause hypothyroidism through either the Wolff-Chaikoff effect or type 2 destructive thyroiditis. Lithium inhibits thyroid hormone release, causing hypothyroidism in 5 to 15% of chronic users. Interferon-alpha induces autoimmune thyroiditis in 5 to 10% of treated patients. Immune checkpoint inhibitors, particularly anti-PD-1 agents, cause thyroiditis in 5 to 10% of patients, frequently preceded by a transient thyrotoxic phase. Tyrosine kinase inhibitors such as sunitinib and sorafenib can cause destructive thyroiditis and increased T4 clearance. Bexarotene increases T4 clearance via glucuronidation and directly suppresses TSH.

Infiltrative diseases including amyloidosis, hemochromatosis, sarcoidosis, scleroderma, and Riedel thyroiditis can replace thyroid parenchyma and impair function. Congenital hypothyroidism is most commonly caused by thyroid dysgenesis (85%), encompassing agenesis, ectopy, and hypoplasia, with dyshormonogenesis accounting for the remaining 15%. Newborn screening programs using TSH or T4 measurement enable early detection and treatment.

Central Hypothyroidism

Central hypothyroidism results from pituitary adenomas, pituitary surgery, radiation, infiltrative diseases, Sheehan syndrome, or traumatic brain injury. A critical diagnostic point is that TSH may be low, normal, or even mildly elevated in central hypothyroidism, the latter reflecting secretion of biologically inactive glycosylation variants of TSH that are immunoreactive but lack full bioactivity. The diagnosis is established by a low free T4, and treatment monitoring must be based on free T4 rather than TSH.

Clinical Features of Hypothyroidism

Symptoms (insidious onset)

The clinical manifestations of hypothyroidism develop insidiously over months to years, often leading to delayed recognition. Cardinal symptoms include fatigue, lethargy, cold intolerance, and weight gain, typically modest at 5 to 10 kilograms and primarily representing fluid accumulation rather than adiposity. Gastrointestinal manifestations include constipation. Integumentary findings include dry skin and hair loss, which is characteristically diffuse, with loss of the lateral third of the eyebrows (madarosis) being a classic but inconsistently present sign.

Neuropsychiatric symptoms encompass cognitive slowing, depression, and memory impairment. Reproductive manifestations include menstrual irregularities, initially manifesting as menorrhagia (from anovulatory cycles) and progressing to oligomenorrhea or amenorrhea, infertility, and hyperprolactinemia resulting from TRH-stimulated prolactin release. Musculoskeletal symptoms include myalgias, arthralgias, carpal tunnel syndrome, and muscle cramps.

Signs

Physical examination in hypothyroidism reveals bradycardia, diastolic hypertension from increased systemic vascular resistance, and narrowed pulse pressure. Non-pitting edema (myxedema), resulting from accumulation of hyaluronic acid and glycosaminoglycans in the skin and subcutaneous tissues, is a characteristic finding. Periorbital edema, macroglossia, and hoarseness from vocal cord edema may be present. A particularly classic sign is the delayed relaxation phase of deep tendon reflexes, sometimes termed pseudo-myotonia or "hung-up reflexes."

The thyroid gland may be enlarged as a firm, lobulated goiter (in Hashimoto thyroiditis or iodine deficiency) or absent (in atrophic thyroiditis or post-surgical patients). Serous effusions are common in severe hypothyroidism: pericardial effusion is present in up to 30% of severe cases, though tamponade is rare given the slow rate of fluid accumulation. Pleural effusions and ascites may also develop.

Laboratory Findings (Beyond TFTs)

Hypothyroidism produces several characteristic non-thyroid laboratory abnormalities. Creatine kinase (CK) is elevated, sometimes up to 10 times the upper limit of normal, reflecting hypothyroid myopathy. The lipid profile typically shows elevated LDL cholesterol, total cholesterol, and triglycerides, as well as increased lipoprotein(a). Hyponatremia from impaired free water excretion mimics SIADH. Anemia may be normocytic (from decreased erythropoietin) or macrocytic (from associated pernicious anemia or folate malabsorption). Homocysteine is elevated. Prolactin may be mildly increased due to TRH-mediated stimulation of lactotrophs.

Diagnosis

Biochemical Diagnosis

The diagnostic workup begins with TSH as the first-line screening test, which is elevated in primary hypothyroidism. Free T4 confirms the diagnosis when TSH is elevated and is essential for diagnosing central hypothyroidism, where TSH is unreliable. Anti-TPO antibodies establish the autoimmune etiology and are positive in more than 90% of Hashimoto thyroiditis cases. Their clinical significance extends to prognostication: the Wickham survey demonstrated that TPO-positive individuals with subclinical hypothyroidism progress to overt hypothyroidism at a rate of approximately 4.3% per year, compared to 2.6% per year in TPO-negative individuals. Anti-thyroglobulin antibodies provide supportive evidence but are less specific.

Thyroid ultrasound is not required for the diagnosis of Hashimoto thyroiditis but may be performed when there is clinical uncertainty or a palpable nodule. The typical sonographic appearance shows diffuse heterogeneity, hypoechogenicity, and increased vascularity in the early inflammatory phase, progressing to decreased vascularity in the late or atrophic stage. Incidental nodules detected on ultrasound require separate evaluation according to standard guidelines.

Subclinical Hypothyroidism - When to Treat

The management of subclinical hypothyroidism requires an individualized approach based on TSH level, symptom burden, and patient-specific risk factors. When TSH exceeds 10 mIU/L, treatment is generally recommended regardless of symptoms, as the risk of progression to overt disease and the potential cardiovascular consequences at this level of TSH elevation favor intervention. For TSH between 4.5 and 10 mIU/L with symptoms, a trial of levothyroxine with reassessment of symptoms is reasonable. For the same TSH range without symptoms, treatment should be considered if anti-TPO antibodies are positive, goiter is present, dyslipidemia is identified, pregnancy is being planned, or other cardiovascular risk factors exist.

In elderly patients over 65 to 70 years of age, a higher threshold for treatment is appropriate. The TRUST trial demonstrated no benefit of levothyroxine therapy for subclinical hypothyroidism (TSH 4.6 to 19.9 mIU/L) in patients 65 years and older, and observational data suggest that higher TSH levels may be associated with longevity in the very elderly population. During pregnancy, treatment is recommended when TSH exceeds the trimester-specific upper limit of normal (generally above 4.0 mIU/L), and some guidelines recommend treating at a lower threshold of 2.5 mIU/L in TPO-positive women attempting conception.

Treatment

Levothyroxine (T4) - Standard of Care

Levothyroxine, a synthetic form of T4 identical to the endogenous hormone, is the standard of care for hypothyroidism. Its long half-life of 6 to 7 days provides stable serum levels with once-daily dosing. The full replacement dose is approximately 1.6 mcg/kg/day based on lean body weight, which translates to 75 to 125 mcg daily for most adults. In young, healthy patients, the full replacement dose can be initiated immediately. In elderly patients or those with cardiac disease, a cautious approach is warranted, starting at 12.5 to 25 mcg daily with increments of 12.5 to 25 mcg every 4 to 6 weeks to avoid precipitating cardiac arrhythmias or angina. The TSH target for most adults is 0.5 to 2.5 mIU/L, narrowing to 0.5 to 2.5 mIU/L during the first trimester of pregnancy, and relaxing to 1 to 4 mIU/L in the elderly.

Administration and Absorption

Optimal levothyroxine absorption requires attention to administration technique. The medication should be taken on an empty stomach, 30 to 60 minutes before breakfast, or alternatively at bedtime at least 3 hours after the last meal. Approximately 70 to 80% of the oral dose is absorbed, primarily in the jejunum and ileum.

Numerous medications and supplements reduce levothyroxine absorption and must be separated by appropriate intervals. Calcium supplements, iron preparations, aluminum-containing antacids, and sucralfate should be separated by at least 4 hours. Cholestyramine, proton pump inhibitors, H2 blockers (which have a modest effect), sevelamer, and coffee (when taken simultaneously) can also impair absorption. Liquid and softgel formulations are less affected by gastric pH and food interactions and represent useful alternatives in patients with malabsorption or those taking PPIs. Intravenous levothyroxine, dosed at 50 to 80% of the oral dose, is used for NPO patients and in the treatment of myxedema coma.

Monitoring

TSH should be rechecked 6 to 8 weeks after initiating or adjusting the dose, allowing adequate time for T4 to reach steady state given its 6- to 7-day half-life. Adjustments based on TSH checked before 6 weeks risk inappropriate dose changes. Once stable, annual TSH monitoring is sufficient. In central hypothyroidism, monitoring must rely on free T4, targeting the upper half of the normal range, as TSH is unreliable for dose titration.

Dose Adjustments in Special Situations

Several clinical situations necessitate dose adjustments. Pregnancy requires an immediate dose increase of approximately 30 to 50%, often empirically implemented as two extra tablets per week, with TSH monitoring every 4 weeks through the first half of gestation. Aging decreases requirements by approximately 25% after age 70. Weight changes require proportional adjustment. Initiation of oral estrogen therapy increases TBG and may necessitate a higher levothyroxine dose, while transdermal estrogen has less effect. Malabsorptive conditions, including celiac disease, bariatric surgery, and short bowel syndrome, may require higher doses, and liquid formulations may improve bioavailability. Medications that increase T4 clearance, including phenytoin, carbamazepine, rifampin, and sertraline (modest effect), may necessitate dose increases.

<image>A patient education infographic about levothyroxine administration best practices. Show a timeline of a typical morning routine: (1) Wake up and take levothyroxine with a full glass of water on an empty stomach, (2) Wait at least 30-60 minutes, (3) Then have breakfast, coffee, and other medications. Include icons for common interfering substances with minimum separation times: calcium supplements (4 hours), iron supplements (4 hours), antacids (4 hours), PPI (take together is OK but may need higher dose), coffee (30 min if separated). Include a box about liquid/softgel formulations as alternatives when absorption is an issue. Use clean, patient-friendly infographic style.</image>

Combination T4/T3 Therapy (Controversy)

Some patients report persistent symptoms despite achieving normal TSH on levothyroxine monotherapy, leading to interest in combination T4/T3 therapy using liothyronine. However, multiple randomized controlled trials and meta-analyses have failed to demonstrate consistent superiority of combination therapy over T4 monotherapy. The 2014 ATA guidelines do not routinely recommend combination therapy but acknowledge that a subgroup of patients may benefit, possibly including carriers of the D2 Thr92Ala polymorphism, though the evidence for this pharmacogenomic approach remains weak.

When combination therapy is attempted, the T4-to-T3 ratio should approximate the physiologic ratio of 13 to 20:1, using liothyronine at 5 to 10 mcg daily in divided doses with a corresponding reduction in T4 dose. Sustained-release T3 formulations, when available, may provide more stable T3 levels. Desiccated thyroid extract (Armour Thyroid, NP Thyroid), derived from porcine thyroid, contains T4 and T3 in a ratio of approximately 4:1, which delivers supraphysiologic T3 relative to T4. While some patients report preference for desiccated thyroid, concerns about T3 peaks, batch variability, and the absence of supporting evidence from major clinical trials preclude endorsement by major professional guidelines.

Persistent Symptoms Despite Normal TSH

When patients continue to report symptoms despite a normalized TSH, a systematic evaluation should be undertaken before attributing the symptoms to thyroid disease. Depression accounts for 40 to 60% of residual symptoms and should be specifically assessed. Other conditions to exclude include sleep apnea, iron deficiency, vitamin B12 deficiency, adrenal insufficiency (particularly in the setting of pituitary disease), celiac disease, and other coexisting autoimmune conditions. Dose optimization targeting the lower portion of the TSH range and selenium supplementation (200 mcg daily, with modest evidence of benefit for autoimmune symptom burden in some studies) represent additional therapeutic considerations. Patients should be counseled that most weight gained during hypothyroidism reflects fluid retention, with expected weight loss of 3 to 5 kilograms following treatment; persistent obesity is usually not attributable to thyroid dysfunction.

Hypothyroidism in Pregnancy

Maternal and Fetal Risks of Untreated Hypothyroidism

Untreated hypothyroidism during pregnancy carries significant risks for both mother and fetus. Maternal complications include miscarriage, preeclampsia, placental abruption, preterm delivery, and gestational hypertension. Fetal risks center on impaired neurocognitive development, particularly during the first trimester when the fetus depends entirely on transplacental transfer of maternal T4 for brain development, as well as low birth weight. Although the CATS trial and other randomized trials did not demonstrate improved childhood IQ with screening and treatment of subclinical hypothyroidism during pregnancy, this may reflect the relatively late gestational timing of treatment initiation in these studies.

Management

Preconception optimization involves targeting TSH below 2.5 mIU/L and ensuring adequate iodine intake of 250 mcg daily. Upon pregnancy confirmation, levothyroxine should be increased by approximately 30 to 50%, with the practical approach of taking two additional tablets per week. TSH monitoring should occur every 4 weeks until 20 weeks of gestation, then at least once at approximately 30 weeks. After delivery, patients should return to their pre-pregnancy dose, with TSH rechecked at 6 weeks postpartum. Euthyroid women who are TPO-positive carry a 16% risk of developing hypothyroidism during pregnancy and should have TSH monitored every 4 weeks.

Key Clinical Pearls

• Hashimoto thyroiditis is by far the most common cause of hypothyroidism in iodine-sufficient regions; however, iodine deficiency remains the most common cause worldwide
• The Wickham survey demonstrated that TPO-positive women with TSH >2.0 mIU/L have a 4.3% annual rate of progression to overt hypothyroidism, supporting monitoring of this subgroup
• A rapidly enlarging goiter in a patient with longstanding Hashimoto should raise concern for primary thyroid lymphoma; core needle biopsy (not FNA) is preferred for diagnosis (need tissue architecture)
• Myxedema coma mortality remains 30-60% despite treatment; high clinical suspicion in hypothermic, obtunded patients is essential (covered separately in thyroid emergencies)
• In patients with new hypothyroidism and adrenal insufficiency (e.g., panhypopituitarism), always start glucocorticoid replacement BEFORE levothyroxine to avoid precipitating adrenal crisis
• Hashimoto thyroiditis can coexist with Graves disease; some patients oscillate between hypothyroidism and hyperthyroidism depending on the balance of stimulating vs blocking antibodies

References

1. Garber JR, et al. "Clinical Practice Guidelines for Hypothyroidism in Adults: Cosponsored by the American Association of Clinical Endocrinologists and the American Thyroid Association." Thyroid. 2012;22(12):1200-1235.
2. Jonklaas J, et al. "Guidelines for the Treatment of Hypothyroidism." Thyroid. 2014;24(12):1670-1751.
3. Stott DJ, et al. "Thyroid Hormone Therapy for Older Adults with Subclinical Hypothyroidism." N Engl J Med. 2017;376(26):2534-2544.
4. Caturegli P, et al. "Hashimoto Thyroiditis: Clinical and Diagnostic Criteria." Autoimmun Rev. 2014;13(4-5):391-397.
5. Alexander EK, et al. "2017 Guidelines of the ATA for the Diagnosis and Management of Thyroid Disease During Pregnancy and the Postpartum." Thyroid. 2017;27(3):315-389.

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