Hashimoto's: the blood tests your doctor should be running.

Why TSH alone is not enough

If you have Hashimoto's thyroiditis, there is a good chance your doctor monitors your thyroid with a single number: TSH (thyroid-stimulating hormone). TSH is a pituitary hormone that rises when thyroid hormones are low. It is useful. It is also insufficient.

TSH tells you whether your pituitary thinks you have enough thyroid hormone. It does not tell you how aggressive the autoimmune attack is, whether your thyroid is converting T4 to T3 properly, or whether nutrient deficiencies are undermining thyroid function. A normal TSH with raging antibodies, low free T3, and depleted ferritin is not a healthy thyroid — it is a thyroid that has not yet failed enough for TSH to flag it[1].

Here is what the complete Hashimoto's blood panel looks like, and why each marker matters.

Thyroid antibodies: the disease markers

TPO antibodies (anti-thyroid peroxidase)

Thyroid peroxidase is the enzyme responsible for synthesizing thyroid hormones. In Hashimoto's, the immune system produces antibodies against this enzyme, gradually destroying thyroid tissue. TPO antibodies (TPO-Ab) are elevated in approximately 90-95% of Hashimoto's patients[2].

TPO-Ab levels correlate with the degree of lymphocytic infiltration of the thyroid — in other words, higher antibodies generally mean more aggressive autoimmune attack. They also predict future thyroid failure: a person with elevated TPO-Ab and normal TSH has a 4.3% annual risk of developing overt hypothyroidism, compared to 2.6% for someone with elevated TSH but normal antibodies[3].

Thyroglobulin antibodies (TgAb)

Thyroglobulin is the protein scaffold on which thyroid hormones are assembled. Antibodies against thyroglobulin (TgAb) are elevated in approximately 60-80% of Hashimoto's patients. In about 5-10% of Hashimoto's cases, TgAb is elevated while TPO-Ab is normal — so testing both improves diagnostic sensitivity[2].

TSH: useful but misunderstood

TSH remains important but needs context. The standard reference range for TSH is typically 0.4-4.0 mIU/L, but this range is based on population statistics that include people with subclinical thyroid disease. The National Academy of Clinical Biochemistry (NACB) has suggested that 95% of rigorously screened euthyroid individuals have a TSH below 2.5 mIU/L[4].

For Hashimoto's patients on levothyroxine, many endocrinologists target a TSH between 0.5-2.0 mIU/L, acknowledging that a TSH of 3.5 — while technically "normal" — may leave patients symptomatic. The American Thyroid Association guidelines recommend maintaining TSH in the lower half of the reference range for patients on replacement therapy[5].

Free T4 and Free T3: the active hormones

Free T4 (thyroxine)

Free T4 is the unbound, bioavailable form of thyroxine — the primary hormone produced by the thyroid. It is also what levothyroxine replacement provides. In Hashimoto's, free T4 drops as thyroid tissue is destroyed. A low or low-normal free T4 with elevated TSH confirms hypothyroidism.

Free T3 (triiodothyronine)

T3 is the active thyroid hormone — it is approximately 3-5x more biologically potent than T4. About 80% of T3 is produced by peripheral conversion of T4 to T3 by deiodinase enzymes (mainly in the liver, kidneys, and muscles)[5].

Some Hashimoto's patients have normal TSH and free T4 but low free T3. This "poor converter" phenotype may explain why 5-10% of hypothyroid patients on levothyroxine (which is T4-only) report persistent symptoms despite "normal" labs[6].

Factors that impair T4-to-T3 conversion include: selenium deficiency, iron deficiency, chronic stress (elevated cortisol), inflammation, caloric restriction, and certain medications. These are all modifiable — but you cannot address what you do not measure.

The nutrient connection

Selenium

Selenium is critical for thyroid function in two ways: it is a cofactor for glutathione peroxidase (which protects thyroid tissue from oxidative damage during hormone synthesis) and for deiodinase enzymes (which convert T4 to T3)[7].

Multiple randomized controlled trials have shown that selenium supplementation (200 mcg/day of selenomethionine) reduces TPO antibody levels by 20-40% over 3-12 months in Hashimoto's patients[8]. A 2010 Cochrane-style systematic review confirmed this finding, though noted that the clinical significance (symptom improvement, slowed progression to hypothyroidism) is less well established[7].

Much of Europe is selenium-deficient due to low soil selenium content. The recommended daily intake is 55 mcg, but the therapeutic dose used in Hashimoto's trials is 200 mcg. Brazil nuts are the richest food source (one nut contains approximately 70-90 mcg), but content varies widely.

Vitamin D

Vitamin D deficiency is significantly more prevalent in Hashimoto's patients compared to controls. A 2013 meta-analysis found that Hashimoto's patients had significantly lower 25(OH)D levels and higher rates of vitamin D deficiency[9].

Multiple studies show an inverse correlation between vitamin D levels and TPO antibody titers — lower vitamin D, higher antibodies. Supplementation studies show modest but consistent antibody reductions when vitamin D is optimized to 40-60 ng/mL. The relationship is bidirectional: Hashimoto's may impair vitamin D metabolism, and low vitamin D may worsen autoimmune activity.

Iron and ferritin

Iron is a cofactor for thyroid peroxidase — the very enzyme that Hashimoto's antibodies attack. Iron deficiency impairs thyroid hormone synthesis, impairs T4-to-T3 conversion, and reduces the effectiveness of levothyroxine replacement[8].

Women with Hashimoto's are particularly vulnerable to iron deficiency because hypothyroidism often causes heavier menstrual periods. A ferritin below 30 ng/mL — which many labs call "normal" — is associated with impaired thyroid function and persistent hypothyroid symptoms. Targeting ferritin above 50-70 ng/mL is a reasonable goal for Hashimoto's patients[10].

B12

Autoimmune conditions tend to cluster. Hashimoto's patients have a higher prevalence of pernicious anemia (autoimmune B12 deficiency) and atrophic gastritis. B12 deficiency causes fatigue, brain fog, and numbness — symptoms that overlap with hypothyroidism and may be misattributed to inadequate thyroid replacement[1].

The complete Hashimoto's blood panel

Stages of Hashimoto's progression

Hashimoto's does not begin with hypothyroidism. It progresses through stages that are visible on blood work years before TSH becomes abnormal[3]:

1. Stage 1 — Genetic susceptibility: No lab abnormalities yet. Family history of autoimmune thyroid disease.
2. Stage 2 — Immune activation: TPO-Ab and/or TgAb elevated. TSH, free T4, free T3 all normal. This stage can persist for years or decades. Some people never progress beyond it.
3. Stage 3 — Subclinical hypothyroidism: TSH elevated (4.0-10.0 mIU/L). Free T4 and free T3 still in the normal range but often low-normal. Symptoms may be present.
4. Stage 4 — Overt hypothyroidism: TSH elevated, free T4 low. Unambiguous diagnosis. Levothyroxine required.
5. Stage 5 — Thyroid atrophy: Long-standing disease. Thyroid tissue replaced by fibrosis. Antibodies may actually decrease (less tissue to react against). Fully dependent on replacement.

The clinical opportunity is in stages 2 and 3 — when intervention (selenium, vitamin D, gluten assessment, stress management) may slow or prevent progression.

The gluten question

The connection between Hashimoto's and celiac disease is well established — celiac disease is 4-5x more common in Hashimoto's patients than in the general population[1]. Screening for celiac (tissue transglutaminase IgA antibodies) is warranted in all Hashimoto's patients.

The more controversial question is whether non-celiac gluten sensitivity affects Hashimoto's. Some interventional studies show that gluten-free diets reduce TPO antibody levels in Hashimoto's patients who do not have celiac disease, but the evidence is limited and mixed[11]. A reasonable approach: screen for celiac disease (blood test), and if celiac is ruled out, consider a 3-6 month gluten-free trial with antibody monitoring.

Hashimoto's and pregnancy

Thyroid management during pregnancy is critical. TSH targets are trimester-specific: below 2.5 mIU/L in the first trimester, below 3.0 mIU/L in the second and third[12]. Levothyroxine dose typically needs to increase by 30-50% early in pregnancy.

Elevated TPO antibodies during pregnancy increase the risk of miscarriage, preterm delivery, and postpartum thyroiditis — even when TSH is normal. Levothyroxine treatment in TPO-Ab-positive women with subclinical hypothyroidism reduces these risks[12].

Monitoring schedule during pregnancy: TSH every 4-6 weeks in the first trimester, then every trimester. Free T4 as needed. Postpartum check at 6 weeks and 6 months (postpartum thyroiditis flares are common).

Hashimoto's encephalopathy

A rare but important condition: Hashimoto's encephalopathy (also called steroid-responsive encephalopathy associated with autoimmune thyroiditis, or SREAT). Symptoms include confusion, seizures, tremor, and psychiatric disturbance. TSH may be normal. The diagnosis is suggested by very high TPO antibody titers in the setting of neurological symptoms and confirmed by response to corticosteroid treatment[13].

Other autoimmune screening

Hashimoto's rarely travels alone. Consider screening for:

• Celiac disease: tTG-IgA (4-5x higher prevalence in Hashimoto's)
• Type 1 diabetes: Fasting glucose, HbA1c, possibly GAD antibodies if clinically indicated
• Pernicious anemia: B12, intrinsic factor antibodies
• Addison's disease: AM cortisol if symptoms suggest (rare but serious)
• Vitiligo, alopecia areata: Clinical assessment

Medication interactions and absorption

Levothyroxine absorption is affected by numerous factors[5]:

• Timing: Take on an empty stomach, 30-60 minutes before food or other medications.
• Iron and calcium supplements: Separate by at least 4 hours — they bind levothyroxine in the gut.
• PPIs (omeprazole, etc.): Reduce absorption. May need dose increase.
• Coffee: Reduces absorption by up to 30% if taken simultaneously. Wait at least 30 minutes.
• GLP-1 agonists (semaglutide, tirzepatide): Slowed gastric emptying may affect absorption. Recheck TSH 6-8 weeks after starting.

Monitoring schedule

• Dose adjustment phase: TSH, free T4, free T3 every 6-8 weeks until stable
• Stable on treatment: TSH, free T4, free T3 every 6-12 months
• Annually: TPO-Ab (track trend), vitamin D, ferritin, B12
• Pregnancy: TSH every 4-6 weeks first trimester, then each trimester
• After medication/lifestyle change: Recheck full panel in 6-8 weeks