This document discusses the interpretation of thyroid function tests and scan reports. It begins by describing the hypothalamic-pituitary-thyroid axis and how TSH, T4, and T3 levels are regulated. It then discusses various thyroid function tests including total T4 and T3, free T4 and T3, and conditions where measuring TSH alone may be misleading. It provides examples of interpreting test results in different clinical cases. It also discusses potential pitfalls in interpretation including non-thyroidal illness, pregnancy, biotin supplementation, and isolated hypothyroxinemia. Finally, it covers thyroid radionuclide uptake scans and what various uptake patterns may indicate such as Graves' disease, sub

1. Interpretation of Thyroid Function
Tests and Scan Reports
Dr. Animesh Maiti
Associate Professor
Dept Of Endocrinology
Medical college &Hospital

2. Hypothalamic Pituitary Thyroid Axis
• Thyroxine (T4) and triiodothyronine(T3)
production is stimulated by pituitary TSH,
the synthesis of which is regulated by
hypothalamic TRH
• In turn , T4 and T3 inhibit TRH and TSH
production by negative feedback , thus
establishing an equilibrium

3. Thyroid function tests (TFTs)
• One of the most commonly requested laboratory tests
• Important role because of varying clinical manifestations of thyroid disorders
• Most TFTs are straightforward to interpret and confirm the clinical impression of
euthyroidism, hypothyroidism or hyperthyroidism.
• However, in an important subgroup of patients the results of TFTs can seem
confusing, either by virtue of being discordant with the clinical picture or because
they appear incongruent with each other

4. Total T4 and T3 –
• T4 is the principal hormone secreted by the thyroid gland , while most of T3 is
formed enzymatically in nonthyroidal tissues by monodeiodination of T3.
• Easy to measure but problematic in patients with abnormalities of hormone
binding proteins
• Measured by automated RIA kits or chemiluminescent enzyme immunoassay
FT4 and FT3 –
• Free hormone level is responsible for biological activity at cellular level
• Sepration from proteins by equilibrium dialysis or ultrafiltration followed by
measurement of hormones by immunoassays or LC/MS

5. Conditions in which measuring TSH alone may be misleading
• Central hypothyroidism
• Non thyroidal illness
• TSH assay interference
• Recent treatment for thyrotoxicosis
• TSH secreting pituitary adenoma
• Resistance to thyroid hormone

6. • A 24 years old lady presents with dysmenorrhoea and increased flow. also has
associated weight gain, Cold intolerance, Constipation
• O/E : small diffuse goitre
• Thyroid Function Test:
- Free T4= 0.67 ng/dl
- TSH = 50 mIU/L
- Anti TPO ab : positive
• Primary Hypothyroidism

7. FT4 / FT3 ; TSH

8. • A 29-year-old woman in good health discovered her serum TSH to be 6.8 mIU/L
on routine laboratory investigations. All other tests, including complete blood
count and lipid profile, were within normal limits. The patient’s serum TSH was
repeated after 3 months and was found to be 6.5 mIU/L and FT4 was 1.4 ng/dL.
• No significant past or medical history ; not planning pregnancy
• On examination: Her thyroid gland was not enlarged. Rest of the examination was
unremarkable
• Subclinical hypothyroidism

9. FT4 / FT3 ; TSH

10. FT4/FT3 ; TSH or

11. • 35 yr old male
• palpitations , heat intolerance and increased stool frequency * 3 months
• O/E - Gd II Goitre ; PR – 104/ min ; fine tremors + in b/l hands
• Thyroid Function Test:
- Free T4= 3.0 ng/dl
- TSH =0.01 mIU/L
• Grave’s disease

12. FT4/ FT3 ; TSH

13. • T3 toxicosis –
- state in which patients have a high level of T3 and low TSH but a normal level
of T4
- caused by iodine deficiency or the earliest stages of disease caused by an
autonomously functioning thyroid nodule, multinodular goiter or Graves' disease
• T4 Toxicosis
- T4 level is relatively high if thyrotoxicosis is caused by thyroiditis or intake of
levothyroxine
 A total T3 to total T4 ratio less than 20 ng/ mg in thyrotoxic patients before
therapy is a laboratory signal of destruction-induced thyrotoxicosis

14. • A 28 years old asymptomatic lady on routine health screening
• No past h/o thyroid problems
• Menstrual cycle: regular but has not had menses the last cycle.
• Thyroid Function Test:
- Free T4= 1.4 ng/dl
- TSH =0.14 mIU/L
• Normal pregnancy : physiological

15. FT4/ FT3 ; TSH

16. • 35 years old lady
• Presented with headache, nausea and visual distubances
• Secondary amenorrhea * 6 months
• Thyroid Function Test:
- Free T4= 0.6 ng/dl
- TSH =5.25 mIU/L
• Central hypothyroidism

17. or
FT4/ FT3 ; TSH

18. Thyroid autoantibodies

19. Thyroglobulin
• Normally present in serum ( mean – 20 ng /ml )
• Elevated levels seen in –
- Goiter and thyroid hyperfunction
- Inflammatory injury to thyroid
- Differentiated follicular cell derived thyroid tumors
• Important role in management of differentiated thyroid carcinoma and establishing
diagnosis of congenital hypothyroidism

20. Some common pitfalls….

21. Non-thyroidal illness (Sick euthyroid syndrome )
• relatively common finding following any acute or chronic illness
• defined by the absence of an intrinsic abnormality of HPT function
Most commonly –
• Decreased T3
• Increased rT3
• Normal f T4 and TSH

22. Pregnancy
• significant impact on HPT physiology
• Under normal circumstances, about two thirds of circulating T4 is bound to TBG. During
pregnancy TBG levels rise as a consequence of oestrogen-induced increased hepatic
synthesis, together with reduced degradation
• serum total T4 and T3 concentrations increase to approximately 150% of non-pregnant
values
• Free T4 concentrations also change during pregnancy: in the first trimester a transient rise is
often observed and has been ascribed to the stimulatory effects of high circulating levels of
HCG acting on the TSH receptor ; TSH maybe transiently suppressed

23. • In subjects with an intact HPT axis , altered TBG levels result in changes in total
but not free thyroid hormone concentrations

24. Effect of biotin supplementation
• common component of multivitamin preparations
• reported to cause interference in immunoassays resulting in abnormal thyroid function tests
• In general, two-site “sandwich” or noncompetitive immunoassays are used to measure larger
molecules such as thyroid-stimulating hormone (TSH), while competitive assays are used to
measure small molecules such as thyroid and steroid hormone.
• Exogenous biotin can interfere with biotin streptavidin linkage interaction resulting in
- low TSH (noncompetitive assay) combined with high T3 and T4 (competitive assay) could
lead to the biochemical diagnosis of hyperthyroidism

25. Isolated Hypothyoxinemia
• presence of a decreased free thyroxine (FT4) value with a thyrotropin (TSH) level
within the reference range
• Common in pregnancy
• No interventional data has shown beneficial effect of LT4 therapy
• Recommendation – should not be routinely treated

26. O. Koulouri et al. Pitfalls in the measurement and interpretation of thyroid function tests. Best Practice & Research Clinical Endocrinology & Metabolism 27 (2013) 745–762

27. Thyroid radionuclide uptake scan

28. • Measurement of the fractional uptake by the thyroid of a tracer dose of radioiodine
• Less commonly used due to better availability of immunoassays and more
frequent use of thyroid ultrasound and needle biopsies
• Situations when there is concern for overactivity or autonomy of nodules

30. Radiotracers used
Radiotracer Dose Mode of
administration
Time of imaging
Tc-99m
Pertechnetate
3 to 5 mCi Intravenously 20 min. after tracer
administration
Iodine-123
Sodium Iodide
100 to 400 µCi Orally At 4 to 6 hours, sometimes
24 hours after tracer
administration.
Iodine-131
Sodium Iodide
50 to 100 µCi Orally At 24 hours after tracer
administration.

32. Subacute thyroiditis
32 yr old lady relatively rapid
onset of palpitations, insomnia,
anxiety, preceded by an upper
respiratory tract infection.
- neck tenderness +
- free T4 = 2.5 ng/dL,
free T3 = 420 pg/ml
TSH < 0.01 μIU/mL.
- The 24-hour RAIU was 0.5%.

33. Grave’s disease
bilateral enlarged thyroid lobes
with diffusely increased uptake
of the tracer (arrow)

34. Grave’s disease Subacute thyroiditis

35. Toxic adenoma

36. Toxic Multinodular goiter

37. • Cold Nodule

38. Lingual thyroid

39. Thyroid agenesis /
Post thyroidectomy

40. •
Ant Post
I -131 Scan post
therapy
demonstrating
Residual
Thyroid tissue

41. 56 year old patient with
Follicular thyroid carcinoma
post thyroidectomy - whole
body 131I images show
significant 131I uptake in
residual thyroid tissue and
in extensive skeletal and
pulmonary metastases

42. Happy Durga
Pujo 2018