Investigation of Thyroid Disease

Thyroid Physiology

Two Hormones ‐ T3 & T4

T4‐Only thyroidal source

T3‐20% from thyroid secretion

80% from T4 in other tissues

> 99% protein – bound hormones

FT4‐0.05%

FT3‐0.5%

Reverse T3

Thyroid Regulation

Stimulation by TSH

Negative feed‐back.

TSH under TRH control

Thyroid Hormone Estimation

(1)T3, T4, FT3, FT4 & TSH.

Radio‐immunoassay

ELISA

Chemiluminiscence

(2)Binding ratio (Resin uptake)

(3)TBG level

Why Free hormone levels?

Metabolically active forms

Not affected by binding protein disorders.

TBG excess ‐ Estrogen therapy, pregnancy, hepatitis,   congenital.

Low TBG ‐ Androgen therapy, chronic liver disease.

Thyroid Stimulating Hormone

Single best test for thyroid disease.

When T3 & T4 are high‐TSH is low.

When t3 & T4 are low ‐ TSH is high

Use of TSH level ‐ Diagnosis & follow ‐ up of thyroid dysfunction.

TRH Stimulation Test.

Best  Assays To Order

TSH

FT4

T3 if T3 toxicosis is suspected.

TFT Patterns

T3 T4 TSH Diagnosis

Low             low high         Primary hypothyroidism

Low             low low Pituitary hypothyroidism

High           high low           Hyperthyroidism

High           high high TSH secreting tumourResistance to Thyroid   

hormones

Antithyroid Antibodies

Inhibiting  antibodies:

TMA or TPO antibody

Anti‐thyroglobulin antibody

In Hashimotos Thyroiditis ‐ TMA + ve in 95%TGA + ve in 55%

TPO + ve ‐ future hypothyroidism

Stimulating Antibodies

Thyroid Stimulating Immunoglobulins

Positive in Graves disease

Measured by c‐AMP production by Graves serum 

In thyroid cells

Useful in predicting neo ‐ natal  thyrotoxicosis

Thyroglobulin

Found in colloid

Site of T3 & T4 synthesis.

High TG – HyperthyroidismThyroiditisSmoking

Low TG‐Athyreosis

Use of TG estimation – Follow ‐ up of thyroid cancers

Radioactive Iodine Uptake

Thyroid gland concentrates Iodine

Normal – 25 % after 24 hours

Use ‐ in thyrotoxicosis only ‐

High in Graves disease,toxic MNG, toxic  solitary nodule.

Low in factitious  thyrotoxicosis, sub ‐ acute & post ‐ partum thyroiditis

Iodine induced thyrotoxicosis

Thyroid Scan

I 123 or Technitium ‐ 99m

Use ‐ to assess function in thyroid nodules

Hot ‐ hyperfunctioning

Cold – non ‐ functioning

20 % malignant

Ultrasonography

1. Diagnostic‐For thyroid nodules

Benign‐anechoic,floating debris+‐ or hyperechoicwith eggshell calcification

Malignant ‐ Hypoechoic

Irregular margins

Microcalcification

2. Follow‐up                                                    

Fine Needle Aspiration  Cytology

Safe OP procedure.

90 – 95 % accurate

Useful in solitary nodule

Dominant nodule in MNG

Thyroiditis

? US   guided

Calcitonin

From para ‐ follicular C cells

? physiological   role

High in Medullary carcinoma,

MEN   II

Use‐in families with MTC, MEN   IIFor early diagnosis & Follow ‐ up

Genetic studies